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

Michael Sweet mike at
Thu Jan 19 08:25:53 PST 2006

Owen Taylor wrote:
> ...
> Hmmm. In earlier discussions we've already pretty much decided
> against supporting various advanced printing features - spot
> colors, CMYK and so forth. Things get a lot simpler when there
> is a single unique representation of a color. 
> (As sRGB not restricted to [0,1])
> It may be that this falls into the same category.

If you won't be supporting CMYK color, then there is no reason
to support a destination-alpha PS/PDF surface.  Basically, no
prepress app will be able to use your API to produce PS and PDF
output, and those are the apps that might use destination alpha
to do masking in the output.

> But if not, should be noted that what is currently implemented for 
> CAIRO_CONTENT_COLOR_ALPHA is *not* compatible with the white
> ink model. What you get is an entire page printing with the
> a base of the white ink, and no possibility for showing the
> underlying media.

Can't you clear the page to some color with alpha=0?

> Does PS or PDF support more than 1-bit of alpha in this context?

PDF - yes, PS - not generally (although certain interpreters seem
to support 8-bit mask images with values other than 0 and 255)

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

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