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14 January 2003

1 Introduction


Contents


 

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 in XML [XML10]. 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 XHTML 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.

1.1.1 Modularization

The modularization of SVG included here is a decomposition of SVG 1.0 and errata into a collection of abstract modules that provide specific units of functionality. These modules may be combined with each other and with modules defined in other specifications (such as XHTML) to create SVG subset and extension document types that qualify as members of the SVG family of document types. See Conformance for a description of SVG family documents, and [XHTMLplusMathMLplusSVG] for a profile that combines XHTML, MathML and SVG.

Each major section of the SVG specification produces a module named after that section, e.g. "Text Module" or "Basic Structure Module". A module without the "Basic" prefix implies that the module includes the complete set of elements and attributes, with no restrictions, from the corresponding section of the specification. If there is a need to provide a subset of the functionality of the complete module, then a Basic module is created with the "Basic" prefix added to the name of the complete module. For example, the "Basic Text Module" is a subset of the "Text Module".

It is an error for a profile of SVG 1.1 to include both the complete module and its basic subset module (e.g. the "Text Module" and the "Basic Text Module").

1.1.2 Element and Attribute collections

Most modules define a named collection of elements or attributes. These collections are used as a shorthand when describing the set of attributes allowed on a particular element (eg. the "Style" attibute collection) or the set of elements allowed as children of a particular element (eg. the "Shape" element collection). All collections have names that begin with an uppercase character.

When defining a profile, it is assumed that all the element and attribute collections are defined to be empty. That way, a module can redefine the collection as it is included in the profile, adding elements or attributes to make them available within the profile. Therefore, it is not a mistake to refer to an element or attribute collection from a module that is not included in the profile, it simply means that collection is empty.

The exception to this is the collection Presentation.attrib, which is the union of all the presentation attribute collections (i.e. all the attribute collections with the string "Presentation" in their name). Presentation.attrib is not defined in any module, but it exists in every profile.

A subset module (ie. a Basic module) may define a different named collection from a superset module. Since it is an error to include a subset and superset module of the same group in a profile, all attribute and element collections will either be defined once by the module that includes them, or will have their default empty value (again, with the exception of Presentation.attrib which is not defined by any module).

1.1.3 Profiling the SVG specification

The modularization of SVG 1.1 allows profiles to be described by listing the SVG modules they allow and possibly a small number of restrictions or extensions on the elements provided by those modules.

The "Full" profile of SVG 1.1 is the collection of all the complete modules listed in this specification (ie. every module that is not a subset module).

When applied to conformance, the unqualified term "SVG" implies the "Full" profile of SVG 1.1 defined by this specification. If an implementation does not implement the Full profile, it must state either the profile to which it conforms, or that it implements a subset of SVG.

1.2 SVG MIME type, file name extension and Macintosh file type

The MIME type for SVG is "image/svg+xml" (see [RFC3023]). The registration of this MIME type is in progress at the W3C.

It is recommended that SVG files have the extension ".svg" (all lowercase) on all platforms. It is recommended that gzip-compressed SVG files have the extension ".svgz" (all lowercase) on all platforms.

It is recommended that SVG files stored on Macintosh HFS file systems be given a file type of "svg " (all lowercase, with a space character as the fourth letter). It is recommended that gzip-compressed SVG files stored on Macintosh HFS file systems be given a file type of "svgz" (all lowercase).

1.3 SVG Namespace, Public Identifier and System Identifier

The following are the SVG 1.1 namespace, public identifier and system identifier:

SVG Namespace:
http://www.w3.org/2000/svg
Public Identifier for SVG 1.1:
PUBLIC "-//W3C//DTD SVG 1.1//EN"
System Identifier for the SVG 1.1 Recommendation:
http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd

The following is an example document type declaration for an SVG document:

<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
         "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

Note that DTD listed in the System Identifier is a modularized DTD (ie. its contents are spread over multiple files), which means that a validator may have to fetch the multiple modules in order to validate. For that reason, there is a single flattened DTD available that corresponds to the SVG 1.1 modularized DTD. It can be found at http://www.w3.org/Graphics/SVG/1.1/DTD/svg11-flat.dtd.

1.4 Compatibility with Other Standards Efforts

SVG leverages and integrates with other W3C specifications and standards efforts. By leveraging and conforming to other standards, SVG becomes more powerful and makes it easier for users to learn how to incorporate SVG into their Web sites.

The following describes some of the ways in which SVG maintains compatibility with, leverages and integrates with other W3C efforts:

In environments which support [DOM2] for other XML grammars (e.g., XHTML [XHTML]) and which also support SVG and the SVG DOM, a single scripting approach can be used simultaneously for both XML documents and SVG graphics, in which case interactive and dynamic effects will be possible on multiple XML namespaces using the same set of scripts.


1.5 Terminology

Within this specification, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 (see [RFC2119]). However, for readability, these words do not appear in all uppercase letters in this specification.

At times, this specification recommends good practice for authors and user agents. These recommendations are not normative and conformance with this specification does not depend on their realization. These recommendations contain the expression "We recommend ...", "This specification recommends ...", or some similar wording.


1.6 Definitions

basic shape
Standard shapes which are predefined in SVG as a convenience for common graphical operations. Specifically: 'rect', 'circle', 'ellipse', 'line', 'polyline', 'polygon'.
canvas
A surface onto which graphics elements are drawn, which can be real physical media such as a display or paper or an abstract surface such as a allocated region of computer memory. See the discussion of the SVG canvas in the chapter on Coordinate Systems, Transformations and Units.
clipping path
A combination of 'path', 'text' and basic shapes which serve as the outline of a (in the absence of anti-aliasing) 1-bit mask, where everything on the "inside" of the outline is allowed to show through but everything on the outside is masked out. See Clipping paths.
container element
An element which can have graphics elements and other container elements as child elements. Specifically: 'svg', 'g', 'defs' 'symbol', 'clipPath', 'mask', 'pattern', 'marker', 'a' and 'switch'.
current innermost SVG document fragment
The XML document sub-tree which starts with the most immediate ancestor 'svg' element of a given SVG element.
current SVG document fragment
The XML document sub-tree which starts with the outermost ancestor 'svg' element of a given SVG element, with the requirement that all container elements between the outermost 'svg' and this element are all elements in the SVG language.
current transformation matrix (CTM)
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. The current transformation matrix (CTM) defines the mapping from the user coordinate system into the viewport coordinate system. See Coordinate system transformations.
fill
The operation of painting the interior of a shape or the interior of the character glyphs in a text string.
font
A font represents an organized collection of glyphs in which the various glyph representations will share a common look or styling such that, when a string of characters is rendered together, the result is highly legible, conveys a particular artistic style and provides consistent inter-character alignment and spacing.
glyph
A glyph represents a unit of rendered content within a font. Often, there is a one-to-one correspondence between characters to be drawn and corresponding glyphs (e.g., often, the character "A" is rendered using a single glyph), but other times multiple glyphs are used to render a single character (e.g., use of accents) or a single glyph can be used to render multiple characters (e.g., ligatures). Typically, a glyph is defined by one or more shapes such as a path, possibly with additional information such as rendering hints that help a font engine to produce legible text in small sizes.
graphics element
One of the element types that can cause graphics to be drawn onto the target canvas. Specifically: 'path', 'text', 'rect', 'circle', 'ellipse', 'line', 'polyline', 'polygon', 'image' and 'use'.
graphics referencing element
A graphics element which uses a reference to a different document or element as the source of its graphical content. Specifically: 'use' and 'image'.
local URI reference
A Uniform Resource Identifier [URI] that does not include an <absoluteURI> or <relativeURI> and thus represents a reference to an element within the current document. See References and the 'defs' element.
mask
A container element which can contain graphics elements or other container elements which define a set of graphics that is to be used as a semi-transparent mask for compositing foreground objects into the current background. See Masks.
non-local URI reference
A Uniform Resource Identifier [URI] that includes an <absoluteURI> or <relativeURI> and thus (usually) represents a reference to a different document or an element within a different document. See References and the 'defs' element.
paint
A paint represents a way of putting color values onto the canvas. A paint might consist of both color values and associated alpha values which control the blending of colors against already existing color values on the canvas. SVG supports three types of built-in paint: color, gradients and patterns.
presentation attribute
An XML attribute on an SVG element which specifies a value for a given property for that element. See Styling.
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 [CSS2]. See Styling.
shape
A graphics element that is defined by some combination of straight lines and curves. Specifically: 'path', 'rect', 'circle', 'ellipse', 'line', 'polyline', 'polygon'.
stroke
The operation of painting the outline of a shape or the outline of character glyphs in a text string.
SVG canvas
The canvas onto which the SVG content is rendered. See the discussion of the SVG canvas in the chapter on Coordinate Systems, Transformations and Units.
SVG document fragment
The XML document sub-tree which starts with an 'svg' element. An SVG document fragment can consist of a stand-alone SVG document, or a fragment of a parent XML document enclosed by an 'svg' element. When an 'svg' element is a descendant of another 'svg' element, there are two SVG document fragments, one for each 'svg' element. (One SVG document fragment is contained within another SVG document fragment.)
SVG viewport
The viewport within the SVG canvas which defines the rectangular region into which SVG content is rendered. See the discussion of the SVG viewport in the chapter on Coordinate Systems, Transformations and Units.
text content element
One of SVG's elements that can define a text string that is to be rendered onto the canvas. SVG's text content elements are the following: 'text', 'tspan', 'tref', 'textPath' and 'altGlyph'.
transformation
A modification of the current transformation matrix (CTM) 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 (CTM) and Coordinate system transformations.
URI Reference
A Uniform Resource Identifier [URI] which serves as a reference to a file or to an element within a file. See 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 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 space
A synonym for user coordinate system.
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.
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 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 space
A synonym for viewport coordinate system.
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.

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