July 18, 2010

Scalable Vector Graphics (SVG)

Scalable Vector Graphics (SVG) is a family of specifications of XML-based file format for describing two-dimensional vector graphics, both static and dynamic (interactive or animated).
The SVG specification is an open standard that has been under development by the World Wide Web Consortium (W3C) since 1999. SVG images and their behaviours are defined in XML text files. This means that they can be searched, indexed, scripted and, if required, compressed. SVG is also well-suited to small and mobile devices. The SVG Basic and SVG Tiny specifications were developed with just such uses in mind and many current mobile devices support them. Since they are XML files, SVG images can be edited with any text editor, but specialized SVG-based drawing programs are also available.

All major modern web browsers except Microsoft Internet Explorer support and render SVG markup directly. To view SVG files in Internet Explorer, users have to download and install a browser plugin.

SVG has been in development since 1999 by a group of companies within the W3C after the competing standards PGML (developed from Adobe's PostScript) and VML (developed from Microsoft's RTF) were submitted to W3C in 1998. SVG drew on experience designing both those formats.
SVG allows three types of graphic objects:

  • Vector graphics
  • Raster graphics
  • Text

Graphical objects can be grouped, styled, transformed, and composited into previously rendered objects. SVG does not directly support z-indices that separate drawing order from document order for objects, which is different from other vector markup languages like VML. Text can be in any XML namespace suitable to the application, which enhances searchability and accessibility of the SVG graphics. The feature set includes nested transformations, clipping paths, alpha masks, filter effects, template objects and extensibility.
Since 2001, the SVG specification has been updated to version 1.1 (current Recommendation) and 1.2 (still a Working Draft). The SVG Mobile Recommendation introduced two simplified profiles of SVG 1.1, SVG Basic and SVG Tiny, meant for devices with reduced computational and display capabilities. SVG Tiny later became an autonomous Recommendation (current version 1.2) and the basis for SVG 1.2. In addition to these variants and profiles, the SVG Print specification (still a Working Draft) contains guidelines for printable SVG 1.2 and SVG Tiny 1.2 documents.

While being primarily designated as a vector graphics markup language, the specification is also designed with the basic capabilities of a page description language, like Adobe's PDF. It contains provisions for rich graphics, and is also compatible with the CSS specification's properties for styling purposes; thus, unlike XHTML and XSL-FO which are layout-oriented languages, SVG is a fully presentational language.  A much more print-specialized subset of SVG (SVG Print, authored by Canon, HP, Adobe and Corel) is currently a W3C Working Draft.

SVG images, being XML, contain many repeated fragments of text and are thus particularly suited to compression by gzip, though other compression methods may be used effectively. Once an SVG image has been compressed by gzip it may be referred to as an "SVGZ" image; with the corresponding filename extension. The resulting file may be as small as 20% of the original size.

Development history
SVG was developed by the W3C SVG Working Group starting in 1998, after Macromedia and Microsoft introduced Vector Markup Language (VML) whereas Adobe Systems and Sun Microsystems submitted a competing format known as PGML. The working group was chaired by Chris Lilley of the W3C.

  • SVG 1.0 became a W3C Recommendation on September 4, 2001.

  • SVG 1.1 became a W3C Recommendation on January 14, 2003.  The SVG 1.1 specification is modularized in order to allow subsets to be defined as profiles. Apart from this, there is very little difference between SVG 1.1 and SVG 1.0.

  • SVG Tiny and SVG Basic (the Mobile SVG Profiles) became W3C Recommendations on January 14, 2003. These are described as profiles of SVG 1.1.

  •  SVG Tiny 1.2 became a W3C Recommendation on December 22, 2008.

  • SVG Full 1.2 is a W3C Working Draft. SVG Tiny 1.2 was initially released as a profile, and later refactored to be a complete specification, including all needed parts of SVG 1.1 and SVG 1.2. SVG 1.2 Full adds modules onto the SVGT 1.2 core.

  • SVG Print adds syntax for multi-page documents and mandatory color management support.
Mobile profiles
Because of industry demand, two mobile profiles were introduced with SVG 1.1: SVG Tiny (SVGT) and SVG Basic (SVGB). These are subsets of the full SVG standard, mainly intended for user agents with limited capabilities. In particular, SVG Tiny was defined for highly restricted mobile devices such as cellphones, and SVG Basic was defined for higher-level mobile devices, such as PDAs.

In 2003, the 3GPP adopted SVG Tiny as the mandatory vector graphics media format for next-generation phones. SVGT is the required vector graphics format and support of SVGB is optional for MMS (Multimedia Messaging Service) and PSS (Packet-Switched Streaming Service).  It was later added as required format for vector graphics in 3GPP IP Multimedia Subsystem (IMS).

Neither mobile profile includes support for the full DOM, while only SVG Basic has optional support for scripting, but because they are fully compatible subsets of the full standard most SVG graphics can still be rendered by devices which only support the mobile profiles.

SVGT 1.2 adds a microDOM (μDOM), allowing all mobile needs to be met with a single profile.

The SVG 1.1 specification defines 14 important functional areas or feature sets:

Simple or compound shape outlines drawn with curved or straight lines can be filled in or outlined (or used as a clipping path) and are expressed in a highly compact coding in which, for example, M (from 'move to') precedes the initial numeric X and Y coordinates and L (line to) will precede a subsequent point to which a line should be drawn. Further command letters (C, S, Q, T and A) precede data that is used to draw various Bézier and elliptical curves. Z is used to close a path. In all cases, absolute coordinates follow capital letter commands and relative coordinates are used after the equivalent lower-case letters.

Basic Shapes
Straight-line paths or paths made up of a series of connected straight-line segments (polylines), as well as closed polygons, circles and ellipses can be drawn. Rectangles and round-cornered "rectangles" are other standard elements.

Unicode character text included in an SVG file is expressed as XML character data. Many visual effects are possible, and the SVG specification automatically handles bidirectional text (as when composing a combination of English and Arabic text, for example), vertical text (as Chinese was historically written) and characters along a curved path (such as the text around the edges of the Great Seal of the United States).

SVG shapes can be filled and/or outlined (painted with a color, a gradient or a pattern). Fills can be opaque or have various degrees of transparency. "Markers" are end-of-line features, such as arrowheads, or symbols which can appear at the vertices of a polygon.

Colors can be applied to all visible SVG elements, either directly or via the 'fill', 'stroke' and other properties. Colors are specified in the same way as in CSS2, i.e. using names like black or blue, in hexadecimal such as #2f0 or #22ff00, in decimal like rgb(255,255,127) or as percentages of the form rgb(100%,100%,50%).

Gradients and Patterns
SVG shapes can be filled or outlined with solid colors as above, or with color gradients or with repeating patterns. Color gradients can be linear or radial (circular), and can involve any number of colors as well as repeats. Opacity gradients can also be specified. Patterns are based on predefined raster or vector graphic objects, which can be repeated in x and/or y directions. Gradients and patterns can be animated and scripted.

Since 2008, there has been discussion among professional users of SVG that either gradient meshes or preferably diffusion curves could usefully be added to the SVG specification. It is said that a "simple representation [using diffusion curves] is capable of representing even very subtle shading effects" and that "Diffusion curve images are comparable both in quality and coding efficiency with gradient meshes, but are simpler to create (according to several artists who have used both tools), and can be captured from bitmaps fully automatically."

Clipping, Masking and Compositing
Graphic elements, including text, paths, basic shapes and combinations of these, can be used as outlines to define both 'inside' and 'outside' regions that can be painted (with colors, gradients and patterns) independently. Fully opaque clipping paths and semi-transparent masks are composited together to calculate the color and opacity of every pixel of the final image, using simple alpha blending.

SVG images can interact with users in many ways. In addition to hyperlinks as mentioned below, any part of an SVG image can be made to trigger events representing changes in focus, mouse clicks, scrolling or zooming the image and other pointer, keyboard and document events. Event handlers may start, stop or alter animations and trigger any other scripts in response to these events.

All aspects of an SVG document can be accessed and manipulated using scripts in a similar way to HTML. The default scripting language is ECMAScript (closely related to JavaScript) and there are defined Document Object Model (DOM) objects for every SVG element and attribute. Scripts are enclosed in

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