How do I use graphics in XML?

Interview Questions

XML Interview Questions and Answers

45. How do I use graphics in XML?

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 question C.18, ‘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, the SVG specification (see the tip below, by Peter Murray-Rust) 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 slowly becoming supported in browsers.
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).
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 the tip below, by Peter Murray-Rust).
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. This is available only when using a DTD, not a Schema.

(Continued on next question...)