Chapter 1: Introduction

Contents

• • 1.1. About SVG
  • 1.2. SVG MIME type and file name extensions
  • 1.3. SVG namespace and DTD
  • 1.4. Compatibility with other standards efforts
  • 1.5. Terminology
  • 1.6. Definitions

1.1. About SVG

This specification defines the features and syntax for Scalable Vector Graphics (SVG).

SVG is a language for describing two-dimensional graphics. As a standalone format or when mixed with other XML, it uses the XML syntax [XML10]. When mixed with HTML5, it uses the HTML5 syntax [HTML]. SVG allows for three types of graphic objects: vector graphic shapes (e.g., paths consisting of straight lines and curves), images and text. Graphical objects can be grouped, styled, transformed and composited into previously rendered objects. The feature set includes nested transformations, clipping paths, alpha masks, filter effects and template objects.

SVG drawings can be interactive and dynamic. Animations can be defined and triggered either declaratively (i.e., by embedding SVG animation elements in SVG content) or via scripting.

Sophisticated applications of SVG are possible by use of a supplemental scripting language which accesses SVG Document Object Model (DOM), which provides complete access to all elements, attributes and properties. A rich set of event handlers such as ‘onmouseover’ and ‘onclick’ can be assigned to any SVG graphical object. Because of its compatibility and leveraging of other Web standards, features like scripting can be done on HTML and SVG elements simultaneously within the same Web page.

SVG is a language for rich graphical content. For accessibility reasons, if there is an original source document containing higher-level structure and semantics, it is recommended that the higher-level information be made available somehow, either by making the original source document available, or making an alternative version available in an alternative format which conveys the higher-level information, or by using SVG's facilities to include the higher-level information within the SVG content. For suggested techniques in achieving greater accessibility, see Accessibility.

1.6. Definitions

There should be a separate section that lists element and attribute categories and their members.

invalid value

An invalid value specified for a property, either in a style sheet or a presentation attribute, is one that is either not allowed according to the grammar defining the property's values, or is allowed by the grammar but subsequently disallowed in prose. A CSS declaration with an invalid value is ignored; see Declarations and properties ([CSS21], section 4.1.8).

lacuna value

A lacuna value is a defined behavior used when an attribute or property is not specified, or when an attribute or property has an unsupported value. This value is to be used for the purposes of rendering, calculating animation values, and when accessing the attribute or property. As opposed to an XML default value, however, the attribute or property and its value are not visible in the DOM, and cannot be accessed with DOM methods. For properties, if the property is inherited and there is no inherited value (for example, on the root element), the lacuna value is the initial value as specified in the definition of that property. For non-inherited properties, the lacuna value is always the initial value.

"Unsupported value" should be a term that is linked to.

Note that a lacuna value is distinct from the XML term default value, which uses DTD lookup to determine whether an attribute is required and what its value is, and inserts required attributes and their values into the DOM ([XML10], section 3.3.2). At the XML parser level, SVG Tiny 1.2 does not have default values; lacunae are part of the SVG application layer, and their values are derived from the UA.

local URL reference

An Internationalized Resource Identifier [RFC3987] that does not include an <absoluteURL> or <relativeURL> and thus represents a reference to an element within the current document. See References and the ‘defs’ element.

Unify this with other specifiction defintions of URL

markable element

A markable element is one that can have markers painted on it either through the use of the marker properties or by having a ‘marker’ element as a child. The following elements are markable elements: ‘line’, ‘path’, ‘polygon’ and ‘polyline’

non-local URL reference

An Internationalized Resource Identifier [RFC3987] that includes an <absoluteURL> or <relativeURL> and thus (usually) represents a reference to a different document or an element within a different document. See References and the ‘defs’ element.

Unify this with other specifiction defintions of URL. But see HTML5 refs section on URL which is complex.

presentation attribute

An attribute on an SVG element which specifies a value for a given property for that element. See Styling. Note that although any property may be specified on any element, not all properties will apply to (affect the rendering of) a given element. The definition of each property states to what set of elements it applies.

property

A parameter that helps specify how a document should be rendered. A complete list of SVG's properties can be found in Property Index. Properties are assigned to elements in the SVG language either by presentation attributes on elements in the SVG language or by using a styling language such as CSS [CSS21]. See Styling.

structural element

The structural elements are those which define the primary structure of an SVG document. Specifically, the following elements are structural elements: ‘defs’, ‘g’, ‘svg’, ‘symbol’ and ‘use’.

structurally external element

Elements that define its structure by reference to an external resource. Specifically, the following elements are structurally external elements when they have an ‘xlink:href’ attribute: ‘audio’, ‘foreignObject’, ‘iframe’, ‘image’, ‘script’, ‘use’ and ‘video’.

SVG user agent

An SVG user agent is a user agent that is able to retrieve and render SVG content.

transformation

A modification of the current transformation matrix by providing a supplemental transformation in the form of a set of simple transformations specifications (such as scaling, rotation or translation) and/or one or more transformation matrices. See Coordinate system transformations.

transformation matrix

Transformation matrices define the mathematical mapping from one coordinate system into another using a 3x3 matrix using the equation [x' y' 1] = [x y 1] * matrix. See current transformation matrix and Coordinate system transformations.

URL reference

An URL reference is an Internationalized Resource Identifier with an optional fragment identifier, as defined in Internationalized Resource Identifiers [RFC3987]. An URL reference serves as a reference to a resource or (with a fragment identifier) to a secondary resource. See References and References and the ‘defs’ element.

user agent

The general definition of a user agent is an application that retrieves and renders Web content, including text, graphics, sounds, video, images, and other content types. A user agent may require additional user agents that handle some types of content. For instance, a browser may run a separate program or plug-in to render sound or video. User agents include graphical desktop browsers, multimedia players, text browsers, voice browsers, and assistive technologies such as screen readers, screen magnifiers, speech synthesizers, onscreen keyboards, and voice input software.

A "user agent" may or may not have the ability to retrieve and render SVG content; however, an "SVG user agent" retrieves and renders SVG content.

user space

user coordinate system

In general, a coordinate system defines locations and distances on the current canvas. The current user coordinate system is the coordinate system that is currently active and which is used to define how coordinates and lengths are located and computed, respectively, on the current canvas. See initial user coordinate system and Coordinate system transformations.

user units

A coordinate value or length expressed in user units represents a coordinate value or length in the current user coordinate system. Thus, 10 user units represents a length of 10 units in the current user coordinate system. If not specified otherwise, one user unit is also equal to one CSS pixel in current user coordinate system.

viewport

A rectangular region within the current canvas onto which graphics elements are to be rendered. See the discussion of the SVG viewport in the chapter on Coordinate Systems, Transformations and Units.

viewport space

viewport coordinate system

In general, a coordinate system defines locations and distances on the current canvas. The viewport coordinate system is the coordinate system that is active at the start of processing of an ‘svg’ element, before processing the optional ‘viewBox’ attribute. In the case of an SVG document fragment that is embedded within a parent document which uses CSS to manage its layout, then the viewport coordinate system will have the same orientation and lengths as in CSS, with the origin at the top-left on the viewport. See The initial viewport and Establishing a new viewport.

viewport units

A coordinate value or length expressed in viewport units represents a coordinate value or length in the viewport coordinate system. Thus, 10 viewport units represents a length of 10 units in the viewport coordinate system.

People might mix this up with the CSS viewport which is relevant in SVG as well. We should be explicit here.

white space characters

The following characters are considered white space characters: U+0009 CHARACTER TABULATION, U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), U+000A LINE FEED (LF) and U+0020 SPACE.

XLink attributes

The XLink attributes are two of the seven attributes defined in the XML Linking Language specification [XLINK], which are used on various SVG elements that can reference resources. The most important XLink attribute is ‘xlink:href’, whose definition can be found on each element that allows it. ‘xlink:title’ is also allowed on any element that supports the ‘xlink:href’ attribute. The remaining five XLink attributes – ‘xlink:type’, ‘xlink:role’, ‘xlink:arcrole’, ‘xlink:show’ and ‘xlink:actuate’ – are not used in SVG.