Re[2]: [cgmo-webcgm] Issue CL-d3: 2.2.2 Drawing model

[Benoit Bezaire <benoit@itedo.com>]

Some good and bad news...

First the good news; Escape 45 is well defined since it defines the
compositing equation: result = alpha*foreground + (1-alpha)*background.

The bad news is that I don't know how a 4 channel RGB color model (say
RGBA or sRGBA) works in combination with Escape 45. I can't find the
answer in the pointers given to me. RGBA says the following:
"Conceptually, values of the alpha transparency are real numbers
between 0.0 (fully transparent) and 1.0 (fully opaque), inclusive, and
intermediate values have meaning identical to the alpha transparency
defined in Escape 45." I dunno, it still seems vague.

My question is as follows:
Can I use the RGBA color model and also use Escape 45 in the same
WebCGM file?
- If Yes...
  - How are they combined?
    i)   multiply the two alpha values.
    ii)  A of RGBA is used.
    iii) Escape 45 is used.
- If No...
  - Who wins?
    i)  RGBA
    ii) Escape 45

Thougths?

-- 
 Benoit   mailto:benoit@itedo.com

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Friday, March 24, 2006, 11:26:00 AM, you wrote:

> 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.