Re: [cgmo-webcgm] Issue CL-d3: 2.2.2 Drawing model

[Lofton Henderson <lofton@rockynet.com>]

At 05:08 PM 3/23/2006 -0500, Benoit Bezaire wrote:
>Link: 
>http://www.oasis-open.org/apps/org/workgroup/cgmo-webcgm/download.php/17342/CL-comments.html#CL-d3
>
>Doh! The short version didn't work eh ;)
>
>I can take this one if nobody wants it.

Thanks!

(Btw, I have the impression that Forrest and Don have more interest in 
colour and compositing, with their petro application backgrounds.  I'd like 
to see their feedback on these questions.)

>Lofton, you state that WebCGM is pretty unspecified here... what about
>ISO CGM, what _does it_ specify? I wouldn't mind a couple of pointers
>in the ISO CGM spec as a starting point:
>
>Section 2.5.3 Color and transparency of Webcgm 2.0 states:
>- "The normal behavior of CGM:1999 viewers is to render later occurring
>primitives completely opaquely on top of earlier primitives." Where is
>this said in CGM:1999?

Amazingly, I can't find the explicit statement.  ***ANYONE*** ... can you 
provide a reference to a precise statement in CGM:1999?

("6.11 State model" certainly implies it, but actually is about an abstract 
state machine, as opposed to a viewer with rendering).

However, I assert that these are true, and *everyone* has implemented them:
1.) drawing occurs in element order (painters model);
2.) drawing is unaffected by APS grouping, i.e., one does not composite 
into an off-screen bitmap and then draw (like SVG).

(See 6.13.4, 'statelist' being the only inheritance value that is allowed 
in WebCGM.)

I think this is also true:  WebCGM 1.0 & 2.0 people do not care about 
precise colour rendering -- it is not a requirement of the WebCGM 
constituency.    COLOUR CALIBRATION is the CGM element that gives precise 
control, and it was excluded from both profiles.

Here are a couple of useful ISO CGM references:
ISO 6.7.6 (Colour attributes)
ISO 6.5.7 (Transparent Cell Colour)


>- the alpha-transparency Escape element... where is that element
>specified? Can we get the wording?

Two general references, plus the esc045 specific reference:

http://www.cgmopen.org/technical/registry/
http://jitc.fhu.disa.mil/nitf/graph_reg/class_pages/escape.html

http://jitc.fhu.disa.mil/nitf/graph_reg/diagrams/esc045.html


>- same question for sRGB/sRGBA, RGB/RGBA.

Two general references:
http://www.cgmopen.org/technical/registry/
http://jitc.fhu.disa.mil/nitf/graph_reg/class_pages/colour.html

and the three specific ones:
http://jitc.fhu.disa.mil/nitf/graph_reg/diagrams/col006.html
http://jitc.fhu.disa.mil/nitf/graph_reg/diagrams/col007.html
http://jitc.fhu.disa.mil/nitf/graph_reg/diagrams/col008.html


>Would it be fair to say that Esc 45 maps to the 'opacity' attribute in
>SVG?

Same general idea.  (Maybe inverted?  I.e., transparency vs. opacity?  I 
haven't looked at SVG to see which one it defines.)

-Lofton.