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

[Russell Shaw]

Owen Taylor wrote:
> I can't really object too strongly to this change, since the the model
> where a PS/PDF surface matches a CAIRO_CONTENT_COLOR surface is still
> available.
> 
> But I'm really not very comfortable with the with the idea of a 
> CAIRO_CONTENT_COLOR_ALPHA PS/PDF surface; I have concerns about 
> efficiency, concerns about consistency with the rest of cairo,
> and concerns about utility.
> 
> What is destination alpha good for?
> 
>  A) It's good if you want to end up with an object with 
>     destination alpha. You want to draw a transparent
>     clock that you'll combine with the rest of your 
>     desktop. This doesn't really seem relevant to printing
>     to me. In the end, you are going to end up with an 
>     opaque piece of paper.

What about overhead transparencies or other transparent film printers?

...

> The final result of drawing to a PS/PDF surface does *not* 
> have alpha, no matter what your best intention. (Even you print
> to overhead slides, the alpha there is just the color white,
> and has no relation to the alpha of your surface.)

If you print red with alpha=0.5, you should get a semi transparent
printout onto clear film.