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Defs and forward references

From: Russ Shotts <rshotts@jasc.com>
Date: Tue, 14 Mar 2000 15:44:54 -0600
Message-ID: <E580CB8FBC72D211A94A00A0C9B57292503971@EXCHANGE_SERVER>
To: "'www-svg@w3.org'" <www-svg@w3.org>

The latest spec removes the restriction on forward references and the
requirement that all referenced elements be defined in a 'defs'.  I will
admit that I dislike these changes due to the additional complexity they
introduce.  The spec does not give examples to justify the changes; it
simply states that the changes were made for compatibility with
SMIL/Animation.

I cannot locate details or examples which define the expected behavior.
Would you please clarify the following:

1. Section 6.3.4 states that graphics elements within a 'defs' will not be
drawn.  While the spec does not state, I would expect some items defined
outside a 'defs' to not be drawn.  Specifically, it would seem that items
which are always referenced (e.g., 'linearGradient', a 'pattern', a
'symbol', or a 'marker') but are not definied in a 'defs' should not be
drawn.

2. When (or how) are references to be resolved?  When an individual shape is
read?  Once all shapes are read?

3. What if multiple gradients (or patterns) use the same id?  The spec does
not indicate whether the reference resolves to the first definition, the
most recent, or the last.  

4. What if one of the definitions is before the referencing shape and one is
after?

5. What if the multiple definitions are spread across this group and its
ancestors?  And if they are all forward references?

6. What happens when a gradient and a shape each use the same id?  Is there
a single namespace for all ids, or separate namespaces for shapes and fills?

The previous version of the spec had a simpler model, yet the Adobe plug-in
and the IBM viewer had two very different implementations.  If two gradients
were defined with the same id, the plug-in uses the first definition while
the viewer uses the most recent.

Things get far more complex with the above changes.  For example, consider
the following case:


<g>
  <ellipse style="fill:url(#id01)" cx="100" cy="50" rx="75" ry="20" />

  <linearGradient id="id01".../>
    <stop offset="0%" style="stop-color:red"/>
    <stop offset="100%" style="stop-color:blue" />
  </linearGradient>

  <circle style="fill:url(#id01)" cx="100" cy="175" r="75" />

  <linearGradient id="id01".../>
    <stop offset="0%" style="stop-color:blue"/>
    <stop offset="100%" style="stop-color:green" />
  </linearGradient>

  <rect style="fill:url(#id01)" x="20" y="300" width="400" height="50" />

  <linearGradient id="id01".../>
    <stop offset="0%" style="stop-color:green"/>
    <stop offset="100%" style="stop-color:red" />
  </linearGradient>

  <polygon style="fill:url(#id01)" points="0,500,200,500,100,400" />

  <linearGradient id="id01".../>
    <stop offset="0%" style="stop-color:black"/>
    <stop offset="100%" style="stop-color:blue" />
  </linearGradient>

</g>

What gradient is used for each shape?


Russ Shotts
Senior Software Engineer
Jasc Software, Inc.
Received on Tuesday, 14 March 2000 16:46:02 GMT

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