PS/PDF API Change Proposal: (Re: [cairo] Semantics of transparent objects)

[Carl Worth]

On Thu, 19 Jan 2006 12:37:58 -0800, Bill Spitzak wrote:
> 
> This is done by defining the PostScript surface as containing the 
> composite of whatever you have drawn over opaque white. This is done by 
> replacing all the compositing math with these substitutions:

What is implemented now does give exactly the composite of whatever
you have drawn over opaque white. The difference between that and what
you suggest is that in the current implementation "whatever you have
drawn" is semantically equivalent to rendering to a COLOR_ALPHA surface
or a COLOR surface.

> 1. Whenever the alpha of the destination is wanted, 1 is substituted.
> 
> 2. When a color,alpha pair of c,a is written to the desitnation, instead 
> the value c+1-a is written, and there is no need to store the a.

This, I believe, would introduce a _third_ semantic, used uniquely for
printing. That gets us further from consistent output, not closer.

-Carl
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