Image Library Files

Norman Walsh

17 Nov 1998

Additional information about graphics files can be used to improve the presentation of images. In particular, passing the height and width of a graphic to a web browser improves layout and alternative text should always be used. Image library files provide a mechanism for passing this information to the DocBook DSSSL Stylesheets.

Image library files can be used to pass additional information about images to the stylesheet. The HTML stylesheet uses this information in the IMG tag; the print stylesheet may someday be able to use this information to get proper spacing around images, but I don't quite have that worked out yet.

Image Library File Format

Image library files are XML documents conforming the image library DTD shown in .

Figure 0. The Image Library DTD

<!-- $Id: imagelib.dtd,v 1.1 2001/04/02 21:40:27 nwalsh Exp $

     DTD for the image-library files used by the Modular DocBook
     Stylesheets.

     Common declaration:
   
     <?xml version='1.0'?>
     <!DOCTYPE image-library 
               PUBLIC "-//Norman Walsh//DTD Image Library 1.0//EN" "">

     Instances of this DTD are usually XML.
-->

<!-- An image-library is a collection of images -->
<!ELEMENT image-library	(image+)>

<!-- Each image has some number of properties and may have a short -->
<!-- text description and a long test description -->
<!ELEMENT image		(properties?, alttext?, altdesc?)>

<!-- The stylesheets resolve the requested graphic to a filename -->
<!-- and search for an image with that filename in the image -->
<!-- library. Images may refer to each other with 'id' and -->
<!-- 'sameas' attributes. -->
<!ATTLIST image
	filename	CDATA	#REQUIRED
	id		ID	#IMPLIED
	sameas		IDREFS	#IMPLIED
>

<!-- A few image properties are universal, the rest are stored -->
<!-- in HTML-style 'meta' elements. -->
<!ELEMENT properties	(meta*)>
<!ATTLIST properties
	format		(gif|jpeg|png|other)	#IMPLIED
	otherformat	CDATA	#IMPLIED
	width		CDATA	#IMPLIED
	height		CDATA	#IMPLIED
>

<!-- Any additional properties that specify 'imgattr="yes"', -->
<!-- will automatically be copied to the attribute list of -->
<!-- the HTML IMG tag. -->
<!ELEMENT meta EMPTY>
<!ATTLIST meta
	name		CDATA	#REQUIRED
	content		CDATA	#REQUIRED
	imgattr		(yes|no)	"no"
>

<!-- The content of this element will be used as the HTML -->
<!-- 'ALT' text, unless the alt text can be found in the -->
<!-- instance. -->
<!ELEMENT alttext	(#PCDATA)>

<!-- The 'altdesc' is a longer description of the image. -->
<!-- It is not used at this time. -->
<!ELEMENT altdesc	(para+)>
<!ELEMENT para		(#PCDATA)>

<!--EOF-->

The stylesheets use the filename of the graphic file (either the name entered in the fileref attribute or the system identifier of the entity in the entityref attribute) to locate the appropriate image in the library.

Creating Image Library Files

At present, the files have to be created by hand. It's easy to imagine a tool that could scan the graphics and build the file (or at least part of the file) automatically, but none have been written yet.

Image Library File Location

The default location for image library files is imagelib/imagelib.xml. Using a subdirectory allows the image library file to be in XML while the documents may be in SGML. If you're formatting XML documents, you can move the image library into the same directory as your documents.

The Catalog Trick

If you're formatting SGML documents, you have to employ a catalog trick in order to get Jade to properly parse the image library file. Add a catalog to the imagelib directory that contains the single line:

SGMLDECL "path/to/xml.dcl"

Jade will use that declaration to process all the documents in the imagelib directory.

Using the Image Library

You can use the image library in one of two ways:

Turn on image-library in your stylesheet:
```
(define image-library #t)
```
Use the -V image-library option when you run jade.

An Example

See the examples in the test directory.