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Subject: Suggestion for graphic scaling improvements in DITA 1.1

I spoke to Don, and he suggested I send a specific
suggestion for a DITA 1.1 work item to the list
in the area of graphic scaling.

DITA's support for graphic sizing--which just allows
a number of pixels to be specified for the graphic
image size via the image element's height and width
attributes--does not meet our customer requirements. 
I suggest we make some upward compatible changes to
address at least a subset of these requirements.


Current situation
The DITA spec is vague on the topic of what are valid
values for the image element's height and width
attributes and what are the semantics of these values.

In checking both the toolkit and the collective IBM
knowledge, I understand that, for HTML generation, the 
image height and width values are passed through as-is 
implying that DITA's image's height and width attributes 
must be unitless integers giving the number of pixels to 
which to scale the graphic image (in each direction) in
the browser displayed results.  For IBM PDF processing,
the pixel values are converted to points (assuming 96 pixels
per inch and 72 points per inch), and that value is used.

Requirements for graphic scaling

1. It should be possible for users to specify a percentage
   value to which to scale the intrinsic size of the image.

2. It should be possible for users to specify an absolute size
   for the image height and/or width using a numeric value and
   unit of measure.  Allowable units should include: pc (pica),
   pt (point), px (pixels), in (inches), cm (centimeters), 
   mm (millimeters), and em (ems) as allowed by CSS and XSL-FO.

3. To aid in reuse and minimize hardwiring style to the source,
   it may be a requirement to allow a user to request that an
   image be scaled to some percentage of the "available space".

One specific proposal

1.  Add the "scale" attribute to the "image" element, and
    change the declared type of the scale attribute to NMTOKEN.
    Define the allowable value space of the scale attribute
    to be any unsigned integer.  This would be an upward
    compatible change.

2.  No change needs to be made to the image element's height
    and width DTD declarations, as NMTOKEN would allow all
    values we wish to allow.  We would just augment the spec
    to say that the value space of these attributes is a
    length which is a real number with an optional unit
    from the set of pc, pt, px, in, cm, mm, em.  An omitted
    unit implies pixels so that existing documents will
    continue to be treated the same way. 

3.  This requirement could be met by allowing image element's
    height and width attributes to take % values along with
    some work to define what the "available space" is.  The
    specific details of a suggested solution depend on the
    exact user requirements which I'm still researching (and
    would appreciate input from other users).


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