a question about Esc.45

[Lofton Henderson <lofton@rockynet.com>]

Hi all --

I'm working on the 2.1 drafting assignment, specified in the minutes as:

[[[
·	Clarify transparency interactions
Not assigned  Ben did this before  is there anything more here?
]]]

First, consider it assigned -- I'll take it.  Right now, I'm still reading 
and understanding the details.  One such detail question is the main 
purpose of this mail.

There are three references relevant to my question:

[1] Sec. 
2.2.2: 
http://docs.oasis-open.org/webcgm/v2.0/OS/WebCGM20-Concepts.html#webcgm_2_2_2
[2] Esc.45:  http://jitc.fhu.disa.mil/nitf/graph_reg/diagrams/esc045.html
[3] Sec. 
2.2.3: 
http://docs.oasis-open.org/webcgm/v2.0/OS/WebCGM20-Concepts.html#webcgm_2_2_3

[2] says:  "This escape function may occur in the Picture Descriptor or 
Picture Body, in the definition of segments of all kinds, and in the 
METAFILE DEFAULTS REPLACEMENT element."  But it doesn't say what happens if 
it occurs in the PD.

[1] talks about "Primitives which have a value for [...Esc.45...] other 
than fully opaque...", but doesn't mention the PD (which of course has no 
primitives).

[3] says: "...the registered Escape 45 (alpha transparency) element may be 
included in the Picture Descriptor and applied to the background color of 
the picture."

QUESTION:  Is that the clarification/interpretation that should apply to 
the (underspecified) definition of Esc.45 in the PD [1]?

OPINION:  Yes.  It seems to conform to our (1999) thinking as written into 
[1] and [3], and it would allow the main use cases involving non-trivial 
alpha values on foreground stuff and background to be handled either by 
Esc.45 alone (in RGB metafile), or by RGBa alone.  I.e., two separate and 
complete mechanisms.  The only sensible alternative meaning would be 
"applies to everything in the picture body", but this can be done by 
putting Esc.45 at the beginning of the picture body.

-Lofton.