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

[Owen Taylor]

On Thu, 2006-01-19 at 15:40 +1100, Russell Shaw wrote:
> 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.

The question is, do you get something different with:

 alpha=0.5 red
 alpha=1.0 pink

My understanding is that with PS/PDF, no, you don't. The infrastructure
just isn't really there for supporting printers with white opaque
inks. (Such things might exist for printing labeling onto colored
plastic, say.)

Regards,
						Owen

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