PS/PDF API Change Proposal: (Re: [cairo] Semantics of transparent
spitzak at d2.com
Thu Jan 19 13:37:22 PST 2006
Carl Worth wrote:
>>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.
I guess my proposal is that the above math *replaces* the current COLOR
surface math. Thus there are still only two types of surfaces (COLOR
with my new math, and COLOR_ALPHA).
I thought about instead saying the printer is a COLOR_ALPHA surface,
with a final composite over white after all the drawing is done. But it
seems this will make it too difficult to convert Cairo to PostScript
without reverting to an image. Changing the math as stated above I
believe will make all the destination composites emulatable with
PostScript, so only the use of intermediate surfaces will cause images
to be used. This makes it much easier to avoid creating intermediate
images by accident.
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