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Graphics have traditionally just been links which happen to have a picture file at the end rather than another piece of text. They can therefore be implemented in any way supported by the XLink and XPointer specifications (see How will XML affect my document links?), including using similar syntax to existing HTML images. They can also be referenced using XML's built-in NOTATION and ENTITY mechanism in a similar way to standard SGML, as external unparsed entities.
However, Scalable Vector Graphics (an XML specification for vector graphics) lets you use XML markup to draw vector graphics objects directly in your XML file. This provides enormous power for the inclusion of portable graphics, especially interactive or animated sequences, and it is now becoming supported in browsers, and can be exported from standard graphics (drawing) programs like GIMP.
The XML linking specifications for external images give you much better control over the traversal and activation of links, so an author can specify, for example, whether or not to have an image appear when the page is loaded, or on a click from the user, or in a separate window, without having to resort to scripting.
XML itself doesn't predicate or restrict graphic file formats: GIF, JPG, TIFF, PNG, CGM, EPS, and SVG at a minimum would seem to make sense; however, vector formats (EPS, SVG) are normally essential for non-photographic images (diagrams).
Embedded binary graphics
You cannot embed a raw binary graphics file (or any other binary [non-text] data) directly into an XML file because any bytes happening to resemble markup would get misinterpreted: you must refer to it by linking (see below). It is, however, possible to include a text-encoded transformation of a binary file as a CDATA Marked Section, using something like UUencode with the markup characters ], & and > removed from the map so that they could not occur as an erroneous CDATA termination sequence and be misinterpreted. You could even use simple hexadecimal encoding as used in PostScript. For vector graphics, however, the solution is to use SVG (see Peter Murray-Rust’s tip).
Sound files are binary objects in the same way that external graphics are, so they can only be referenced externally (using the same techniques as for graphics). Music files written in MusiXML or an XML variant of SMDL could however be embedded in the same way as for SVG.
The point about using entities to manage your graphics is that you can keep the list of entity declarations separate from the rest of the document, so you can re-use the names if an image is needed more than once, but only store the physical file specification in a single place. External entities are available only when using a DTD, not a Schema.
Bob DuCharme writes:
All the data in an XML document entity must be parsable XML. You can define an external entity as either a parsed entity (parsable XML) or an unparsed entity (anything else). Unparsed entities can be used for picture files, sound files, movie files, or whatever you like. They can only be referenced from within a document as the value of an attribute (much like a bitmap picture on an HTML Web page is the value of the img element's src attribute) and not part of the actual document. In an XML document, this attribute must be declared to be of type ENTITY, and the entity's declaration must specify a declared NOTATION, because if the entity isn't XML, the XML processor needs to know what it is. For example, in the following document, the colliepic entity is declared to have a JPEG notation, and it's used as the value of the empty dog element's picfile attribute.<?xml version="1.0"?> <!DOCTYPE dog [ <!NOTATION JPEG SYSTEM "Joint Photographic Experts Group"> <!ENTITY colliepic SYSTEM "lassie.jpg" NDATA JPEG> <!ELEMENT dog EMPTY> <!ATTLIST dog picfile ENTITY #REQUIRED> ]> <dog picfile="colliepic"/>
The Entity method is particularly useful when you have many images, or many repeated uses of the same images, because you only declare them once, at the top of the document, making image management much easier.
The XLink and XPointer linking specifications describe other ways to point to a non-XML file such as a graphic. These offer more sophisticated control over the external entity's position, handling, and appearance within the XML document.
Peter Murray-Rust writes:
GIFs and JPEGs cater for bitmaps (pixel representations of images: all made up of coloured dots). Vector graphics (scalable, made up of drawing specifications) are addressed in the W3C's graphics activity as Scalable Vector Graphics (see http://www.w3.org/Graphics/SVG). With the specification now complete, it is possible to transmit the graphical representation as vectors directly within the XML file. For many graphics objects this will mean greatly decreased download time and scaling without loss of detail.
Max Dunn writes:
SVG has really taken off recently, and is quite an XML success story […] there are [many] conformant implementations. We recently started an SVG FAQ at http://www.svgfaq.com/.
XSLT can be used to generate SVG from XML; details are at http://www.svgfaq.com/xslt.asp (be careful to use XSLT, not Microsoft's obsolete WD-xsl). Documents can also interact with SVG images (see http://www.xml.com/pub/a/2000/03/22/style/index.html).