SVG Tiny 1.2 – 20080912

8 Paths

Contents

8.1 Introduction

Paths represent the outline of a shape which can be filled or stroked. (See Filling, Stroking and Paint Servers.)

A path is described using the concept of a current point. In an analogy with drawing on paper, the current point can be thought of as the location of the pen. The position of the pen can be changed, and the outline of a shape (open or closed) can be traced by dragging the pen in either straight lines or curves.

Paths represent the geometry of the outline of an object, defined in terms of moveto (set a new current point), lineto (draw a straight line), curveto (draw a curve using a cubic Bézier) and closepath (close the current shape by drawing a line to the last moveto) elements. Compound paths (i.e., a path with multiple subpaths) are possible to allow effects such as "donut holes" in objects.

This chapter describes the syntax and behavior for SVG paths. Various implementation notes for SVG paths can be found in 'path' element implementation notes.

A path is defined in SVG using the 'path' element.

8.2 The 'path' element

Schema: path
    <define name='path'>
      <element name='path'>
        <ref name='path.AT'/>
        <zeroOrMore><ref name='shapeCommon.CM'/></zeroOrMore>
      </element>
    </define>

    <define name='path.AT' combine='interleave'>
      <ref name='svg.ShapeCommon.attr'/>
      <ref name='svg.D.attr'/>
      <optional>
        <attribute name='pathLength' svg:animatable='true' svg:inheritable='false'>
          <ref name='Number.datatype'/>
        </attribute>
      </optional>
    </define>

Attribute definitions:

d = "<path-data>"

The definition of the outline of a shape. See Path data. An empty attribute value (d="") disables rendering of the element. The lacuna value is the empty string.

Animatable: yes, but see restrictions described in Animating path data.

pathLength = "<number>"

The authoring length of the path, in user units. This value is used to calibrate the user agent's own distance-along-a-path calculations with that of the author. The user agent shall scale all distance-along-a-path computations by the ratio of 'pathLength' to the user agent's own computed value for total path length. 'pathLength' potentially affects calculations for motion animation and various stroke operations.

A negative value is an unsupported value.

Animatable: yes.

focusable = "true" | "false" | "auto"

See attribute definition for description.

Animatable: yes.

Navigation Attributes

See definition.

8.2.1 Animating path data

Interpolated path data animation is only possible when each normalized path data specification within an animation specification has exactly the same list of path data commands as the 'd' attribute after normalization. This means that each path data specification and the 'd' attribute would have the exact same list of commands if normalized as defined in Path Normalization. If an animation is specified and the list of path data commands is not the same, then the animation specification must be ignored as unsupported. The animation engine shall interpolate each parameter to each path data command separately based upon the attributes to the given animation element.

Non-interpolated (i.e. calcMode="discrete") path data animation is always possible.

8.3 Path data

8.3.1 General information about path data

A path is defined by including a 'path' element which contains a 'd' attribute, where the 'd' attribute contains the moveto, line, curve (both cubic and quadratic Béziers) and closepath instructions.

Example 08_01 specifies a 'path' in the shape of a triangle. (The M indicates a moveto, the L's indicate lineto's, and the z indicates a closepath).

Example: 08_01.svg
<?xml version="1.0"?>
<svg width="4cm" height="4cm" viewBox="0 0 400 400"
     xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny">
  <title>Example triangle01- simple example of a 'path'</title>
  <desc>A path that draws a triangle</desc>
  <rect x="1" y="1" width="398" height="398"
        fill="none" stroke="blue" />
  <path d="M 100 100 L 300 100 L 200 300 z"
        fill="red" stroke="blue" stroke-width="3" />
</svg>
Rendering of 08_01.svg

Path data can contain newline characters and thus can be broken up into multiple lines to improve readability.

The syntax of path data is concise in order to allow for minimal file size and efficient downloads, since many SVG files will be dominated by their path data. Some of the ways that SVG attempts to minimize the size of path data are as follows:

The path data syntax is a prefix notation (i.e., commands followed by parameters). The only allowable decimal point is a Unicode U+002E FULL STOP (".") character (also referred to in Unicode as PERIOD, dot and decimal point) [UNICODE] and no other delimiter characters are allowed. (For example, the following is an invalid numeric value in path data: "13,000.56". Instead, say: "13000.56".)

For the relative versions of the commands, all coordinate values shall be relative to the current point at the start of the command.

In the tables below, the following notation is used:

The following sections list the commands.

8.3.2 The "moveto" commands

The 'moveto' commands (M or m) establish a new current point. The effect is as if the "pen" were lifted and moved to a new location. A path data segment (if there is one) must begin with a 'moveto' command. Subsequent 'moveto' commands (i.e., when the 'moveto' is not the first command) represent the start of a new subpath:

Command Name Parameters Description
M (absolute)
m (relative)
moveto (x y)+ A new sub-path at the given (x,y) coordinate shall be started. This shall also establish a new current point at the given coordinate. If a relative 'moveto' (m) appears as the first element of the 'path', then it shall treated as a pair of absolute coordinates. If a 'moveto' is followed by multiple pairs of coordinates, the subsequent pairs shall be treated as implicit 'lineto' commands.

8.3.3 The "closepath" command

A straight line shall be drawn from the current point to the initial point of the current subpath, and shall end the current subpath. If a 'closepath' (Z or z) is followed immediately by any other command, then the next subpath must start at the same initial point as the current subpath.

When a subpath ends in a 'closepath', it differs in behavior from what happens when "manually" closing a subpath via a 'lineto' command in how 'stroke-linejoin' and 'stroke-linecap' are implemented. With 'closepath', the end of the final segment of the subpath shall be "joined" with the start of the initial segment of the subpath using the current value of 'stroke-linejoin'. If instead the subpath is closed "manually" via a 'lineto' command, the start of the first segment and the end of the last segment are not joined but instead shall each be capped using the current value of 'stroke-linecap'. At the end of the command, the new current point shall be set to the initial point of the current subpath.

Command Name Parameters Description
Z or
z
closepath (none) The current subpath shall be closed by drawing a straight line from the current point to current subpath's initial point, which then shall become the new current point. Since the Z and z commands take no parameters, they have an identical effect.

8.3.4 The "lineto" commands

The various 'lineto' commands draw straight lines from the current point to a new point:

Command Name Parameters Description
L (absolute)
l (relative)
lineto (x y)+ A line shall be drawn from the current point to the given (x,y) coordinate, which then shall become the new current point. If more than one coordinate pair is specified, a polyline shall be drawn. At the end of the command, the new current point shall be set to the final set of coordinates provided.
H (absolute)
h (relative)
horizontal lineto x+ A horizontal line shall be drawn from the current point (cpx, cpy) to (x, cpy). If more than one x value is specified, multiple horizonal lines shall be drawn (although usually this doesn't make sense). At the end of the command, the new current point shall be (x, cpy) for the final value of x.
V (absolute)
v (relative)
vertical lineto y+ A vertical line shall be drawn from the current point (cpx, cpy) to (cpx, y). If more than one y value is specified, multiple vertical lines shall be drawn (although usually this doesn't make sense). At the end of the command, the new current point shall be (cpx, y) for the final value of y.

8.3.5 The Curve commands

These groups of commands draw curves:

8.3.6 The Cubic Bézier curve commands

The 'Cubic Bézier' commands are as follows:

Command Name Parameters Description
C (absolute)
c (relative)
curveto (x1 y1 x2 y2 x y)+ A cubic Bézier curve shall be drawn from the current point to (x,y) using (x1,y1) as the control point at the beginning of the curve and (x2,y2) as the control point at the end of the curve. If multiple sets of coordinates are specified, a polybézier shall be drawn. At the end of the command, the new current point shall be the final (x,y) coordinate pair used in the polybézier.
S (absolute)
s (relative)
shorthand/smooth curveto (x2 y2 x y)+ A cubic Bézier curve shall be drawn from the current point to (x,y). The first control point shall be the reflection of the second control point on the previous command relative to the current point. (If there is no previous command or if the previous command was not an C, c, S or s, the first control point shall be coincident with the current point.) (x2,y2) shall be used as the second control point (i.e., the control point at the end of the curve). If multiple sets of coordinates are specified, a polybézier shall be drawn. At the end of the command, the new current point shall be the final (x,y) coordinate pair used in the polybézier.

Example 08_02 shows some simple uses of 'Cubic Bézier' commands within a 'path'. Note that the control point for the "S" command is computed automatically as the reflection of the control point for the previous "C" command relative to the start point of the "S" command.

Example: 08_02.svg
<?xml version="1.0"?>
<svg width="5cm" height="4cm" viewBox="0 0 500 400"
     xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny">
  <title>Example cubic01- cubic Bézier commands in path data</title>
  <desc>Picture showing a simple example of path data
        using both a "C" and an "S" command,
        along with annotations showing the control points
        and end points</desc>
  <rect fill="none" stroke="blue" stroke-width="1" x="1" y="1" width="498" height="398" />
  <polyline fill="none" stroke="#888888" stroke-width="1" points="100,200 100,100" />
  <polyline fill="none" stroke="#888888" stroke-width="1" points="250,100 250,200" />
  <polyline fill="none" stroke="#888888" stroke-width="1" points="250,200 250,300" />
  <polyline fill="none" stroke="#888888" stroke-width="1" points="400,300 400,200" />
  <path fill="none" stroke="red" stroke-width="5" d="M100,200 C100,100 250,100 250,200
                                       S400,300 400,200" />
  <circle fill="#888888" stroke="none" stroke-width="2" cx="100" cy="200" r="10" />
  <circle fill="#888888" stroke="none" stroke-width="2" cx="250" cy="200" r="10" />
  <circle fill="#888888" stroke="none" stroke-width="2" cx="400" cy="200" r="10" />
  <circle fill="#888888" stroke="none" cx="100" cy="100" r="10" />
  <circle fill="#888888" stroke="none" cx="250" cy="100" r="10" />
  <circle fill="#888888" stroke="none" cx="400" cy="300" r="10" />
  <circle fill="none" stroke="blue" stroke-width="4" cx="250" cy="300" r="9" />
  <text font-size="22" font-family="Verdana" x="25" y="70">M100,200 C100,100 250,100 250,200</text>
  <text font-size="22" font-family="Verdana" x="325" y="350"
        text-anchor="middle">S400,300 400,200</text>
</svg>
Rendering of 08_02.svg

The following picture shows some how cubic Bézier curves change their shape depending on the position of the control points. The first five examples illustrate a single cubic Bézier path segment. The example at the lower right shows a "C" command followed by an "S" command.

Example cubic02 - cubic Bézier commands in path data

View this example as SVG (SVG-enabled browsers only)
 

8.3.7 The Quadratic Bézier curve commands

The 'Quadratic Bézier' commands are as follows:

Command Name Parameters Description
Q (absolute)
q (relative)
quadratic Bézier curveto (x1 y1 x y)+ A quadratic Bézier curve is drawn from the current point to (x,y) using (x1,y1) as the control point. If multiple sets of coordinates are specified, a polybézier shall be drawn. At the end of the command, the new current point shall be the final (x,y) coordinate pair used in the polybézier.
T (absolute)
t (relative)
Shorthand/smooth quadratic Bézier curveto (x y)+ A quadratic Bézier curve is drawn from the current point to (x,y). The control point shall be the reflection of the control point on the previous command relative to the current point. (If there is no previous command or if the previous command was not a Q, q, T or t, the control point shall be current point.) If multiple sets of coordinates are specified, a polybézier shall be drawn. At the end of the command, the new current point shall be the final (x,y) coordinate pair used in the polybézier.

Example quad01 shows some simple uses of 'Quadratic Bézier' commands within a path. Note that the control point for the "T" command is computed automatically as the reflection of the control point for the previous "Q" command relative to the start point of the "T" command.

Example: 08_03.svg
<?xml version="1.0"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="12cm" height="6cm" viewBox="0 0 1200 600"
     xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny">
  <title>Example quad01 - quadratic Bezier commands in path data</title>
  <desc>Picture showing a "Q" a "T" command,
        along with annotations showing the control points
        and end points</desc>
  <rect x="1" y="1" width="1198" height="598"
        fill="none" stroke="blue" stroke-width="1" />
  <path d="M200,300 Q400,50 600,300 T1000,300"
        fill="none" stroke="red" stroke-width="5"  />
  <!-- End points -->
  <g fill="black" >
    <circle cx="200" cy="300" r="10"/>
    <circle cx="600" cy="300" r="10"/>
    <circle cx="1000" cy="300" r="10"/>
  </g>
  <!-- Control points and lines from end points to control points -->
  <g fill="#888888" >
    <circle cx="400" cy="50" r="10"/>
    <circle cx="800" cy="550" r="10"/>
  </g>
  <path d="M200,300 L400,50 L600,300 
           L800,550 L1000,300"
        fill="none" stroke="#888888" stroke-width="2" />
</svg>
Rendering of 08_03.svg

8.3.8 The grammar for path data

The following description of the grammar for path data uses Extended Backus-Naur Form [EBNF]:

path-data ::=
    wsp* moveto-drawto-command-groups? wsp*
moveto-drawto-command-groups ::=
    moveto-drawto-command-group
    | moveto-drawto-command-group wsp* moveto-drawto-command-groups
moveto-drawto-command-group ::=
    moveto wsp* drawto-commands?
drawto-commands ::=
    drawto-command
    | drawto-command wsp* drawto-commands
drawto-command ::=
    closepath
    | lineto
    | horizontal-lineto
    | vertical-lineto
    | curveto
    | smooth-curveto
    | quadratic-bezier-curveto
    | smooth-quadratic-bezier-curveto
moveto ::=
    ( "M" | "m" ) wsp* moveto-argument-sequence
moveto-argument-sequence ::=
    coordinate-pair
    | coordinate-pair comma-wsp? lineto-argument-sequence
closepath ::=
    ("Z" | "z")
lineto ::=
    ( "L" | "l" ) wsp* lineto-argument-sequence
lineto-argument-sequence ::=
    coordinate-pair
    | coordinate-pair comma-wsp? lineto-argument-sequence
horizontal-lineto ::=
    ( "H" | "h" ) wsp* horizontal-lineto-argument-sequence
horizontal-lineto-argument-sequence ::=
    coordinate
    | coordinate comma-wsp? horizontal-lineto-argument-sequence
vertical-lineto ::=
    ( "V" | "v" ) wsp* vertical-lineto-argument-sequence
vertical-lineto-argument-sequence ::=
    coordinate
    | coordinate comma-wsp? vertical-lineto-argument-sequence
curveto ::=
    ( "C" | "c" ) wsp* curveto-argument-sequence
curveto-argument-sequence ::=
    curveto-argument
    | curveto-argument comma-wsp? curveto-argument-sequence
curveto-argument ::=
    coordinate-pair comma-wsp? coordinate-pair comma-wsp? coordinate-pair
smooth-curveto ::=
    ( "S" | "s" ) wsp* smooth-curveto-argument-sequence
smooth-curveto-argument-sequence ::=
    smooth-curveto-argument
    | smooth-curveto-argument comma-wsp? smooth-curveto-argument-sequence
smooth-curveto-argument ::=
    coordinate-pair comma-wsp? coordinate-pair
quadratic-bezier-curveto ::=
    ( "Q" | "q" ) wsp* quadratic-bezier-curveto-argument-sequence
quadratic-bezier-curveto-argument-sequence ::=
    quadratic-bezier-curveto-argument
    | quadratic-bezier-curveto-argument comma-wsp?
        quadratic-bezier-curveto-argument-sequence
quadratic-bezier-curveto-argument ::=
    coordinate-pair comma-wsp? coordinate-pair
smooth-quadratic-bezier-curveto ::=
    ( "T" | "t" ) wsp* smooth-quadratic-bezier-curveto-argument-sequence
smooth-quadratic-bezier-curveto-argument-sequence ::=
    coordinate-pair
    | coordinate-pair comma-wsp? smooth-quadratic-bezier-curveto-argument-sequence
coordinate-pair ::=
    coordinate comma-wsp? coordinate
coordinate ::=
    number
nonnegative-number ::=
    integer-constant
    | floating-point-constant
number ::=
    sign? integer-constant
    | sign? floating-point-constant
flag ::=
    "0" | "1"
comma-wsp ::=
    (wsp+ comma? wsp*) | (comma wsp*)
comma ::=
    ","
integer-constant ::=
    digit-sequence
floating-point-constant ::=
    fractional-constant exponent?
    | digit-sequence exponent
fractional-constant ::=
    digit-sequence? "." digit-sequence
    | digit-sequence "."
exponent ::=
    ( "e" | "E" ) sign? digit-sequence
sign ::=
    "+" | "-"
digit-sequence ::=
    digit
    | digit digit-sequence
digit ::=
    "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
wsp ::=
    (#x20 | #x9 | #xD | #xA)

The processing of the EBNF must consume as much of a given EBNF production as possible, stopping at the point when a character is encountered which no longer satisfies the production. Thus, in the string 'M 100-200', the first coordinate for the "moveto" consumes the characters "100" and stops upon encountering the minus sign because the minus sign cannot follow a digit in the production of a "coordinate". The result is that the first coordinate will be "100" and the second coordinate will be "-200".

Similarly, for the string 'M 0.6.5', the first coordinate of the "moveto" consumes the characters "0.6" and stops upon encountering the second decimal point because the production of a "coordinate" only allows one decimal point. The result is that the first coordinate will be "0.6" and the second coordinate will be ".5".

Note that the EBNF allows the path 'd' attribute to be empty. This is not an error, instead it disables rendering of the path. Values of the 'd' that do not match the EBNF are treated as unsupported.

8.4 Distance along a path

Various operations, including motion animation and some stroke operations, require that the user agent compute the distance along the geometry of a graphics element, such as a 'path'.

To aid hand authoring by allowing convenient round numbers to be used, the 'pathLength' attribute can be used to provide the author's computation of the total length of the path so that the user agent can scale distance-along-a-path computations by the ratio of 'pathLength' to the user agent's own computed value for total path length.

A "moveto" operation within a 'path' element is defined to have zero length. Only the various "lineto" and "curveto" commands contribute to path length calculations.