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Referencing Binary Entities

This section contains no programming exercises. Instead, it discusses the options for referencing binary files like image files and multimedia data files.

Using a MIME Data Type

There are two ways to go about referencing an unparsed entity like a binary image file. One is to use the DTD's NOTATION-specification mechanism. However, that mechanism is a complex, non-intuitive holdover that mostly exists for compatibility with SGML documents. We will have occasion to discuss it in a bit more depth when we look at the DTDHandler API, but suffice it for now to say that the combination of the recently defined XML namespaces standard, in conjunction with the MIME data types defined for electronic messaging attachments, together provide a much more useful, understandable, and extensible mechanism for referencing unparsed external entities.

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Note: The XML described here is in slideshow1b.dtd. We won't actually be echoing any images. That's beyond the scope of this tutorial's Echo program. This section is simply for understanding how such references can be made. It assumes that the application which will be processing the XML data knows how to handle such references.

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To set up the slideshow to use image files, add the text highlighted below to your slideshow.dtd file:

<!ELEMENT slide (image?, title, item*)>	
<!ATTLIST slide 	
      type   (tech | exec | all) #IMPLIED	
>	
<!ELEMENT title (#PCDATA)>	
<!ELEMENT item (#PCDATA | item)* >	
<!ELEMENT image EMPTY>	
<!ATTLIST image 	
      alt    CDATA    #IMPLIED	
      src    CDATA    #REQUIRED	
      type   CDATA    "image/gif"	
>
 

These modifications declare image as an optional element in a slide, define it as empty element, and define the attributes it requires. The image tag is patterned after the HTML 4.0 tag, img, with the addition of an image-type specifier, type. (The img tag is defined in the HTML 4.0 Specification.)

The image tag's attributes are defined by the ATTLIST entry. The alt attribute, which defines alternate text to display in case the image can't be found, accepts character data (CDATA). It has an "implied" value, which means that it is optional, and that the program processing the data knows enough to substitute something like "Image not found". On the other hand, the src attribute, which names the image to display, is required.

The type attribute is intended for the specification of a MIME data type, as defined at ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/. It has a default value: image/gif.

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Note: It is understood here that the character data (CDATA) used for the type attribute will be one of the MIME data types. The two most common formats are: image/gif, and image/jpeg. Given that fact, it might be nice to specify an attribute list here, using something like: type ("image/gif", "image/jpeg") That won't work, however, because attribute lists are restricted to name tokens. The forward slash isn't part of the valid set of name-token characters, so this declaration fails. Besides that, creating an attribute list in the DTD would limit the valid MIME types to those defined today. Leaving it as CDATA leaves things more open ended, so that the declaration will continue to be valid as additional types are defined.

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In the document, a reference to an image named "intro-pic" might look something like this:

<image src="image/intro-pic.gif", alt="Intro Pic", 
type="image/gif" />
 

The Alternative: Using Entity References

Using a MIME data type as an attribute of an element is a mechanism that is flexible and expandable. To create an external ENTITY reference using the notation mechanism, you need DTD NOTATION elements for jpeg and gif data. Those can of course be obtained from some central repository. But then you need to define a different ENTITY element for each image you intend to reference! In other words, adding a new image to your document always requires both a new entity definition in the DTD and a reference to it in the document. Given the anticipated ubiquity of the HTML 4.0 specification, the newer standard is to use the MIME data types and a declaration like image, which assumes the application knows how to process such elements.

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