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

Michael Sweet mike at
Thu Jan 19 05:10:54 PST 2006

Owen Taylor wrote:
> 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.)

The ALPS MD-series printers are the most recent consumer printers to
support white ink, and a lot of CD duplicators use digital printers
with white ink (often used as a flood fill in the background, but
it is not unusual to have a pattern or design that shows the
reflective surface underneath...

Michael Sweet, Easy Software Products           mike at easysw dot com
Internet Printing and Publishing Software

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