W3C

Ink Markup Language (InkML)

W3C Recommendation 20 September 2011

This version:
http://www.w3.org/TR/2011/REC-InkML-20110920/
Latest version:
http://www.w3.org/TR/InkML
Previous version:
http://www.w3.org/TR/2011/PR-InkML-20110510/
Editors:
Stephen M. Watt, University of Western Ontario and Maplesoft
Tom Underhill, Microsoft
Authors:
Yi-Min Chee (until 2006 while at IBM)
Katrin Franke (until 2004 while at Fraunhofer Gesellschaft)
Max Froumentin (until 2006 while at W3C)
Sriganesh Madhvanath (until 2009 while at HP)
Jose-Antonio Magaña (until 2006 while at HP)
Grégory Pakosz (until 2007 while at Vision Objects)
Gregory Russell (until 2005 while at IBM)
Muthuselvam Selvaraj (until 2009 while at HP)
Giovanni Seni (until 2003 while at Motorola)
Christopher Tremblay (until 2003 while at Corel)
Larry Yaeger (until 2004 while at Apple)

Please refer to the errata for this document, which may include some normative corrections.

See also translations.


Abstract

This document describes the syntax and semantics for the Ink Markup Language.  The Ink Markup Language serves as the data format for representing ink entered with an electronic pen or stylus. The markup allows for the input and processing of handwriting, gestures, sketches, music and other notational languages in applications. It provides a common format for the exchange of ink data between components such as handwriting and gesture recognizers, signature verifiers, and other ink-aware modules.  It may be used in the W3C Multimodal Interaction Framework as proposed by the W3C Multimodal Interaction Activity.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the Recommendation of "Ink Markup Language (InkML)". It has been produced by the Multimodal Interaction Working Group, which is part of the Multimodal Interaction Activity.

Comments are welcome on www-multimodal@w3.org (archive). See W3C mailing list and archive usage guidelines.

The design of InkML has been widely reviewed (see the disposition of comments) and satisfies the Working Group's technical requirements. A list of implementations is included in the InkML 1.0 Implementation Report. There are no substantial changes from the 10 May 2011 Proposed Recommendation.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This specification describes markup for representing ink entered with an electronic pen or stylus and forms part of the proposals for the W3C Multimodal Interaction Framework.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent.


Table of Contents


1 Overview

As more electronic devices with pen interfaces have and continue to become available for entering and manipulating information, applications need to be more effective at using this method of input. Handwriting is a powerful and versatile input modality that is very familiar for most users since everyone learns to write in school. Hence, users will tend to use this as a mode of input and control when available.

A pen-based interface is enabled by a device that allows movements of the pen to be captured as digital ink. A number of methods may be used for ink capture, including those based on radio frequency, optical tracking, physical pressure, or other technologies. Digital ink can be passed on to recognition software that will convert the pen input into appropriate computer actions. Alternatively, the handwritten input can be organized into ink documents, notes or messages that can be stored for later retrieval or exchanged through telecommunications means. Such ink documents are appealing because they capture information as the user composed it, including text in any mix of languages and drawings such as equations and graphs.

Hardware and software vendors have typically stored and represented digital ink using proprietary or restrictive formats. The lack of a public and comprehensive digital ink format has severely limited the capture, transmission, processing, and presentation of digital ink across heterogeneous devices developed by multiple vendors. In response to this need, the Ink Markup Language (InkML) provides a simple and platform-neutral data format to promote the interchange of digital ink between software applications.

InkML supports a complete and accurate representation of digital ink. In addition to the pen position over time, InkML allows recording of information about device characteristics and detailed dynamic behavior to support applications such as handwriting recognition and authentication. For example, there is support to record additional information such as pen tilt and pen tip force (often referred to as "pressure") and information about the recording device such as accuracy and dynamic distortion. InkML also provides features to support rendering of digital ink captured optically to approximate the original appearance. For example, stroke width and color information can be recorded.

It is not within the design of InkML to describe and store semantic information, such as the plain text of ink recognized as handwriting.  Nor is it a goal of InkML to store the contextual information about the ink, such as what kind of field in a form where ink was written.  However, InkML provides means for extension.  InkML can include XML from other schemas at specific locations in a file or stream (see <annotationXML>.)  Additionally, InkML could be embedded within other XML documents.

1.1 Uses of InkML

With the establishment of a non-proprietary ink standard, a number of applications, old and new, are expanded where the pen can be used as a very convenient and natural form of input. Here are a few examples.

1.2 Elements

The current InkML specification defines a set of primitive elements sufficient for all basic ink applications. All content of an InkML document is contained within a single <ink> element. The fundamental data element in an InkML file is the <trace>. A trace represents a sequence of contiguous ink points, where each point captures the values of particular quantities such as the X and Y coordinates of the pen's position. A sequence of traces accumulates to meaningful units, such as characters, words or diagrams.

In its simplest form, an InkML file with its enclosed traces looks like this:

<ink xmlns="http://www.w3.org/2003/InkML">
   <trace>
      10 0, 9 14, 8 28, 7 42, 6 56, 6 70, 8 84, 8 98, 8 112, 9 126, 10 140,
      13 154, 14 168, 17 182, 18 188, 23 174, 30 160, 38 147, 49 135,
      58 124, 72 121, 77 135, 80 149, 82 163, 84 177, 87 191, 93 205
   </trace>
   <trace>
      130 155, 144 159, 158 160, 170 154, 179 143, 179 129, 166 125,
      152 128, 140 136, 131 149, 126 163, 124 177, 128 190, 137 200,
      150 208, 163 210, 178 208, 192 201, 205 192, 214 180
   </trace>
   <trace>
      227 50, 226 64, 225 78, 227 92, 228 106, 228 120, 229 134,
      230 148, 234 162, 235 176, 238 190, 241 204
   </trace>
   <trace>
      282 45, 281 59, 284 73, 285 87, 287 101, 288 115, 290 129,
      291 143, 294 157, 294 171, 294 185, 296 199, 300 213
   </trace>
   <trace>
      366 130, 359 143, 354 157, 349 171, 352 185, 359 197,
      371 204, 385 205, 398 202, 408 191, 413 177, 413 163,
      405 150, 392 143, 378 141, 365 150
   </trace>
</ink>

These traces consist simply of X and Y value pairs, and may look like this when rendered:

a possible rendering of the sample trace above

Figure 1: Example of trace rendering

Figure 1 shows a trace of a sampled handwriting signal. The dots mark the sampling positions which were interpolated by the blue line. Green points represent pen-downs whereas red dots indicate pen-ups.

More generally, traces consist of sequences of points. Each point consists of a number of coordinate values whose meanings are given by a <traceFormat> element. These coordinates may provide values for such quantities as pen position, angle, tip force, button states and so on.

Information about the device used to collect the ink (e.g., the sampling rate and resolution) may be specified with the <inkSource> element.

Ink traces can have certain attributes such as color and width, writer identification, pen modes (eraser versus writing), and so on. These and other attributes are captured using the <brush> element. Traces that share the same characteristics, such as being written with the same brush, can be grouped together with the <traceGroup> element.

Ink traces may also be organized into collections for application-specific purposes either by grouping the traces objects themselves, using the <traceGroup> element, or by reference, using the <traceView> element.

Certain applications, such as collaborative whiteboards (where ink coming from different devices is drawn on a common canvas) or document review (where ink annotation from various sources are combined), will require ink sharing. The <context> element allows representation and grouping of the pertinent information, such as the trace format, brush, and canvas. Canvas transformations allow ink from different devices to be combined and manipulated by multiple parties.

InkML supports the semantic labeling of traces with attributes on traces or collections of traces. These may be given with either <annotation>, for text, or <annotationXML>, for XML, using application-defined encodings.

In all appropriate cases, the InkML specification defines default values for elements that are not specified, and rules that establish the scope of a given attribute.

Finally, the InkML specification is limited in scope: It is currently oriented to fixed Cartesian coordinate systems, it does not support sophisticated compression of trace data, and it does not support non-ink events (although the later could be handled via annotations).

1.3 Exchange Modes

Most ink-related applications fall into two broad categories: "Streaming" and "Archival". Archival ink applications capture and store digital ink for later processing, such as document storage/retrieval applications and batch forms processing . In these applications, an entire <ink> element is written prior to processing. For ease of implementation in archival mode, referenced elements should be defined inside a declaration block using the <definitions> element (see The Default Context section, the Definitions section, and the Archival Applications section).

Streaming ink applications, on the other hand, transmit digital ink as it is captured, such as in the electronic whiteboard example mentioned above. In order to support a streaming style of ink markup generation, the InkML language supports the notion of a "current" state (e.g., the current brush) and allows for incremental changes to this state.

1.4 Conventions used in this document

This document uses the following conventions:

Syntax of element contents
The syntax of the contents of InkML elements is expressed in Backus-Naur Form, using the notation defined in the Trace section. Non-literal symbols represent InkML markup and are linked to the relevant section in this document. For example:
Syntax of attribute contents
In this specification attributes definitions are formatted as:
default = xsd:decimal | xsd:boolean
The left hand side of the '=' sign is the name of the attribute and the right hand side describes the syntax of the attribute's contents, using the same Backus-Naur Form notation as used for element definitions. In addition, a non-literal symbol will represent a data type name. By convention, this specification uses the prefix 'xsd:' to indicate that the following name is that of a datatype formally defined in the XML Schema Part 2: Datatypes Recommendation [XMLSCHEMA2]. The 'xsd' prefix is used only as a notation in this specification, and does not mandate any prefix when using XML Schema names in InkML.

2 Structure

InkML documents are well-formed XML documents which comply to the syntax rules of this specification.

The namespace URI of InkML is http://www.w3.org/2003/InkML

The media type of InkML document is application/inkml+xml. See the Media Type definition for details. This media type is expected to be registered with IETF.

2.1 <ink> element

The ink element is the root element of any InkML instance. When combining InkML and other XML elements within applications, elements from different namespaces must be disambiguated by use of the namespace qualifier. The allowed sub-elements of the ink element can occur any number of times, in any order.

Attributes:
documentID = xsd:anyURI
The unique identifier for this document.
Required: no, Default: none

A URI that uniquely identifies this document. No two documents with a distinct application intent may have the same documentID contents. The value of this property is an opaque URI whose interpretation is not defined in this specification.

Contents:
trace ( definitions | context | trace | traceGroup | traceView | annotation | annotationXML )*
Example:
<ink xmlns="http://www.w3.org/2003/InkML"
    documentID="uuid:6B29FC40-CA47-1067-B31D-00DD010662DA">
   ...
</ink>

3 Traces and Trace Formatting

<trace> is the basic element used to record the trajectory of a pen as the user writes digital ink. More specifically, these recordings describe sequences of connected points. On most devices, these sequences of points will be bounded by pen contact change events (pen-up and pen-down), although some devices may simply record proximity and force data without providing an interpretation of pen-up or pen-down state.

The simplest form of encoding specifies the X and Y coordinates of each sample point. For compactness, it may be desirable to specify absolute coordinates only for the first point in the trace and use delta-x and delta-y values to encode subsequent points. Some devices record acceleration rather than absolute or relative position; some provide additional data that may be encoded in the trace, including Z coordinates or tip force, or the state of side switches or buttons.

These variations in the information available from different ink sources, or needed by different applications, are supported in InkML through the <traceFormat> and <trace> elements. The <traceFormat> element specifies the encoding format for each sample of a recorded trace, while <trace> elements are used to represent the actual trace data. If no <traceFormat> is specified, a default encoding format of X followed by Y coordinates is assumed.

Traces generated by different devices, or used in differing applications, may contain different types of information. InkML defines channels to describe the data that may be encoded in a trace.

A channel can be characterized as either regular, meaning that its value is recorded for every sample point of the trace, or intermittent, meaning that its value may change infrequently and thus will not necessarily be recorded for every sample point. X and Y coordinates are examples of likely regular channels, while the state of a pen button is likely to be an intermittent channel.

3.1 Trace Formats

3.1.1 <traceFormat> element

Attributes:
xml:id = xsd:ID
The unique identifier for this trace format.
Required: no, Default: none
Contents:
channel* intermittentChannels?

The <traceFormat> element describes the format used to encode points within <trace> elements. In particular, it defines the sequence of channel values that occurs within <trace> elements. The order of declaration of channels in the <traceFormat> element determines the order of appearance of their values within <trace> elements.

Regular channels appear first in the <trace>, followed by any intermittent channels. Correspondingly, the <traceFormat> element contains an ordered sequence of <channel>s, giving the regular channels (if any), followed by an optional <intermittentChannels> section. The order of the coordinates in each point of a trace is determined by the order of the <channel> elements in the trace format, including those from the intermittent channels part.

The <context> element may use the traceFormatRef attribute to refer to a <traceFormat> by it's id.  If no <traceFormat> is specified in an InkML file, an application defined default trace format is used.  The default trace has the reserved id "DefaultTraceFormat" and may be explicitly referenced using the URI "#DefaultTraceFormat".

3.1.2 <channel> element

Attributes:
xml:id = xsd:ID
The unique identifier for this element.
Required: no, Default: none

name = xsd:string
The case sensitive name of this channel.
Required: yes

type = "integer" | "decimal" | "double" | "boolean"
The data type of the point values for this channel.
Required: no, Default: "decimal"

default = xsd:decimal | xsd:boolean
The default value of the point data for this channel. This only applies to intermittent channels.
Required: no, Default: 0 (for integer or decimal channel), F (for boolean channel)

min = xsd:number
The lower boundary for the values of this channel.
Required: no, Default: none

max = xsd:number
The upper boundary for the values of this channel.
Required: no, Default: none

orientation = "+ve" | "-ve"
The orientation of increasing channel values with respect to the default direction of the channel's coordinate axis, where applicable.
Required: no, Default: "+ve"

respectTo = xsd:anyURI
Specifies that the values are relative to another reference point.  The reference point may be the URI of a <timestamp> for time channels, or an application defined URI for application specific channels.
Required: no, Default: none

units = xsd:string
The units in which the values of the channel are expressed (numerical channels only).
Required: no, Default: none
Contents:
mapping?

Channels are described using the <channel> element, with various attributes.

The required name attribute specifies the interpretation of the channel in the trace data. The following case sensitive channel names, with their specified meanings, are reserved:

channel name dimension default unit interpretation
X length mm X coordinate. This is the horizontal pen position on the writing surface, increasing to the right for +ve orientation.
Y length mm Y coordinate. This is the vertical position on the writing surface, increasing downward for +ve orientation.
Z length mm Z coordinate. This is the height of pen above the writing surface, increasing upward for +ve orientation.
F force % pen tip force
S     tip switch state (touching/not touching the writing surface)
B1...Bn     side button states
OTx angle deg tilt along the x-axis
OTy angle deg tilt along the y-axis
OA angle deg azimuth angle of the pen (yaw)
OE angle deg elevation angle of the pen (pitch)
OR angle deg rotation (counter-clockwise rotation about pen axis )
C     color value as an RGB octet triple (i.e. #000000 to #FFFFFF).
CR,CG,CB     color values (Red/Green/Blue)
CC,CM,CY,CK     color values (Cyan/Magenta/Yellow/Black)
A     transparency (device-specific encoding)
W length mm stroke width (orthogonal to stroke)
BW length mm brush width
BH length mm brush height
T time ms time (of the sample point)

The type attribute defines the encoding type for the channel (either boolean, decimal, or integer). If type is not specified, it defaults to decimal.

A default value can be specified for the channel using the default attribute; the use of default values within a trace is described in the next section. If no default is specified, it is assumed to be zero for integer and decimal-valued channels, and false for boolean channels.

The min and max attributes, if given, specify the minimum and maximum possible values for a channel of type integer or decimal. If neither is given, then there is no a prior bound on the channel values. If one is given, then the channel values are bounded above or below but unbounded in the other direction. If both are given, then all channel values must fall within the specified range.

The orientation attribute is applicable to channels of integer or decimal type. It gives the meaning of increasing value. For example, whether X increases to the left or the right. The value may be given as "+ve" or "-ve", with "+ve" being the default.

The respectTo attribute specifies the origin for channels of integer or decimal type. For time channels, this is given as a URI for a <timestamp> element. For other application defined channels the URI is application-dependent.

Typically, a channel in the <traceFormat> will map directly to a corresponding channel provided by the digitizing device, and its values as recorded in the trace data will be the original channel values recorded by the device. However, for some applications, it may be useful to store normalized channel values instead, or even to remap the channels provided by the digitizing device to different channels in the trace data. This correspondence between the trace data and the device channels is recorded using a <mapping> element (described in the Mappings section) within the <channel> element. If no mapping is specified for a channel, it is assumed to be unknown.

3.1.3 <intermittentChannels> element

Attributes:
none
Contents:
channel*

The <intermittentChannels> element lists those channels whose value may optionally be recorded for each sample point.  The order of the enclosed channel declarations gives the order of the intermittent channel data samples within traces having this format.  The <intermittentChannels> section is optional and must appear after the regular <channel> elements (if any) within a <traceFormat> element.

3.1.4 Orientation Channels

The channels OTx, OTy, OA, OE and OR record pen orientation data. Implementers may choose to use either pen azimuth OA and pen elevation OE, or alternatively tilt angles OTx and OTy. The latter are the angles of projections of the pen axis onto the XZ and YZ planes, measured from the vertical. It is often useful to record the sine of this angle, rather than the angle itself, as this is usually more useful in calculations involving angles. The <mapping> element can be employed to specify an applied sine transformation. While it is not forbidden to use channels from different groups together (i.e. from more than one of {OA, OE} and {OTx, OTy}), applications will not normally do this.

The third degree of freedom in orientation is generally defined as the rotation of the pen about its axis. This is potentially useful (in combination with tilt) in application such as illustration or calligraphy, and signature verification.

Diagram showing the azimuth and elevation of the penDiagram showing the tilt angles of the pen

Figure 2: (a) azimuth and elevation angles, (b) tilt angles

Diagram showing the pen orientation decompositionDiagram showing the rotation angle around the axis of the pen

Figure 3: (a) pen orientation decomposition, (b) pen rotation

Figure 2a displays the pen orientation using Azimuth and Elevation. The origin of the Azimuth is at the Y-axis. Azimuth increases anticlockwise up to 360 degrees. The origin of Elevation is located within the XY-plane. Elevation increases up to 90 degrees, at which point the pen is perpendicular to the XY-plane.

Figure 2b explains the definition of the Tilt-X and the Tilt-Y angles. For both the origin is along the Z-axis. Tilt-X increases up to +90 degrees for inclinations along the positive X-axis and decreases up to -90 degrees for inclinations along the negative X-axis. Respectively, Tilt-Y is defined for pen inclinations along the Y-axis.

Figure 3a displays the pen orientation decomposition as functions of Azimuth/Elevation or alternatively as function of Tilt-X/Tilt-Y. Thereby, elevations of the pen which are mapped to the XZ- and to the YZ- plane lead to Tilt-X and Tilt-Y.

Figure 3b shows the Rotation of the pen along its longitudinal axis.  The departure of a reference mark or meridian on the pen barrel from the nominal 'up' direction which may be constructed by a ray perpendicular to the pen barrel (somewhere not at the tip) and intersecting a pure-Z ray arising from the surface of the pen passing through the tip. This angle is measured in a clockwise direction when viewing the pen barrel from tail to tip, in degrees.

3.1.5 Color Channels

The channels CR, CG, CB, CC, CM, CY, CK, C and A are defined to record color and transparency data as captured by an optical device, as generated by software or by other means.

The channels CR, CG, CB provide an additive color model for the colors red, green and blue. The channels CC, CY, CM, CK provide a subtractive color model for the colors cyan, magenta, yellow and black. The channel C provides a mechanism to give color as a single numerical value in the range #000000..#FFFFFF that encodes the colors red, green and blue as three octets. While it is not forbidden to use channels from different groups together (i.e. from more than one of {C}, {CR, CG, CB} and {CC, CY, CM, CK}), applications will not normally do this. The A channel records transparency as an integer. The value 0 represents opaque ink and the maximum permissible value represents complete transparency.

Color channels are intended for use when these values are part of the data itself and hence potentially changing from one sample to the next. Strokes with constant color may more economically be described with reference to a <brush> element.

It is legitimate for an application to have an accessibility mode or alternative rendering mode where the explicit color values in the InkML are reinterpreted as other colors for better accessibility or suitability of the rendering device. Examples of this would be mapping color to black and white for monochrome devices or to high-contrast colors for greater visibility.

3.1.6 Width Channels

Three channels are provided to provide stroke width information. 

The channel W is provided for recording stroke width. The value is in length units and is the diameter of the larger circle that can be inscribed within the trace locus. This allows optical devices to record measured stroke width and allows applications that generate InkML to specify desired width for rendering.

The channels BW and BH are defined to record the brush width and height at each point.  The meaning of the width and height is defined by the brush tip shape, as given by a <brushProperty>. 

As with the color channels, the width channels are intended for use when this quantity is part of the data itself and hence potentially changing from one sample to the next. Strokes with constant width may more economically be described with reference to a <brush> element with width and height properties.

3.1.7 Time Channel

The time channel allows for detailed recording of the timing information for each sample point within a trace. This can be useful if the digitizing device has a non-uniform sampling rate, for example, or in cases where duplicate point data is removed for the sake of compactness.

The time channel can be specified as either a regular or intermittent channel. When specified as a regular channel, the single quote prefix can be used to record incremental time between successive points.  The value of the time channel for a given sample point is defined to be the timestamp of that point in the units and frame of reference specified by the respectTo attribute of the time channel that is defined in the associated <traceFormat> of the trace.

As with the other predefined channels, the meaning of the integer or decimal values recorded by the time channel in a given trace is defined by the trace's associated <traceFormat>. In the case of the time channel, its <channel> element contains both a units and respectTo attribute.

The units attribute gives the units of the recorded time values, and the respectTo attribute describes the frame of reference for those recorded values. The value of the respectTo attribute is a reference to a time stamp. If it is not given, the time channel values are relative to the beginning timestamps of the individual traces in which they appear.

The following example defines a time channel whose values for a given point are the relative to the timestamp referred to by #ts1:

<channel name="T"
         type="integer"
         units="ms"
         respectTo="#ts1" />

If no <traceFormat> information is provided, or if no value is specified for the respectTo attribute, the ink processor cannot make any assumption about the relative timing of points within different traces. Likewise, if no units are specified, no assumption can be made about the units of the time channel data.

3.1.8 User Defined Channels

In addition to the pre-defined channels, user-defined channels are allowed, although their interpretation is not required by conforming ink markup processors.

When specifying a number of related channels, it is recommended to use a common prefix. For example, direction-sensitive stylus force could be named FX, FY, FZ.

User defined channels may be used to describe ink traces in non-Cartesian coordinate systems, using various compression schemes, or with supplementary information. Channels need not describe properties of the digital ink, per se, but may be used to provide additional information in the ink stream.   For example, a user defined channels could give  information about changing lighting conditions.

3.1.9 Specifying Trace Formats

The following example defines a <traceFormat> which reports decimal-valued X and Y coordinates for each point, and intermittent boolean values for the states of two buttons B1 and B2, which have default values of F ("false"):

<traceFormat xml:id="xyb1b2">
   <channel name="X" type="decimal">
      <mapping type="identity"/>
   </channel>
   <channel name="Y" type="decimal">
      <mapping type="identity"/>
   </channel>

   <intermittentChannels>
      <channel name="B1" type="boolean" default="F">
         <mapping type="identity"/>
      </channel>
      <channel name="B2" type="boolean" default="F">
         <mapping type="identity"/>
      </channel>
   </intermittentChannels>
</traceFormat>

The appearance of a <traceFormat> element in an InkML file both defines the format and installs it as the current format for subsequent traces except within a <definitions> block (see Specifying Trace Formats). The id attribute of a <traceFormat> allows the format to be reused by multiple contexts (see the Context section). If no <traceFormat> is specified, the following default format is assumed:

<traceFormat xml:id="DefaultTraceFormat">
    <channel name="X" type="decimal"/>
    <channel name="Y" type="decimal"/>
</traceFormat>

Thus, in the simplest case, an InkML file may contain nothing but <trace> elements within an <ink> element.

3.2 Traces

3.2.1 <trace> element

Attributes:
xml:id = xsd:ID
The identifier for this trace.
Required: no, Default: none

type = "penDown" | "penUp" | "indeterminate"
The type of this trace.
Required: no, Default: "penDown"

continuation = "begin" | "middle" | "end"
This attribute indicates whether this trace is a trace fragment, and if so, where this trace is located in the set of continuation traces.
Required:
no, Default: none

priorRef = xsd:anyURI
The URI of the trace this one is a continuation of.
Required: if and only if continuation has values "end" or "middle", Default: none

contextRef = xsd:anyURI
The context for this trace. Any values in this context over-ride the values in the inherited context.
Required: no, Default: "#DefaultContext," unless this <trace> is contained within a <traceGroup>, then inherit from the <traceGroup>.

brushRef = xsd:anyURI
The brush for this trace.
Required: no, Default: Inherited from context.

duration = xsd:decimal
The duration of this trace, in milliseconds.
Required: no, Default: none

timeOffset = xsd:decimal
The relative timestamp or time-of-day for the start of this trace, in milliseconds.
Required: no, Default: none
Contents:

The following grammar defines the syntax of the data that appears within a <trace> element.  It is described using the subset of Extended Backus-Naur Form defined in the Notation section of the Extensible Markup Language (XML) 1.0 (Fourth Edition) specification [EBNF].  This subset of EBNF includes the following notation:

The grammar is as follows:

trace   ::= point ("," point)* ","? wsp*

point   ::= (wsp* value)+ wsp*

value   ::= difference_order?  wsp* "-"? wsp* number | "T" | "F" | "*" | "?"

number  ::= (decimal | double | hex)

double  ::= decimal ("e"|"E") ("+"|"-")? digit+ 

decimal ::= digit+ ("." digit*)? | "." digit+

hex     ::= "#" (digit | "A" | "B" | "C" | "D" | "E" | "F")+

difference_order ::= ("!" | "'" | '"')

digit   ::= ("0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9")

wsp     ::= (#x20 | #x9 | #xD | #xA)
Additionally, wsp may occur anywhere except within a decimal, float or hex and must occur if required to separate two values. Otherwise the longest token is matched. For example, "3245" requires an internal wsp character if it is to be interpreted as two decimal numbers, "32" and "45". On the other hand, "0.923.45" will be interpreted as "0.923" and ".45".

The number of value tokens appearing within each point must be at least equal to the number of regular channels and be no more than the number of regular channels plus the number of intermittent channels.

The <trace> element is used to record the data captured by the digitizer. It contains a sequence of points encoded according to the specification given by the <traceFormat> element.

The type attribute of a <trace> indicates the pen contact state (either "penUp" or "penDown") during its recording. A value of "indeterminate" is used if the contact-state is neither pen-up nor pen-down, and may be either unknown or variable within the trace. For example, a signature may be captured as a single indeterminate trace containing both the actual writing and the trajectory of the pen between strokes.  The values of the tip switch state channel "S", if present in the trace, overrides the value of the type attribute.

If a continuation attribute is present, it indicates that the current trace is a continuation trace, i.e. its points are a temporally contiguous continuation of (and thus should be connected to) another trace element. The possible values of the attribute are:

If the current trace is a continuation trace but is not the first trace in the set (i.e. the continuation attribute has value middle or end) then a priorRef attribute must be present and must contain the URI of the trace of which the current trace is a continuation. A begin or middle trace can be the prior trace for exactly one trace. An end trace cannot be the prior trace of any other trace.

Regular channels may be reported as explicit values, differences, or second differences: Prefix symbols are used to indicate the interpretation of a value: a preceding exclamation point (!) indicates an explicit value, a single quote (') indicates a single difference, and a double quote prefix (") indicates a second difference. If there is no prefix, then the channel value is interpreted as explicit, difference, or second difference based on the last prefix for the channel. If there is no last prefix, the value is interpreted as explicit.

A second difference encoding must be preceded by a single difference representation; which, in turn, must be preceded with an explicit encoding.

All traces must begin with an explicit value, not with a first or second difference. This is true of continuation traces as well. This allows the location and velocity state information to be discarded at the end of each trace, simplifying parser design.  This is true for continuation traces.

Both regular and intermittent channels may be encoded with the wildcard character "*". This wildcard character means either that the value of the channel remains at the previous channel value (if explicit), or that the channel continues integrating with the previous velocity or acceleration values, as appropriate.

Intermittent channels may be encoded with the wildcard character "?". This means that a value of a channel is not given at that point. It is useful when there are several independent intermittent channels, and they do not always report simultaneously, e.g.

<trace> 11 12 9, 21 22 ? T, 31 32, 41 42 5, 51 52 ? F</trace>

Booleans are encoded as "T" or "F".

For each point in the trace, regular channel values are reported first in the order given by the <channel> elements of the applicable <traceFormat>. All regular channels must be reported, if only with the explicit wildcard "*".  If any intermittent values are reported for the point, they are given next, in the order given by the <intermittentChannels> elements of the applicable <traceFormat>. Unreported intermittent channels are interpreted as though they were given by the wildcard "*".

Here is an example of a trace of 11 points, using the following traceFormat:

<traceFormat>
   <channel name="X" type="decimal"/>
   <channel name="Y" type="decimal"/>

   <intermittentChannels>
      <channel name="B1" type="boolean" default="F"/>
      <channel name="B2" type="boolean" default="F"/>
   </intermittentChannels>
</traceFormat>

<trace xml:id="id4525abc">
   1125 18432,'23'43,"7"-8,3-5,7 -3,6 2,6 8,3 6 T,2 4*T,3 6,3-6 F F
</trace>

The trace is interpreted as follows:

Trace X Y vx vy B1 B2 Comments
1125 18432 1125 18432 ? ? F F button default values
'23'43 1148 18475 23 43 F F velocity values
"7"-8 1178 18510 30 35 F F acceleration Values
3-5 1211 18540 33 30 F F implicit acceleration
no whitespace needed
7 -3 1251 18567 40 27 F F optional whitespace
6 2 1297 18596 46 29 F F whitespace required
6 8 1349 18633 52 37 F F  
3 6 T 1404 18676 55 43 T F an optional value
2 4*T 1461 18723 57 47 T T wildcard
3 6 1521 18776 60 53 T T optional keep last
3-6 F F 1584 18823 63 47 F F optionals

An ink markup generator might also include additional whitespace formatting for clarity. The following trace specification is identical in meaning to the more compact version shown above:

<trace xml:id="id4525abc">
   1125 18432,
   '23 '43,
   "7 "-8,
   3 -5,
   7 -3,
   6 2,
   6 8,
   3 6 T,
   2 4 * T,
   3 6,
   3 -6 F F
</trace>

Note: see Appendix B Implementation Guidelines for information about reducing file or stream size.

3.3 Trace Collections

InkML provides mechanisms to gather and combine traces into structured collections via the <traceGroup> and <traceView> elements. These allow multiple traces or groups to be treated as single units for the purposes of referencing, attaching context information, semantic labeling, or application-specific needs. The <traceGroup> element gathers <trace> other <traceGroup> or <traceView> elements into a unit. The <traceView> element refers to existing <trace>, <traceGroup> or other <traceView> elements to provide alternative views or organization on the ink. For example, a diagramming application may record a stream of fixed-length <trace> packages, organized as continuations, and use <traceGroup> elements containing <traceView> elements to record the logical structure of the diagram.

3.3.1 <traceGroup> element

Attributes:
xml:id = xsd:ID
The identifier for this traceGroup.
Required: no, Default: none

contextRef = xsd:anyURI
The context associated with this traceGroup.
Required: no, Default: "#DefaultContext," unless this <traceGroup> is contained within another <traceGroup>, then inherit from the containing <traceGroup>.

brushRef = xsd:anyURI
The brush associated with this <traceGroup>.
Required: no, Default: Inherited from context
Contents:
( trace | traceGroup | traceView| annotation | annotationXML )*

The <traceGroup> element is used to group successive traces which share common characteristics, such as the same <traceFormat>. The brush and context sections describe other contextual values that can be specified for a <traceGroup>. In the following example the two traces enclosed in the <traceGroup> share the same brush (see the Brushes section for a description of brushes).

<traceGroup brushRef="#penA">
    <trace>...</trace>
    <trace>...</trace>
</traceGroup>

The <traceGroup> element may be used for various purposes, such as to group traces according to their properties at the time of capture or according to computed recognition results. The element may be nested, and it may be used as a generic grouping mechanism, e.g. for the semantic labeling of traces.

Trace groups are the primary mechanism for assigning <context> to traces in archival ink markup. For additional details about this usage, see the Archival Applications section.

3.3.2 <traceView> element

Attributes:
xml:id = xsd:ID
The identifier for this traceView.
Required: no, Default: none

traceDataRef = xsd:anyURI
A URI reference to a <trace>, <traceGroup> or <traceView> element.
Required: yes, Default: none

from = xsd:integer[ ':' xsd:integer ]*
The index of the first item (point, trace or group) in the trace or trace group that this <traceView> element references.
Required: no, Default: the index of the first referenced point (see prose)

to = xsd:integer[ ':' xsd:integer ]*
The index of the last item (point, trace or group) in the trace or trace group that this <traceView> element references.
Required: no, Default: the index of the last referenced point (see prose)
Contents:
EMPTY

The <traceView> element is used to include traces by reference from the current document or other documents. A common use is to group a collection of <traceView> elements in a <traceGroup> to provide annotations.

Together, traceDataRef, from and to refer to another element and select part of it. A traceDataRef attribute may refer to a <trace>, a <traceGroup> or another <traceView>.

A missing from attribute is equivalent to selecting the first point in the (recursively) first child of the referenced element. A missing to attribute is equivalent to selecting the last point in the (recursively) last child of the referenced element. With these defaults, the <traceView> selects the portion of the referenced element from the first point to the last point, inclusive. If neither a to nor from attribute is given, this implies the entire referenced element is selected.

Any value of a from or to attribute is a colon-separated list of integers, whose meaning is defined as follows: An empty list of integers selects the entire referenced object (point, <trace>, <traceGroup> or <traceView>). If the list is non-empty, then its first element is taken as a 1-based index into the referenced object, and the remaining list is used to select within the object. It is an error to try to select within a single point. The rationale to allow selection using this colon-separated-integer indexing scheme is that the desired ink selections in a referenced document might not have id attributes on the desired nodes.

If the referenced object is a <traceView>, then the indexing is relative to the tree selected by the <traceView>, not relative to the original object.

If a <traceGroup> contains continuation traces, they are counted independently.

Examples:

Suppose we have the following ink element:

<ink xmlns="http://www.w3.org/2003/InkML">
   <trace xml:id="L1">911 912, 921 922, 931 932</trace>

   <traceGroup xml:id="L2">
      <trace>111 112, 121 122</trace>
      <traceGroup xml:id="L2-Larry">
         <trace>221 212, 221 222</trace>
         <trace>311 312, 321 322</trace>
      </traceGroup>
      <trace>411 412, 421 422</trace>
      <traceGroup>
         <traceGroup>
            <trace xml:id="L2-Moe">521 512, 521 522</trace>
            <trace>611 612, 621 622</trace>
         </traceGroup>
      </traceGroup>
      <trace>711 712, 721 722</trace>
   </traceGroup>

   <traceGroup xml:id="L3">
      <traceView traceDataRef="#L1" from="2"/>
      <traceView traceDataRef="#L2" from="2" to="4:1:1"/>
   </traceGroup>

   <traceView xml:id="L4" traceDataRef="#L3" from="1:2" to="2:1:2:1"/>
</ink>

With traceDataRef "#L1", the from index "2" refers to the point (921, 922). With traceDataRef "#L2", the from index "2" refers to the <traceGroup> with id "L2-Larry", the index "4:1:1" refers to the element with id "L2-Moe", the index "4:1:1:2" refers to the point (521, 522), and the index "4:1:1:2:1" is illegal.

The <traceGroup> with id "L3" selects the following structure

<traceGroup>
   <trace>921 922, 931 932</trace>

   <traceGroup>
      <traceGroup>
         <trace>221 212, 221 222</trace>
         <trace>311 312, 321 322</trace>
      </traceGroup>
      <trace>411 412, 421 422</trace>
      <traceGroup>
         <traceGroup>
            <trace>521 512, 521 522</trace>
         </traceGroup>
      </traceGroup>
   </traceGroup>

</traceGroup>
and the <traceView> with id "L4" selects
<traceGroup>
   <trace>931 932</trace>
   <traceGroup>
      <traceGroup>
         <trace>221 212, 221 222</trace>
         <trace>311 312</trace>
      </traceGroup>
   </traceGroup>
</traceGroup>

4 Contexts

The context in which ink is written and recorded comprises many details. Examples include the size of the surface the traces were recorded on, the pen tip used or the accuracy of the pressure measurements. This contextual information needs to be captured by InkML in order to fully characterize the recorded ink data. This section defines markup that provides a way to describe this information, including the <context> element which provides a means to associate a defined context with trace data.

The format of trace data -- both in the channels available and their particulars -- may vary from device to device, including from stylus to stylus with the same tablet. Therefore, the <context> element may refer to or contain a specific <traceFormat> and <inkSource> element for the device.

As the ink is generated, there may be various context-dependent attributes associated with the pen. For this, a <brush> element may be used to record the attributes of the pen during the capture of the digital ink.

The start times of traces are often given relative to a specified point in time. A context may provide a <timestamp> element for this.

For applications that require the sharing of ink, contexts may relate their ink to a shared canvas, given by a <canvas> element. The trace format of the ink source is related to the trace format of a shared canvas by means of a <canvasTransform> element.

4.1 The <context> element

This section describes the <context> element and its attributes. The context element both provides access to a useful shared context (canvas) and serves as a convenient agglomeration of contextual attributes. It is used by the <traceGroup> and <traceView> elements to define the complete shared context of a group of traces or may be referred to as part of a context change in streaming mode. In either mode, individual attributes may be overridden at time of use. Additionally, individual traces may refer to a previously defined context (again optionally overriding its attributes) to describe a context change that persists only for the duration of that trace.

Although the use of the <context> element and attributes is strongly encouraged, default interpretations are provided so that they are not required in an InkML file if all trace data is recorded in the same virtual coordinate system, and its relationship to device coordinates is either not needed or unknown.

Attributes:
xml:id = xsd:ID
The unique identifier for this context.
Required: no, Default: none

contextRef = xsd:anyURI
A previously defined context upon which this context is to be based.
Required: no, Default: none

canvasRef = xsd:anyURI
The URI of a canvas element for this context.
Required: no, Default: "#DefaultCanvas", or inherited from contextRef

canvasTransformRef = xsd:anyURI
This is a reference to a mapping from the coordinate system of the trace to the coordinate system of the canvas.
Required: no, Default: identity, or inherited from contextRef

traceFormatRef = xsd:anyURI
A reference to the traceFormat for this context.
Required: no, Default: "#DefaultTraceFormat", or inherited from contextRef

inkSourceRef = xsd:anyURI
A reference to the inkSource for this context.
Required: no, Default: default capture device, or inherited from contextRef

brushRef = xsd:anyURI
A reference to the brush for this context.
Required: no, Default: "#DefaultBrush", or inherited from contextRef

timestampRef = xsd:anyURI
A reference to the timestamp for this context.
Required: no, Default: none, or inherited from contextRef
Contents:
canvas?
canvasTransform?
traceFormat?
inkSource?
brush?
timestamp?

The <context> element consolidates all salient characteristics of one or more ink traces. It may be specified by declaring all non-default attributes, or by referring to a previously defined context and overriding specific attributes. The element is found either in the <definitions> element or as a child of the <ink> element in Streaming InkML

Each constituent part of a context may be provided either by a referencing attribute or as a child element. If both are given, then the child element is used. Thus it is possible to have either a traceFormatRef attribute or a <traceFormat> child element. If both are given, then the <traceFormat> child is used and the attribute is ignored.

4.2 Ink Sources

One of the important requirements for the ink format is to allow accurate recording of metadata about the format and quality of ink as it is reported by the source. The source is typically hardware as embodied in a digitizer device, but may in general be any "virtual" source of ink, such as a software application that is tracking the trajectory of an object. This is accomplished in the <inkSource> element, which supports capture of basic information about the make and model of the device and the ink channels captured, as well as very detailed information about a number of source characteristics.

Some of these characteristics are already commonly used in digitizer specifications, while others are somewhat more esoteric, but nonetheless potentially very useful. In general, these source characteristics describe signal fidelity, allow understanding of the quality of the data, and impose some limits on how the data can be used. They are not intended to be used for repair of bad data from the source.

4.2.1 <inkSource> element

Attributes:
xml:id = xsd:ID
The unique identifier for this <inkSource> element.
Required: yes

manufacturer = xsd:string
String identifying the digitizer device manufacturer.
Required: no, Default: unknown

model = xsd:string
String identifying the digitizer model.
Required: no, Default: unknown

serialNo = xsd:string
Unique manufacturer (or other) serial number for the device.
Required: no, Default: unknown

specificationRef = xsd:anyURI
URI of a page providing detailed or additional specifications.
Required: no, Default: unknown

description = xsd:string
String describing the ink source, especially one implemented in software.
Required: no, Default: unknown
Contents:
traceFormat
sampleRate?
latency?
activeArea?
sourceProperty*
channelProperties?
Examples:
<inkSource xml:id="mytablet"
   manufacturer="Example.com"
   model="ExampleTab 2000 USB"
   specificationRef="http://www.example.com/products/exampletab/2000usb.html">

   <traceFormat>
      <channel name="X" ... />
      <channel name="Y" ... />
      <channel name="F" ... />
   </traceFormat>
   <sampleRate uniform="true" value="200"/>
   <latency value="50"/>
   <activeArea size="A6" height="100" width="130" units="mm"/>
   <sourceProperty name="weight" value="100" units="g"/>

   <channelProperties>
      <channelProperty channel="X" name="resolution" value="5000" units="1/in"/>
      <channelProperty channel="Y" name="resolution" value="5000" units="1/in"/>
      <channelProperty channel="Y" name="peakRate"   value="50"   units="cm/s"/>
      <channelProperty channel="F" name="resolution" value="1024" units="dev"/>
   </channelProperties>

</inkSource>

The <inkSource> element will allow specification of:

4.2.2 <sampleRate> element

The <sampleRate> element captures the rate at which ink samples are reported by the ink source. Many devices report at a uniform rate; other devices may skip duplicate points or report samples only when there is a change in direction. This is indicated using the uniform attribute, which must be designated "false" (non-uniform) if any pen-down points are skipped or if the sampling is irregular.

A time channel should be used to get time information when the sampling rate is not uniform. When the sampling rate is not uniform, the value attribute of the <sampleRate> element specifies the maximum sampling rate.

Attributes:
uniform = xsd:boolean
Sampling uniformity: Is the sample rate consistent, with no dropped points?
Required: no, Default: true

value
= xsd:decimal
The basic sample rate in samples/second.
Required: yes
Contents:
EMPTY
Examples:
<sampleRate uniform="true" value="200"/>

4.2.3 <latency> element

The <latency> element captures the basic device latency that applies to all channels, in milliseconds, from physical action to the API time stamp. This is specified at the device level, since all channels often are subject to a common processing and communications latency.

Attributes:
value = xsd:decimal
Latency in milliseconds.
Required: yes
Contents:
EMPTY
Examples:
<latency value="50"/>

4.2.4 <activeArea> element

Many ink capture devices have a notion of active area, which describes the two-dimensional area within which the device is capable of sensing the pen position. This element allows the specification of a rectangular active area.

Attributes:
size = xsd:string
The active area, described using an international ISO paper sizes standard such as ISO216.
Required: no, Default: unknown

height = xsd:decimal
Height of the active area (corresponding to the Y channel).
Required: yes, Default: unknown

width = xsd:decimal
Width of the active area (corresponding to the X channel).
Required: yes, Default: unknown

units = xsd:string
Units used for width and height.
Required: no, Default: unknown
Contents:
EMPTY
Examples:
<activeArea size="A6" height="100" width="130" units="mm"/>

4.2.5 <sourceProperty> element

The <sourceProperty> element provides a simple mechanism for the capture of additional numeric or string properties of the ink source as a whole.

Attributes:
name = xsd:string
Name of the property of device or ink source.
Required: yes

value = xsd:decimal | xsd:string
Value of named property.
Required: yes

units = xsd:string
Units used for value.  If present, the value must be a numeric property.
Required: no, Default: unknown
Contents:
EMPTY
Examples:
<sourceProperty name="weight" value="100" units="g"/>

4.2.6 <channelProperties> element

The <channelProperties> element is meant for describing properties of specific channels reported by the ink source. Properties such as range and resolution may be specified using corresponding elements. For more esoteric properties (from a lay user's standpoint) the generic <channelProperty> element may be used.

Attributes:
None
Contents:
channelProperty*
Examples:
<channelProperties>
   <channelProperty channel="X" name="resolution" value="5000" units="1/in"/>
   <channelProperty channel="Y" name="resolution" value="5000" units="1/in"/>
   <channelProperty channel="Y" name="peakRate"   value="50"   units="cm/s">
   <channelProperty channel="F" name="resolution" value="1024" units="dev"/>
</channelProperties>

4.2.7 <channelProperty> element

The <channelProperty> element provides a simple mechanism for the capture of additional numeric or string properties of specific channels when known and appropriate. The following channel property names, with their specified meanings, are reserved. Other properties may be defined by the user.

Property name Interpretation
threshold Threshold - e.g. for a binary channel, the threshold force at which the tip switch is activated
resolution Resolution - the scale of the values recorded. This may be expressed as fractions of a unit, e.g. 1/1000 in (inches), 0.1 mm, 1 deg (degrees). It may also be expressed, more popularly, in inverse units, e.g. "1000 points per inch" would be given as 1000 in units 1/in.
quantization Quantization - the unit of smallest change in the reported values. If the value is reported as integer, this is assumed to be the same as the resolution. Note that if decimal values are recorded for resolution, the quantization of the data may be smaller than the "resolution".
noise Noise - the RMS value of noise typically observed on the channel. This is distinct from accuracy! It is an indication of the difference observed in the data from the device when the same path is traced out multiple times (e.g. by a robot).
accuracy Accuracy - the typical accuracy of the data on the channel (e.g. "0.5 mm", "10 degrees" or "0.1 Newton") This is the typical difference between the reported position and the actual position of the pen tip (or tilt ...)
crossCoupling Cross-coupling - the distortion in the data from one channel due to changes in another channel. For example, the X and Y coordinates in an electromagnetic digitizer are influenced by the tilt of the pen. This would be specified by dX/dOTx = ... or max delta X vs. OTx = ... If the influencing channels are also recorded, and the cross-couplings are accurately specified, it may be possible to compensate for the cross-coupling by subtracting the influence, at the expense of higher noise. The cross-coupling is always expressed in the units of the two channels, e.g. if X mm and OTx is in degrees, then cross-coupling is in mm/deg.
skew Skew - the temporal skew of this channel relative to the basic device latency, if any. For example, some devices actually sample X and Y at different points in time, so one might have a skew of -5 millisecond, and the other +5 millisecond.
minBandwidth Minimum bandwidth (in Hz) - the minimum bandwidth of the channel, in Hz (not samples/second), i.e., the frequency of input motion up to which the signal is accurate to within 3dB.
peakRate Peak rate - the maximum speed at which the device can accurately track motion
distortion Dynamic distortion, e.g., how velocity affects position accuracy. This is expressed in inverse seconds, e.g. 0.01 mm / mm / s. This kind of distortion is often cross channel, but this specification only allows a generic, channel-specific value.
Attributes:
channel = xsd:string
The name of the channel. Must be one among those defined by the ink source's trace format.
Required: yes

name = xsd:string
Name of the property of device or ink source.
Required: yes

value = xsd:decimal | xsd:string
Value of named property.
Required: yes

units = xsd:string
Units used for value.  If present, the value must be a numeric property.
Required: no, Default: unknown
Contents:
EMPTY
Examples:
<channelProperty channel="F" name="threshold" value="0.1" units="N"/>
<channelProperty channel="X" name="quantization" value="0.01" units="mm"/>

4.3 Brushes

Along with trace data, it is often necessary to record certain attributes of the pen during ink capture. For example, in a note taking application, it is important to be able to distinguish between traces captured while writing as opposed to those which represent erasures. Because these attributes will often be application specific, this specification does not attempt to enumerate all the brush attributes which can be associated with a trace. It provides a syntax for specifying brush property names, units and values.  Some common brush property names are defined by the specification.  But applications may define other named properties not explicitly named in the specification since it is possible to imagine attributes which are described using complex functions parameterized by time, pen-tip force, or other factors. The specification allows for capturing the fact that a given trace was recorded in a particular brush context, leaving the details of precisely defining specific attributes of that context (such as complex brush geometries and colors in non-RGB color spaces) to a higher-level, application specific layer.

Depending on the application, brush attributes may change frequently. Accordingly, there should be a concise mechanism to assign the attributes for an individual trace. On the other hand, it is likely that many traces will be recorded using the same sets of attributes; therefore, it should not be necessary to explicitly state the attributes of every trace (again, for reasons of conciseness). Furthermore, it should be possible to define entities which encompass these attribute sets and refer to them rather than listing the entire set each time. Since many attribute sets will be similar to one another, it should also be possible to inherit attributes from a prior set while overriding some of the attributes in the set.

4.3.1 <brush> element

Attributes:
xml:id = xsd:ID
The unique identifier for this brush.
Required: no, Default: none.

brushRef = xsd:anyURI
A brush whose attributes are inherited by this brush.
Required: no, Default: "#DefaultBrush"
Contents:
( brushProperty | annotation | annotationXML )*

In the ink markup, brush attributes are described by the <brush> element. This element allows for the definition of reusable sets of brush attributes which may be associated with traces. For reference purposes, a brush specifies an identifier which can be used to refer to the brush. A brush can inherit the attributes of another <brush> element by including a brushRef attribute which contains the id of the referenced brush. The brush attributes are stored in <brushProperty> child elements. Brushes may be used to convey information about how a stroke is to be rendered or simply to distinguish between different types of traces (e.g. an eraser vs. a pen, different writers). In this later case, all that matters is that brushes are distinct so no brush properties are necessary.

Brush attributes are associated with traces using the brushRef attribute. When it appears as an attribute of an individual <trace>, the brushRef specifies the brush attributes for that trace. When it appears as an attribute of a <traceGroup> element, the brushRef specifies the common brush attributes for all traces enclosed in the <traceGroup>. Within the <traceGroup>, an individual trace may still override the traceGroup's brush attributes using a brushRef attribute.

Brush attributes can also be associated with a context by including the brushRef attribute on a <context> element. Any traces which reference the context using a contextRef attribute are assigned the brush attributes defined by the context. If a trace includes both brushRef and contextRef attributes, the brushRef overrides any brush attributes given by the contextRef.

The default brush may be explicitly specified using the URI "#DefaultBrush".  The id "DefaultBrush" is therefore reserved and may not be used as the id of a user defined <brush> element.  The default brush is identical to a user defined brush that has not explicit <brushProperty> child elements.

In streaming ink markup, brushes are assigned to a trace according to the current brush, which can be set using the <context> and <brush> elements. See section Streaming Applications for a detailed description of streaming mode.

4.3.2 <brushProperty> element

The <brushProperty> element provides a mechanism for the storage of named properties of brushes. The following brush property names, with their specified meanings, are reserved. Other properties may be defined by the user.

Property name Interpretation
width Width of the brush.

If the width property is not given and a BW channel is present, the values of the BW channel are used as the brush width.

The default value is defined by the application.
height Height of the brush.

If a height property is not given and a BH channel is present, the values of the BH channel are used as the brush height.

The default value is defined by the application.
color Color of brush as three octets for RGB.

If a color property is not given and color channels are present (C or CR, CG, CB or CC, CM, CY, CK), their values are used for the color.

Default is #000000.
transparency Transparency of brush as an integer: 0 is opaque.

If a transparency property is not given and the transparency channel (A) is present, its value is used.
Default is 0.
tip The type of pen tip: ellipse, rectangle, or drop.

If ellipse, then the width property specifies the horizontal diameter, and the height property specifies the vertical diameter.  If the height property is absent, its default value is the value of width.

If rectangle, the width and height properties specify the width and height of the rectangle.  If the height property is absent, the default value is the value of width making the brush a square.

If drop, the shape is defined by a circle and  two tangent lines to a point outside the circle, located above the circle on the vertical axis, as shown in Figure 4. The width property is the diameter the circle part, and the height property is the maximum diameter of the shape.

Default is ellipse.

If the OR channel is present, the tip shape is rotated counter-clockwise by this amount about its origin.
rasterOp A value that defines how the colors of the pen and background interact.  In the example images below, the original background is white with the black text 'abc' and it is overwritten with a single curved yellow ink stroke.

noOperation noOperation specifies no operation; the background is rendered without ink.
copyPen copyPen specifies that the current pen color property is used and overwrites the background.
maskPen maskPen specifies a combination of the colors common to both the pen and the display.  This value simulates the effect of a highlighter pen.

The default value is copyPen, which indicates that the current pen color is used.  Applications may define additional rasterOp values.
antiAliased The drawn ink is anti-aliased.

Default is true.
fitToCurve The ink is rendered as a series of curves versus as lines between pen sample points.

Default is false.
ignorePressure If true, pressure from the pen tip is ignored and the width of the ink remains the same regardless of the pressure of the pen on the tablet surface.

If false, the width of the ink gets wider with increased pressure of the pen on the tablet surface.

Default is false.
Attributes:
name = xsd:string
Name of property.
Required: yes

value = xsd:decimal | xsd:string
Value of named property.
Required: yes

units = xsd:string
Units used for value.  If present, the value must be a numeric property.
Required: no, Default: unknown
Contents:
( annotation | annotationXML )*
Example:
<brushProperty name="width" value="2" units="cm"/>
<brushProperty name="color" value="#FF0000"/>

4.4 Timestamps

Timestamping of traces is supported by the <timestamp> element and the timestampRef, timeOffset and duration attributes of the <trace> element. For ease of processing, all timestamps are expressed in milliseconds. Finer-grained timestamps are obtained using fractional values.

4.4.1 <timestamp> element

Attributes:
xml:id = xsd:ID
The identifier for this timestamp.
Required: yes

time = xsd:decimal
The absolute time for this timestamp, in milliseconds since 1 January 1970 00:00:00 UTC.
Required: no, Default: none.

timestampRef = xsd:anyURI
The absolute time for this timestamp, given as a reference to another timestamp.
Required: no, Default: none

timeString = xsd:dateTime
The absolute time for this timestamp, given in a human-readable standard format.
Required: no, Default: none.

timeOffset = xsd:decimal
The relative time for this reference timestamp, in milliseconds.
Required: No. Default. 0
Contents:
EMPTY

The <timestamp> element establishes a reference timestamp which can then be used for relative timestamping of traces.

At most one of the attributes time, timestampRef or timeString is used. The time thus given, plus the value of the attribute timeOffset, gives the time value of the timestamp.

If more than one of time, timeString and timestampRef are given, then time is used if present. Failing that, timeString is used.

If none of time, timestampRef or timeString are given, then the timestamp refers to some unspecified moment in time. This is useful when the timestamp is referenced by multiple elements to provide relative timing information.

The four examples below illustrate the establishment of various reference timestamps. The first <timestamp> element, ts001, refers to January 2, 2004 at 7:00am, UTC. The second establishes timestamp ts002 which refers to January 2, 2004 at 7:10am, UTC (10 minutes after the reference timestamp ts001), and the third time stamp, ts003, gives the same time using the timeString attribute. The fourth creates ts004 with time January 2, 2004 at 7:10:04.32, UTC (4.32 seconds after the timestamp of trace ts002).

<timestamp xml:id="ts001" time="1073026800000"/>
<timestamp xml:id="ts002" timeOffset="600000" timestampRef="#ts001"/>
<timestamp xml:id="ts003" timeString="2004-01-02T07:10:00Z"/>
<timestamp xml:id="ts004" timeOffset="4320" timestampRef="#ts002"/>

4.5 The Default Context

Ink traces may specify their contexts explicitly, using a contextRef attribute, or implicitly, in which case they use a default context.

Explicitly referenced <context> elements may occur in a <definitions> element, elsewhere in the same document or in other documents. Explicit contexts are typically used in archival ink applications.

Traces that do not make explicit reference to a context occur in a default context. This is established by the sequence of elements in the <ink> element. Initially the default context is empty and uses defaults for all properties, including a default trace format, default canvas, etc. Then, interspersed with ink data, other elements may occur that alter the default context. These elements are <brush>, <context>, <traceFormat>, <inkSource> and <timestamp>. As the ink is processed from the first child onward, whenever one of these elements is encountered, it is installed as the default to be used by traces. These are used by traces that do not otherwise specify these properties.

The default context may be explicitly specified using the URI "#DefaultContext".  The id "DefaultContext" is therefore reserved and may not be used as the id of a user defined <context> element.

4.6 Context Priority

To describe how contextual information is determined, we start with the notions of "fully resolved context" and "current context" as follows.

A fully resolved context is one for which all the context information (brush, canvas, canvasTransform, inkSource, timestamp, traceFormat) has been obtained either from direct children, by references or inherited. Values are obtained for the context information by giving the contents of the <context> priority over specific references (brushRef, canvasRef, canvasTransformRef, inkSourceRef, timestampRef, traceFormatRef), which take priority over contextRef, which takes priority over the current context.

The current context is a syntactic notion associated to each node in an ink document. Roughly speaking, the current context is changed only by <context>elements that occur directly as children to the <ink> element (i.e. not inside <definitions>). It is defined as follows.

The current context is central to streaming ink applications (see Streaming).

We can now describe how contextual information is determined for ink traces.


5 Canvases

InkML provides support for applications that are required to combine ink from multiple sources. This may arise, for example, from real-time collaboration among several devices, from multiple ink annotations on the same base document or multiple pens operating on the same surface. To support these applications, InkML uses the concept of a shared space, called a canvas.

A canvas is specified using a <canvas> element, and is typically referred to by one or more <context> elements. These contexts may each have their own set of ink capture characteristics and trace formats. In order to map traces from a particular context to a canvas, and vice versa, each context provides its own canvas transform, inverse transform or both.

A context neither referencing nor inheriting a canvas uses a default canvas, sufficient to allow simple single-canvas sharing without further action on the part of devices or applications.

Each canvas defines its dimensions by giving a <traceFormat> element. Its channel declarations may specify minimum and/or maximum values, an orientation and units. If no minimum or maximum is given for a channel of integer or decimal type, then it is unbounded in that direction.

If a canvas is bounded in any direction, then all traces defined on that canvas must be contained inside its limits. There may be applications where strokes appear outside of the canvas. In these cases the processing of out-of-bounds traces is not defined by the specification.

Although canvases are virtual spaces, each of the coordinates may be assigned a unit of measure. This allows collaborating parties to establish a common notion of scale.

An example use for such a shared canvas might be a single ink markup stream or file that contains traces captured on a tablet computer, a PDA device, and an opaque graphics tablet attached to a desktop computer. The size of these traces on each ink source and corresponding display might differ, yet it may be necessary to relate these traces to one another. They could represent scribbles on a shared electronic whiteboard, annotations of a common document, or the markings of two players in a distributed tic-tac-toe game.

The trace data for these different ink sessions could be recorded using the same set of virtual coordinates; however, it is often useful, and may even be necessary at times, to record the data in the ink source coordinates, in order to more precisely represent the original capture conditions, for compactness, or to avoid round-off errors that might be associated with the use of a common coordinate system. Thus we define the concept of a "canvas transform", which can vary according to the ink source. The default transform is the identity. It is also possible to specify the mapping from the canvas back to the coordinates of the original trace format. This is useful in collaborative ink applications where ink added to the canvas from one source must be interpreted in the frame of reference of the other sources. It is not always necessary to specify the inverse transform. If the canvas transform is given as an affine map of full rank, then it may be inverted numerically. Likewise if coordinates are transformed by a lookup table with linear interpolation, then the mapping may be inverted numerically. In all other cases the inverse transformation must be provided if the inverse mapping is required.

5.1 <canvas> element

The <canvas> element provides the virtual coordinate system, which uniquely identifies a shared virtual space for cooperation of ink applications. Together with the trace-to-canvas coordinate transform (discussed below), it provides a common frame of reference for ink collected in multiple sessions on different devices.

Attributes:
xml:id = xsd:ID
The unique identifier for this element.
Required: no, Default: none.

traceFormatRef
= xsd:anyURI
A link to a <traceFormat> element.
Required: no, Default: none.
Contents:
traceFormat?

A <canvas> element must have an associated <traceFormat>, which may either be given as a child element or referred to by a traceFormatRef attribute.  If both a <traceFormat> element and a traceFormatRef attribute are specified, then the element overrides the attribute. The coordinate space of the canvas is given by the regular channels of the trace format and any intermittent channels are ignored.

Example:

<canvas xml:id="A4PaperCanvas">
   <traceFormat>
      <channel name="X" type="decimal" min="0" max="210" units="mm"/>
      <channel name="Y" type="decimal" min="0" max="297" units="mm"/>
   </traceFormat>
</canvas>

5.2 <canvasTransform> element

Attributes:
xml:id = xsd:ID
The identifier for this canvas transform.
Required: no, Default: none

invertible = xsd:boolean
Required: no, Default: false

Contents:
mapping mapping?

The <canvasTransform> element is used to relate two coordinate systems. The source and target coordinate systems are ultimately defined in terms of <traceFormat> elements. These trace formats may either be given directly, or indirectly by <inkSource>, <context> or other <canvas> elements. In general, the source and target coordinate systems may involve a different number and type of coordinates, or have different ranges and orientation for the same dimension.

The contents of the <canvasTransform> consists of one or two <mapping> elements. If there is only one, then it is the mapping from the source to the target coordinate system, where the meaning of "source" and "target" is determined by the use. If there are two children, the first is the mapping from the source to the target and the second is the inverse mapping from the target back to the source.

The transform and its inverse need not be full inverses in the mathematical sense. If a transform is from a trace format to a canvas with fewer coordinates, then the inverse transform may map from the canvas back to the original trace format by supplying default values for the coordinates not in the canvas. This would occur, for example, if a party were sharing ink from a device with a force channel with a canvas with only spatial coordinates.

For certain classes of mappings, the inverse mapping may be determined automatically. These are mappings of type "identity", "affine" (for matrices of full rank), "table" (univariate, with linear interpolation), and "product" mappings of these. In this case, it is possible to specify that an inverse should be determined automatically by giving only the forward transform and specifying a value of true for the invertible attribute.  If two <mapping> elements are specified, then the invertible attribute is ignored.

For an application to give only the inverse transform, it should supply the forward transform as an unknown mapping:

<canvasTransform>
   <mapping type="unknown"/>
   <mapping mappingRef="#map001"/>
</canvasTransform>

5.3 The Default Canvas

The default canvas has two real-valued coordinates X and Y, both unbounded in the positive and negative directions. More precisely, the default canvas is made available as though the following element were included in each InkML document:

<canvas xml:id="DefaultCanvas">
   <traceFormat>
      <channel name="X"
               type="decimal" default="0" orientation="+ve" units="em"/>
      <channel name="Y"
               type="decimal" default="0" orientation="+ve" units="em"/>
   </traceFormat>
</canvas>

The default canvas may be explicitly specified using the URI "#DefaultCanvas".  The id "DefaultCanvas" is therefore reserved and may not be used as the id of a user defined <canvas> element.


6 Generics

This section describes components of the ink markup which are applicable to multiple aspects of the ink markup.

6.1 Mappings

The <mapping> element provides a uniform syntax for the various uses of mappings in the ink markup. The element has an id attribute, which allows a particular mapping to be applied in multiple places. When a previously defined mapping is reused, the mappingRef attribute is used to refer to the <mapping> element, which might be defined in a <definitions> block. Mappings appear in the following different places in InkML:

  1. In a <channel> element of a <traceFormat>, the <mapping> element is used to describe the transformation from the values actually produced by the device to the values recorded in the trace data.
  2. Used by a <canvasTransform>, a mapping may be used to specify the forward or inverse transformations between an ink source and a canvas coordinate system.

InkML supports several types of mappings: unknown, identity, lookup table, affine map, formula (specified using a subset of MathML [MATHML2]) and cross product. The mapping type is indicated by the type attribute of a <mapping> element. Note: If no mapping appears for a <channel>, it defaults to "unknown", which is safer than assuming that 'X' is identical to the device's 'X' since some filtering or modifications could have been applied. Furthermore, one can specify whether the results of a mapping expression are absolute or relative to the current data value. This is done by means of the apply attribute. For lookup table mappings in particular, one can determine how to interpret intermediate mapping values. This is specified using the interpolation attribute.

Some points may have channel values that cannot be mapped.  These may lie outside the domain of a MathML mapping (e.g. division by zero, arcsine of 7) or outside the scope of a lookup table (e.g. below the lowest value when the interpolation scheme is other than "ceiling"). In this situation the behavior is not specified and may vary from implementation to implementation.  For example, an implementation may choose to raise an error or omit the points.

6.1.1 <mapping> element

Attributes
xml:id = xsd:ID
The identifier for this mapping.
Required: no, Default: none

type = "identity" | "product" | "table" | "affine" | "mathml" | "unknown"
The type for the particular mapping.
Required: no, Default: unknown

mappingRef = xsd:anyURI
The ID of a mapping which has previously been defined.
Required: no, Default: none
Contents
( bind* ( table | affine | mathml:math )? ) | mapping*

(The mathml prefix above is declared as "http://www.w3.org/1998/Math/MathML", the MathML schema namespace [MATHML2].)

The identity map

If the type attribute has value identity then the element is empty.

Identity mappings are specified using an empty mapping element:

<mapping xml:id="m01" type="identity" />

<channel name="X" type="decimal" units="pt" default="0">
   <mapping type="identity"/>   
</channel>

They are used, for example, to define a <traceFormat> channel that reports the exact data that is recorded by a corresponding device channel, with no filtering or transformation.

Cross Product Maps

If the type attribute has value product then the contents is a set of <mapping> elements, each giving values for one or more of the coordinates. This allows a multivariate mapping to compute the different coordinate results according to the most convenient means. For example, spatial coordinates may be transformed using an affine map, button states by lookup tables, and color coordinates using formulas.

Lookup Tables

If the type attribute has value table then the mapping is a function specified by a lookup table given as a <table> element containing rows of values separated by commas.

Affine Maps

If the type attribute has value affine then the content is an <affine> element specifying an affine transformation (uM u + b) from n source values to m target values. All of the source and target values must be of the same type, either integer or real (decimal or float). A matrix M containing only the values 0, 1 and -1 may be used to perform arbitrary permutation and reflection of coordinates. If the affine map computes a real number for an integer coordinate, then the value is rounded to the nearest integer.

MathML mappings

If the type attribute has value mathml then the content is a subset of MathML [MATHML2] restricted to the following subset of Content MathML 2.0 elements:

This is a subset has been selected to provide expressions suitable for scalar functions on integers, real numbers and boolean values. A number of restrictions apply:

The content of The arithmetic operators return values whose type depends on the type of the arguments. The logical operators and relations return boolean values. The elementary functions return real values.

Example: The following mapping converts from polar to rectangular coordinates.

<mapping type="product">
   <mapping type="mathml">
      <bind target="X"/>
      <bind source="VR"  variable="r"/>
      <bind source="VTh" variable="theta"/>
      <math xmlns="http://www.w3.org/1998/Math/MathML">
         <apply>
            <times/>
            <ci>r</ci>
            <apply> <cos/> <ci>theta</ci> </apply>
         </apply>
      </math>
   </mapping>
   <mapping type="mathml">
      <bind target="Y"/>
      <bind source="VR"  variable="r"/>
      <bind source="VTh" variable="theta"/>
      <math xmlns="http://www.w3.org/1998/Math/MathML">
         <apply>
            <times/>
            <ci>r</ci>
            <apply> <sin/> <ci>theta</ci> </apply>
         </apply>
      </math>
   </mapping>
</mapping>

6.1.2 <bind> element

Attributes
source = xsd:string
Specifies source data values and/or channel to be considered in the mapping.
Required: no, Default: none

target = xsd:string
Specifies target data values and/or channel to be considered in the mapping.
Required: no, Default: none

column = xsd:integer
Specifies the assigned column within a lookup table either for source or target channels or the assigned position for a channel within the source or target vector of an affine mapping.
Required: for lookup table bindings and affine mappings, Default: none

variable = xsd:string
Specifies the variable within a formula that represents the current source data/channel.
Required: for mathml bindings, Default: none
Contents
EMPTY

The <bind> element is provided for binding channels to entities (variable names, lookup table columns) within a mapping, and thus it supports the reuse of predefined mappings. For each type of mapping, the relevant bindings can be expressed by the combined usage of the <bind> element's attributes, which are source, target, column and variable.

For an identity mapping (type="identity"), if the source channel has a different name than the channel being defined, this can be specified using a <bind> element with a source attribute. In the following markup, the <traceFormat> channel X contains unmanipulated data from the device's devX channel.   When the mapping type is an identity mapping, the <bind> element source attribute is required, and the other attributes target, column, and variable must not be present.

<channel name="X">
   <mapping type="identity">
      <bind source="devX"/>
   </mapping>
</channel>

Within a mapping formula (type="mathml"), the variable names in the formula need to be bound to particular channel names. This is specified using a combination of source and variable attributes for binding inputs of the formula, and target and variable for the output of the formula. This is useful if the same mapping formula is to be reused across multiple channels, like X and Y for example.  When the mapping type is an mathml mapping the column attribute for the <bind> element must not be present.

<mapping xml:id="m06" type="mathml">
   <bind target="X" variable="Q" />
   <math xmlns="http://www.w3.org/1998/Math/MathML">
      <apply>
         <plus/>
         <ci>Q</ci>
         <cn>10</cn>
      </apply>
   </math>
</mapping>

The example shown above means that the channel X is referred to by the variable name Q in the mapping expression "Q+10".

For a lookup table (type="table"), each index column must be bound to the channel that provides the input for the lookup operation. This is done with a <bind> element that specifies source and column attributes. Similarly, each value column must be bound to the channel that receives the output of the lookup. Its <bind> element specifies target and column.   When the mapping type is a lookup mapping the variable attribute for the <bind> element must not be present.

The following example indicates assignments of channels to columns. It means that values for the channels OTx and P are used to look up the value of the cross-coupling for channel X in the table given by the mapping below:

<mapping xml:id="m07" type="table">
   <bind target="X" column="1"/>
   <bind source="OTx" column="2"/>
   <bind source="P" column="3"/>
   <table apply="relative" interpolation="floor">
      10    45    512,
      9     45    400,
      8     45    372,
      7     45    418,

      10    50    510,
      9     50    403,
      8     50    302,
      7     50    407,

      10    55    512,
      9     55    410,
      8     55    303,
      7     55    405,

      10    60    512,
      9     60    420,
      8     60    355,
      7     60    401,
   </table>
</mapping>

For an affine mapping (type="affine"), the column values give the meaning of the rows and columns of the transformation matrix. Suppose an affine mapping is specified by the augmented matrix (M b), corresponding to the transformation v = M . u + b. Then a <bind> element with a source attribute will have a column attribute specifying which index in the vector u corresponds to the named source channel. Likewise, a <bind> element with a target attribute will have a column attribute specifying which index in the vector v corresponds to the named target channel. If the target bindings are the same as the source bindings, then they may be omitted.

6.1.3 <table> element

Attributes
xml:id = xsd:ID
The unique identifier for this table element.
Required: no

apply = "absolute" | "relative"
Specifies whether the mapping values are used from the table/formula, or whether this table/formula needs to be added to the current data value.
Required: no, Default: absolute

interpolation = "floor" | "middle" | "ceiling" | "linear" | "cubic"
Specifies the interpolation between discrete mapping values defined by a lookup table.
Required: no, Default: "linear"
Contents
((number | "T" | "F")+ ",")* (number | "T" | "F")*

where number is defined by the grammar given in the <trace> element section.

The <table> gives a set of points for a mapping. The points are given as comma-separated rows. Each row must have the same number of entries. The final row may optionally be followed by a comma. Each row in the table represents a value of the function at one point. Which columns represent the argument(s) and which the result(s) is determined by <bind> elements.

The entries in the table may either be all numerical or all boolean. They may be derived empirically, by measuring properties of a device, calculated to provide efficient approximation to a numerical function, or give an exhaustive enumeration of values of a function over a finite set of values.

Example:

The following example means that X is a function of OE, given by a lookup table. The value "relative" for the apply, means the table gives an amount to increase X.

<channel name="X"...>
   ...
   <mapping xml:id="m03" type="table">
      <bind source="OE"/>
      <bind target="X"/>
      <table apply="relative" interpolation="floor">
         45  10,
         50   9,
         55   8,
         60   7
      </table>
   </mapping>
   ...
</channel>

Tables may have more than two columns, with some of them (the source columns) determining others (the target columns). If there is more than one source column, then all possible combinations of source values must be given. For example, if there are two source columns with one having 3 distinct values and the other having 5 distinct values, then the table must have 15 rows.

The value of the interpolation attribute defines the behavior for indices that don't appear in a numerical table. The following summarizes the behavior of the above table for the various values of interpolation:

"floor" The value is determined by rounding all source variables down to the nearest specified value.
X += 10  if 45 ≤ OE < 50,
X += 9   if 50 ≤ OE < 55,
...
"middle" The value is constant on regions whose boundaries are mid-way between the given source values.
X += 10  if 45   ≤ OE < 47.5,
X += 9   if 47.5 ≤ OE < 52.5,
...
"ceiling" The value is determined by rounding all source variables up to the nearest specified value.
X += 10  if      OE ≤ 45,
X += 9   if 45 < OE ≤ 50,
...
"linear" Piece-wise linear interpolation.
"cubic" Interpolation by cubic splines. This option may be used only for univariate mappings and requires the table have at least 4 points.

The interpolation attribute may not be used with boolean tables.

6.1.4 <affine> element

Attributes
xml:id = xsd:ID
The unique identifier for this <affine> element.
Required: no
Contents

( number+ ",")* number*

where number is defined by the grammar given in the <trace> element section.

The <affine> element provides the entries for an affine mapping from n source values to m target values. An affine mapping consists of a linear transformation (multiplication by a matrix) and a shift (adding a vector). The content of the <affine> element is text giving a m comma-separated rows of n+1 numbers each. The final row may optionally be followed by a comma. The first n columns specify an m ×n matrix M, and the last column gives a vector b of length m. If u is the source vector of n coordinates, then v = M   u + b is the target vector of m coordinates.

The following is an example of an affine mapping using an <affine> element to describe the transform (X, Y) ↦ (-Y, X+200).

 <mapping xml:id="m01" type="affine">
   <bind source="X" column="1"/>
   <bind source="Y" column="2"/>
   <affine>
      0 -1   0,
      1  0 200,
   </affine>
</mapping>

6.2 Definitions

6.2.1 <definitions> element

Attributes:
none
Contents:
( brush | canvas | canvasTransform | context | inkSource | mapping | timestamp | trace | traceFormat | traceGroup | traceView )*

The <definitions> element is a container which is used to define reusable content. The definitions within a <definitions> block can be referenced by other elements using the appropriate syntax. Content within a <definitions> block has no impact on the interpretation of traces, unless referenced from outside the <definitions> block. In order to allow them to be referenced, elements within a <definitions> block must include an id; attribute. Therefore, an element which is defined inside a <definitions> without an id, or that is never referenced, serves no purpose.

One of the primary uses of <definitions> is to define contextual information. In particular, the elements <brush>, <canvas>, <canvasTransform>, <context>, <inkSource>, <mapping>, <timestamp> and <traceFormat> may be given inside a <definitions>. These may be referenced from other elements by the attributes brushRef, canvasRef, canvasTransformRef, contextRef, inkSourceRef, mappingRef, timestampRef and traceFormatRef, respectively. Timestamps may also be referenced by the respectTo attribute of the <channel>element.

Another use of <definitions> is to define digital ink traces for later reference. These may be given by <trace>, <traceGroup> or <traceView>. These are not considered part of the ink data to be handled by the application until they are referenced from other elements (outside the <definitions>) by a traceDataRef attribute. This is useful in archival applications.

The following simple example illustrates usage of the <definitions> element.

<ink xmlns="http://www.w3.org/2003/InkML">
   <definitions>
      <brush xml:id="redPen"/>
      <brush xml:id="bluePen"/>
      <traceFormat xml:id="normal"/>
      <traceFormat xml:id="noForce"/>
      <context xml:id="context1"
            brushRef="#redPen"
            traceFormatRef="#normal"/>
      <context xml:id="context2"
            contextRef="#context1"
            brushRef="#bluePen"/>
   </definitions>
   <context contextRef="#context2" traceFormatRef="#noForce"/>
   <context xml:id="context3"/>
</ink>

More details on the usage of the <definitions> element are provided in the Archival Applications section.

6.3 Annotations

InkML provides generic ways of assigning metadata or semantics to ink via two elements <annotation> and <annotationXML>, modeled after the corresponding elements in MathML. However since annotations are typically application-specific, InkML does not attempt to prescribe the contents of these elements.  Since the contents of <annotation> or <annotationXML> elements are application defined, implementers should use them with care and remain aware that other implementations may ignore them or fail to round-trip unrecognized annotations.

6.3.1 <annotation> element

Attributes
type = xsd:string
The category of annotation that this element describes, for descriptive purposes only. (Applications may define their own types.)
Required: no
Default: none

encoding = xsd:string
The kind of syntax, standard or convention being used for the values of the annotation, e.g. ISO639 for language codes. Required: no
Default: none

Other attributes in a namespace other than that of InkML are also allowed, such as general metadata properties (e.g. from the Dublin Core vocabulary) or application-specific attributes.

The <annotation> element provides a mechanism for inserting simple textual descriptions in the ink markup. This may be used for multiple purposes. For instance, the text contained in the <annotation> may include additional information provided by the user generating InkML, and may be displayed by an InkML consumer rendering a graphical representation of traces. Or it may be used for the indication of metadata such as the writer, the writing instrument. Another important potential application is the semantic tagging of traces.

Example:
<ink xmlns="http://www.w3.org/2003/InkML"
     xmlns:dc="http://dublincore.org/documents/2001/10/26/dcmi-namespace/">
   <annotation type="description">A Sample of Einstein's Writings</annotation>
   <annotation type="writer">Albert Einstein</annotation>
   <annotation type="contentCategory">Text/en</annotation>
   <annotation type="language" encoding="ISO639">en</annotation>
   <annotation dc:language="en"/>

   <trace xml:id="trace1">
      ...
   </trace>
   <traceGroup xml:id="tg1">
      <annotation type="truth">Hello World</annotation>
      <traceGroup>
         <annotation type="truth">Hello</annotation>
         <trace> ...  </trace>
         ...
      </traceGroup>
      <traceGroup>
         <annotation type="truth">World</annotation>
         <trace> ...  </trace>
         ...
     </traceGroup>
   </traceGroup>
   <traceView traceDataRef="#tg1"/>
</ink>

For semantic tagging, one of the common types of <annotation> is "contentCategory", which describes at a basic level the category of content that the traces represent; e.g., "Text/English", "Drawing", "Math", "Music". Such categories are useful for general data identification purposes, and may be essential for selecting data to train handwriting recognizers in different problem domains.

Although largely application-defined, a number of likely, common categories are suggested below.

The language specification may be made using any of the language identifiers specified in ISO 639, using 2-letter codes, 3-letter codes, or country names. Some text may also require a script specification (such as Kanji, Katakana, or Hiragana) in addition to the language.

For some applications it may be useful to provide additional sub-categories defining the type of the data. For example, some suggested sub-categories for Text include:

Suggested possible sub-categories for Drawing are:

6.3.2 <annotationXML> element

Attributes
type = xsd:string
The category of annotation that this element describes, for descriptive purposes only. (Applications may define their own types.)
Required: no, Default: none

encoding = xsd:string
The kind of syntax, standard or convention being used for the values of the annotation, e.g. ChemML, MathML, RDF, etc.
Required: no, Default: none

href = xsd:anyURI
A reference to XML content giving the annotation. Required: no, Default: none

Other attributes in a namespace other than that of InkML are also allowed, such as general metadata properties (e.g. from the Dublin Core vocabulary) or application-specific attributes.

Contents
Any XML-based annotation

This element allows ink to be annotated with general XML objects. For instance a handwritten equation may be described using a snippet of MathML, or metadata and semantic annotation may be provided using an XML language. These annotations may be given either as the content of an <annotationXML> element or may be referred to by a href attribute, but not both.  If several annotations are desired, several <annotationXML> elements should be given.

When annotations of a parent node include the content of the annotations of the child nodes, then one should consider using <annotationXML> annotations on the children with href attributes referring to sub-trees of the parents annotation in order to maintain linear space complexity in the annotations.

Example:
<ink xmlns="http://www.w3.org/2003/InkML">
   <annotation type="description">A Sample of Einstein's Writings</annotation>    
   <annotationXML type="metadata" encoding="rdf">
      <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
               xmlns:dc="http://purl.org/dc/elements/1.1/" >
         <rdf:Description about=""
            dc:language="en"
            dc:date="2004-04-11"
            dc:creator="InkML Maker v0.1"
            dc:publisher="Famous Handwritings Ltd."/>
      </rdf:RDF>
   </annotationXML>

   <trace> ... </trace>
   ...
   <trace> ... </trace>
</ink>
Example:
<ink xmlns="http://www.w3.org/2003/InkML">
   <annotationXML type="truth" encoding="application/xhtml+xml">
      <html xmlns="http://www.w3.org/1999/xhtml">
         <body>
            <div xml:id="Ch1">
               <h1 xml:id="T1"><span xml:id="W1">Weather </span></h1>
               <p xml:id="P1">
                 <span xml:id="W2">The</span>
                 <span xml:id="W3">rain</span>
                 ... more words
               </p>
               <p xml:id="P2">...</p>
               ... more paragraphs
            </div>
            ... more chapters
         </body>
      </html>
   </annotationXML>

   <traceGroup>
      <annotationXML href="#Ch1"/>
      <traceGroup>
         <annotationXML href="#T1"/>
         <traceGroup>
            <!-- Weather -->
            <annotationXML href="#W1"/>
            <trace>...</trace>
         </traceGroup>
      </traceGroup>

      <traceGroup>
         <annotationXML href="#P1"/>
         <traceGroup>
            <!-- The -->
            <annotationXML href="#W2"/>
            <trace>...</trace>
         </traceGroup>
         <traceGroup>
            <!-- rain -->
            <annotationXML href="#W3"/>
            <trace>...</trace>
         </traceGroup>
         ... more words in paragraph
      </traceGroup>
   </traceGroup>
   <traceGroup>
       <annotationXML href="#P2"/>
       ... words in paragraph
   </traceGroup>
   ... more paragraphs in chapter.
</ink>

If it were not for the sharing of the substructure of the attribute XML data, then each attribute word would be repeated three times (as a word, in a paragraph, and in a chapter), each paragraph would be repeated twice, etc.

6.4 Units

Units are used in several parts of ink mark up. For example channels may report their values with some dimension, such as length, requiring units. Other elements may give values, such as resolution, as quantities in particular units.

The following abbreviations must be recognized as unit attribute values.

Dimension Unit Interpretation
length m meters
  cm centimeters
  mm millimeters
  in inches
  pt points (1pt = 1/72 in)
  pc picas (1pc = 1/22 pt)
  em ems, the width of a letter "M" in a notional normal size
  ex exs, the height of a letter "x" in a notional normal size
time s seconds
  ms milliseconds
mass kg kilograms
  g grams
  mg milligrams
force N Newtons
angle deg degrees
  rad radians
all % percentage, expressed as a fraction (1.0 = 100%) relative to max-min
  dev quanta relative to a device resolution. This can correspond to pixels, force levels, clock ticks, etc.

In addition to the units named above, the following expressions must also be recognized:

unitExpr ::=
      unit    
    | "1"      "/" unit
    | unitExpr "/" unit
    | unitExpr "*" unit

unitPrimitive ::= unit | "(" unitExpr ")"

unit ::= one of the units from the table above,
         with the exception of em, ex, % and dev.

Other units are permitted, but need not be recognized by a compliant application.


7 Archives and Streams

The ink markup is expected to be used in many different scenarios. Ink markup data may be transmitted in substantially real time while exchanging ink messages, or ink documents may be archived for later retrieval or processing. InkML has been designed with both of these uses in mind, and it is natural to use InkML in a particular way in each of these settings.

These settings illustrate two different styles of ink generation and usage. In the later, the markup must facilitate the incremental transmission of a stream of ink data, while in the former, the markup should provide the structure necessary for operations such as search and interpretation. In order to support both cases, InkML provides archival and streaming modes of usage. These are not distinct and incompatible languages, but rather are two stylized ways of using InkML.

7.1 Archival Applications

Archival applications typically handle ink data that has been collected over some span of time and has some structure, organization or interpretation associated to the ink data. These applications may re-organize ink traces so it is preferable that the traces be state-free. That is, in archival applications, to the extent that ink traces make use of context information, this is always done explicitly and never through the "current" context.

In archival usage, contextual elements occur within one or more <definitions> elements and are assigned identifiers using the id attribute. References to defined elements are made using the corresponding brushRef, traceFormatRef, and contextRef attributes. This is illustrated in the following example:

<definitions>
   <brush xml:id="penA"/>
   <brush xml:id="penB"/>
   <traceFormat xml:id="fmt1">
      <channel name="X" type="integer"/>
      <channel name="Y" type="integer"/>
      <channel name="Z" type="integer"/>
   </traceFormat>

   <canvas xml:id="canvasA">
      <traceFormat>
         <channel name="X" type="decimal" min="0" max="200" units="mm"/>
         <channel name="Y" type="decimal" min="0" max="150" units="mm"/>
      </traceFormat>
   </canvas>
   <canvasTransform xml:id="trans1">
      <mapping type="affine">1 0 0 0,0 1 0 0</mapping>
   </canvasTransform>
   <canvasTransform xml:id="trans2">
      <mapping type="affine">2 0 0 0,0 -2 0 0</mapping>
   </canvasTransform>

   <context xml:id="context1"
      canvasRef="#canvasA"
      canvasTransformRef="#trans1"
      traceFormatRef="#fmt1"
      brushRef="#penA"/>

   <context xml:id="context2"
      canvasRef="#canvasA"
      canvasTransformRef="#trans2"
      traceFormatRef="#fmt1"
      brushRef="#penB"/>
</definitions>

This example defines two brushes ("penA" and "penB"), a traceFormat ("fmt1"), and two contexts ("context1" and "context2") which both refer to the same canvas ("canvasA") and traceFormat ("fmt1"), but with different canvas transforms and brushes. Note the use of the brushRef, traceFormatRef, canvasRef and canvasTransformRef attributes to refer to previously defined <brush>, <traceFormat> <canvas> and <canvasTransform> elements.

Within the scope of a <definitions> element, unspecified attributes of a <context> element are assumed to have their default values. The <definitions> block below defines "context1", which is comprised of "canvasA" with the default canvasTransform and traceFormat (the identity mapping and a traceFormat consisting of decimal X-Y coordinate pairs), and "penA".

 <definitions>
    <brush xml:id="penA"/>
    <context xml:id="context1"
      canvasRef="#canvasA"
      brushRef="#penA"/>
</definitions>

A <context> element can inherit and override the values of a previously defined context by including a contextRef attribute, so the following block defines "context2" which shares the same canvas ("canvasA") and traceFormat (the default format) as "context1", but has a different canvasTransform and brush.

<definitions>
   <brush xml:id="penA"/>
   <context xml:id="context1"
      canvasRef="#canvasA"  
      canvasTransformRef="#trans1"/>
   <context xml:id="context2"
      contextRef="#context1"
      canvasTransformRef="#trans2"
      brushRef="#penA"/>
</definitions>

Within archival ink markup, traces can either explicitly specify their context through the use of contextRef and brushRef attributes, or they can have their context provided by an enclosing traceGroup. In the following example, traces "t001" and "t003" have the context defined by "context1", while trace "t002" has a context consisting of the default canvas, canvasTransform and traceFormat, and "penA".

<trace xml:id="t001" contextRef="#context1">...</trace>
<trace xml:id="t002" brushRef="#penA">...</trace>

<traceGroup contextRef="#context1">
   <trace xml:id="t003">...</trace>
</traceGroup>

Traces within a <traceGroup> element can also override the context or brush specified by the traceGroup. In the following example, traces "t001" and "t003" have their context specified by "context1" while trace "t002" overrides the default brush of "context1" with "penA".

<traceGroup contextRef="#context1">
   <trace xml:id="t001">...</trace>
   <trace xml:id="t002" brushRef="#penA">...</trace>
   <trace xml:id="t003">...</trace>
</traceGroup>

A trace or traceGroup can both reference a context and override its brush, as in the following example which assigns the context specified by "context1" to traces "t001" and "t002", but with "penA" instead of the default brush.

<trace xml:id="t001" contextRef="#context1" brushRef="#penA">...</trace>
<traceGroup contextRef="#context1" brushRef="#penA">
    <trace xml:id="t002">...</trace>
</traceGroup> 

In archival mode, the ink markup processor can straightforwardly determine the context for a given trace by examining only the <definitions> blocks within the markup and the enclosing traceGroup for the trace.

7.2 Streaming Applications

Streaming ink applications present digital ink traces in sequential time order. Contextual information is inserted into the stream of ink traces, as needed, to provide interpretation for the ink strokes. These changes to the current trace context are given by <context> elements. These may directly contain brush, trace format and other information or which may refer to previously seen such elements. This corresponds to an event-driven model of ink generation, where events which result in contextual changes map directly to elements in the markup.

The current context consists of the set of canvas, canvasTransform, traceFormat and brush which are associated with subsequent traces in the ink markup. Initially, the current context contains the default canvas, an identity canvasTransform, the default traceFormat, and a brush with no attributes. Each <brush>, <traceFormat>, and <context> element which appears outside of a <definitions> element changes the current context accordingly (elements appearing within a <definitions> block have no effect on the current context, and behave as described above in the archival section).

The appearance of a <brush> element in the ink markup sets the current brush attributes, leaving all other contextual values the same. Likewise, the appearance of a <traceFormat> element sets the current traceFormat, and the appearance of a <context> element sets the current context.

Outside of a <definitions> block, any values which are not specified within a <context> element are taken from the current context. For instance, the <context> element in the following example changes the current brush from "penB" to "penA", leaving the canvas, canvasTransform, and traceFormat unchanged from trace "t001" to trace "t002". That is, each context element is taken to inherit from the previously established context.

<brush xml:id="penA"/>
<brush xml:id="penB"/>
<trace xml:id="t001">...</trace>
<context brushRef="#penA"/>
<trace xml:id="t002">...</trace>

In order to change a contextual value back to its default value, its attribute can be specified with the value "#DefaultCanvas" or "#DefaultBrush". In the following:

<context canvasRef="#canvasA" brushRef="#penA"/>
<trace xml:id="t001">...</trace>
<context canvasRef="#DefaultCanvas" brushRef="#DefaultBrush"/>
<trace xml:id="t002">...</trace>

Trace "t001" is on "canvasA" and has the brush specified by "penA", while trace "t002" is on the default canvas and has the default brush.

Brushes, traceFormats, and contexts which appear outside of a <definitions> block and contain an id attribute both set the current context and define contextual elements which can be reused (as shown above for the brushes "penA" and "penB"). This example:

<context xml:id="context1"
    canvasRef="#canvasA"
    canvasTransformRef="#trans1"
    traceFormatRef="#fmt1"
    brushRef="#penA"/>

defines a context which can be referred to by its identifier "context1". It also sets the current context to the values specified in the <context> element.

A previously defined context is referenced using the contextRef attribute of the <context> element. For example:

<context contextRef="#context1"/>

sets the current context to have the values specified by "context1". A <context> element can also override values of a previously defined context by including both a contextRef attribute and one or more of the canvasRef, canvasTransformRef, traceFormatRef or brushRef attributes. The following:

<context contextRef="#context1" brushRef="#penB"/>

sets the current context to the values specified by "context1", except that the current brush is set to "penB" instead of "penA".

A <context> element which inherits and overrides values from a previous context can itself be reused, so the element:

<context xml:id="context2" contextRef="#context1" brushRef="#penB"/>

defines "context2" which has the same context values as "context1" except for the brush.

Finally, a <context> element with only an id has the effect of taking a "snapshot" of the current context which can then be reused. The element:

<context xml:id="context3"/>

defines "context3", whose values consist of the current canvasRef, canvasTransform, traceFormat, and brush at the point where the element occurs (note that since "context3" does not specify any values, the element has no effect on the current context).

An advantage of the streaming style is that it is easier to express overlapping changes to the individual elements of the context. However, determining the context for a particular trace can require more computation from the ink markup processor, since the entire file may need to be scanned from the beginning in order to establish the current context at the point of the <trace> element.

While it is possible to wait and generate each trace as it is completed, this can lead to considerable latency from the starting time with long strokes.  This may be avoided by generating traces of partial strokes and using continuation traces.

Finally, it should be noted that traces can overlap in time.  This can occur in collaborative applications with several writers or with one user on "multi-touch" devices.  Here it is also possible to generate traces for complete strokes on pen up, but applications may use partial strokes of limited time duration to guarantee that a buffer restricted to a sliding time window sees all simultaneous traces.

7.3 Archival and Streaming Equivalence

The following examples of archival and streaming ink markup data are equivalent, but they highlight the differences between the two styles:

Archival

<ink xmlns="http://www.w3.org/2003/InkML">
   ...
   <definitions>
      <brush xml:id="penA"/>
      <brush xml:id="penB"/>
      <context xml:id="context1"
         canvasRef="#canvas1"
         canvasTransformRef="#trans1"
         traceFormatRef="#format1"/>
      <context xml:id="context2"
         contextRef="#context1"
         canvasTransformRef="#trans2"/>
   </definitions>
   <traceGroup contextRef="#context1">
        <trace>...</trace>
        ...
   </traceGroup>
   <traceGroup contextRef="#context2">
        <trace>...</trace>
        ...
   </traceGroup>
   <traceGroup contextRef="#context2" brushRef="#penB">
        <trace>...</trace>
        ...
   </traceGroup>
   <traceGroup contextRef="#context1" brushRef="#penB">
        <trace>...</trace>
        ...
   </traceGroup>
   <traceGroup contextRef="#context1" brushRef="#penA">
        <trace>...</trace>
        ...
   </traceGroup>
</ink>

Streaming

<ink xmlns="http://www.w3.org/2003/InkML">
   ...
   <definitions>
      <brush xml:id="penA"/>
      <brush xml:id="penB"/>
   </definitions>
   <context xml:id="context1"
      canvasRef="#canvas1"
      canvasTransformRef="#trans1"
      traceFormatRef="#format1"/>
   <trace>...</trace>
   ...
   <context xml:id="context2"
      contextRef="#context1"
      canvasTransformRef="#trans2"/>
   <trace>...</trace>
   ...
   <context brushRef="#penB"/>
   <trace>...</trace>
   ...
   <context contextRef="#context1"/>
   <trace>...</trace>
   ...
   <context brushRef="#penA"/>
   <trace>...</trace>
   ... 
</ink>

In the archival case, the context for each trace is simply determined by the <trace> element, its enclosing traceGroup, and contextual elements defined in the <definitions> block, while in the streaming case, the context for a trace can depend on the entire sequence of context changes up to the point of the <trace> element.

However, the streaming case more simply expresses the changes of context involving "penB", "context1", and "penA", whereas the archival case requires the restatement of the unchanged values in the successive traceGroups.

The two styles of ink markup are equally expressive, but impose different requirements on the ink markup processor and generator. Tools to translate from streaming to archival style might also be of use to applications which work on stored ink markup.

8. Conformance

The contents of this section are normative.

8.1 Conforming InkML Documents

A document is a Conforming InkML Document if it meets both the following conditions:

The InkML specification and these conformance criteria provide no designated size limits on any aspect of InkML documents. There are no maximum values on the number of elements, the amount of character data, or the number of characters in attribute values.

Within this specification, the term URI refers to a Universal Resource Identifier as defined in [RFC3986] and extended in [RFC3987] with the new name IRI. The term URI has been retained in preference to IRI to avoid introducing new names for concepts such as "Base URI" that are defined or referenced across the whole family of XML specifications.

8.2 Using InkML with other Namespaces

The InkML namespace is intended to be used with other XML namespaces as per the Namespaces in XML Recommendation [XMLNS]. Future work by W3C is expected to address ways to specify conformance for documents involving multiple namespaces.

8.3 Conforming InkML Processors

An InkML processor is a program that can process and/or generate Conforming InkML documents.

In a Conforming InkML Processor, the XML parser MUST be able to parse and process all XML constructs defined by XML 1.1 [XML] and Namespaces in XML [XMLNS]. It is not required that a Conforming InkML Processor uses a validating XML parser.

A Conforming InkML Processor MUST correctly understand and apply the semantics of each markup element or attribute as described by this document.

There is, however, no conformance requirement with respect to performance characteristics of the InkML Processor. For instance, no statement is required regarding the accuracy, speed or other characteristics of output produced by the processor. No statement is made regarding the size of input that a InkML Processor is required to support.


A Acknowledgements

We thank our colleagues at IBM for providing their work in 2002 as a starting point for this definition.

We thank all participants in the InkML activity of the Multimodal Interaction Working Group for the many detailed constructive discussions. Without the participants' desire to obtain the best outcome, regardless of corporate affiliation, this work would not have been possible.

We specifically thank the W3C staff who have supported the InkML activity: Max Froumentin who served as a staff member of W3C until 2006 and served as editor of previous working drafts and Kazuyuki Ashimura who then took responsibility for the InkML activity at W3C and has provided continuous energy and support.

Finally, we thank Deborah Dahl, whose stewardship of the W3C Multimodal Interaction Working Group has provided the perfect environment for this work to come to fruition.

B Implementation Guidelines

The following are informative implementation guidelines for reducing InkML file size and environmental interactions.

  1. Gzip compression.

    The lossless gzip compression [RFC1952] will help to reduce the InkML file size considerably.  It is recommend that applications have the facility to compress and decompress InkML files and streams using the gzip algorithm.

    The lossless gzip compression [RFC1952] will help to reduce the InkML file size

  2. Authoring tips

    The elements which define constructs that can be referenced repeatedly such as <brush> definitions, and <traceFormat> definitions should be placed within a <definition> element and referred to in required places such as <trace> elements.

    Applications should take advantage of trace data prefixes (' | " | *) for defining relative coordinate values.  The use of first and second order derivative coordinates can effectively compress <trace> element data losslessly.

    Applications should make use of the 'current context' to cache the context property values and hence reduce the context property markups that are being sent explicitly along with trace data.

  3. InkML transmission

    Any of the usual XML protocols (StAX, SOAP, etc) may be used to transmit InkML documents or fragments between subprograms or distributed programs.

  4. Network streaming

    Client and server applications that wish to stream InkML should have the capability to process the data sent in multiple fragments of InkML packets.  The receipt of such InkML fragments may progressively render on the client or clients.

C References

[DC]
Dublin Core Metadata Element Set, Version 1.1: Reference Description. http://dublincore.org/documents/dces/ .
[RDF-SYNTAX]
RDF/XML Syntax Specification (Revised), D. Beckett, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/ . Latest version available at http://www.w3.org/TR/rdf-syntax-grammar/ .
[RFC1952]
GZIP file format specification version 4.3. IETF RFC 1952. http://www.ietf.org/rfc/rfc1952.txt .
[RFC3023]
XML Media Types. IETF RFC 3023. http://www.ietf.org/rfc/rfc3023.txt .
[XML]
Extensible Markup Language (XML) 1.1 (Second Edition), Tim Bray et al., Editors, W3C Recommendation, 16 August 2006, http://www.w3.org/TR/2006/REC-xml11-20060816/ . Latest version available at http://www.w3.org/TR/xml11/ .
[XMLNS]
Namespaces in XML 1.1 (Second Edition), Tim Bray et al., Editors, W3C Recommendation, 16 August 2006, http://www.w3.org/TR/2006/REC-xml-names11-20060816/ . Latest version available at http://www.w3.org/TR/xml-names11/ .
[XMLSCHEMA2]
XML Schema Part 2: Datatypes Second Edition, A. Malhotra, P. V. Biron, Editors, W3C Recommendation, 28 October 2004, http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/ . Latest version available at http://www.w3.org/TR/xmlschema-2/ .
[MATHML2]
Mathematical Markup Language (MathML) Version 2.0 (Second Edition), David Carlisle, Patrick Ion, Robert Miner, Nico Poppelier, Editors, W3C Recommendation, 21 October 2003, http://www.w3.org/TR/2003/REC-MathML2-20031021/ . Latest version http://www.w3.org/TR/MathML2/ .
[RFC3986]
Uniform Resource Identifier (URI): Generic Syntax. IETF RFC 3986. http://www.ietf.org/rfc/rfc3986.txt, January 2005
[RFC3987]
Internationalized Resource Identifiers (IRIs). IETF RFC 3987, http://www.ietf.org/rfc/rfc3987.txt, January 2005.
[EBNF]
Information technology — Syntactic metalanguage — Extended BNF, International Organization for Standardization, 1996.
Available at http://standards.iso.org/ittf/PubliclyAvailableStandards/s026153_ISO_IEC_14977_1996(E).zip.

D The InkML Media Type

This appendix registers a new MIME media type, "application/inkml+xml".

The "application/inkml+xml" media type is registered with IANA at http://www.iana.org/assignments/media-types/application/.

D.1 Registration of MIME media type application/inkml+xml

MIME media type name:

application

MIME subtype name:

inkml+xml

Required parameters:

None.

Optional parameters:

charset

This parameter has identical semantics to the charset parameter of the application/xml media type as specified in [RFC3023] or its successor.

Encoding considerations:

By virtue of InkML content being XML, it has the same considerations when sent as "application/inkml+xml" as does XML. See RFC 3023 (or its successor), section 3.2.

Security considerations:

InkML elements may include arbitrary URIs. Therefore the security issues of [RFC3986], section 7, should be considered.

In addition, because of the extensibility features for InkML, it is possible that "application/inkml+xml" may describe content that has security implications beyond those described here. However, if the processor follows only the normative semantics of this specification, this content will be ignored. Only in the case where the processor recognizes and processes the additional content, or where further processing of that content is dispatched to other processors, would security issues potentially arise. And in that case, they would fall outside the domain of this registration document.

Interoperability considerations:

This specification describes processing semantics that dictate behavior that must be followed when dealing with, among other things, unrecognized elements.

Because InkML is extensible, conformant "application/inkml+xml" processors MAY expect that content received is well-formed XML, but processors SHOULD NOT assume that the content is valid InkML or expect to recognize all of the elements and attributes in the document.

Published specification:

This media type registration is extracted from Appendix D of the Ink Markup Language (InkML) specification.

Additional information:
Magic number(s):

There is no single initial octet sequence that is always present in InkML documents.

File extension(s):

InkML documents are most often identified with the extensions ".ink" or ".inkml".

Macintosh File Type Code(s):

TEXT

Person & email address to contact for further information:

Kazuyuki Ashimura, <ashimura@w3.org>.

Intended usage:

COMMON

Author/Change controller:

The InkML specification is a work product of the World Wide Web Consortium's Multimodal Interaction Working Group. The W3C has change control over these specifications.

D.2 Fragment Identifiers

For documents labeled as "application/inkml+xml", the fragment identifier notation is exactly that for "application/xml", as specified in RFC 3023.

E XML Schema

This section defines the formal syntax for InkML documents in terms of a normative XML Schema.

The latest version of the XML Schema for InkML is available at http://www.w3.org/TR/InkML/inkml.xsd.

For stability it is RECOMMENDED that you use the dated URI available at http://www.w3.org/TR/2011/REC-InkML-20110920/inkml.xsd.