QUESTION. Transparency (element) and alpha channel.

[Lofton Henderson <lofton@rockynet.com>]

Attention color wonks....

At 01:01 AM 10/11/2005 -0700, Cruikshank, David W wrote:

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T18.4  Transparency  I have a note that indicates there probably should be a note under Other indicating that this element should be prohibited when the colour model has an alpha channel?

Why?  This element says whether (for example) gaps in dashed lines show what is behind, or show the Auxiliary Colour.  It seems unrelated to the presence of an alpha channel.
 
This was a note I found in the document Dieter put together on WebCGM 2.0 for the Houston meeting.  Don't know the rational. 

Recommendation.  Drop it.  TRANSPARENCY (element) and alpha
channel are pretty much orthogonal concepts.

Question.  However, is there an issue about these following and
alpha channel?

1.) T.18.16 TRANSPARENT CELL COLOUR (v3/v4 element)

2.) T.21.1  ESCAPE 22, Transparent Cell Colour (for v1/v2
metafiles)

3.) T.21.1  ESCAPE 45, Alpha Transparency (modal
attribute).

Some clarification might be in order.  To me:

a.) for #1 and #2, this makes sense to me:  if TCC is
"transparent", then that rules over any alpha values on
individual colors in cells; if "opaque", then the alpha values
pertain.

b.) for #3 ... it seems to me that they should combine.  I.e., if
the alpha channel for a color value (e.g., of LINE COLOUR) was 0.5, and
if Esc45 current value was 0.5, then shouldn't the effective alpha value
be 0.25?

If #b is true, then the same rule actually answers #a -- they combine (by
multiplying), where TCC can only have the values 0.0 and 1.0.

Thoughts?

-Lofton.