[cairo] Semantics of transparent objects on opaque targets (image vs. PostScript)

Carl Worth cworth at cworth.org
Fri Jan 13 12:34:30 PST 2006

On Fri, 13 Jan 2006 15:16:22 -0500, Michael Sweet wrote:
> I agree that this is the right thing to do - basically, if you don't
> draw a background then you'll get an implementation/surface-defined
> default background, which for PostScript output is white.

Thanks for the confirmation of the concept.

Note that even if you "draw a background" the default background can
still be made visible, (such as the black that appears when using
CLEAR with an RGB24 target surface).

> On a separate, nit-picky note, your PS output does not conform to
> the Adobe Document Structuring Conventions:

Hey, thanks for the careful look. This is greatly appreciated!

>      1. No Pages: comment in the header (you need at least a
>         "%%Pages: (atend)" in the header)

This one is a recent regression which I committed in the past few
days, (the output had "%%Pages: N" previously), and I've been planning
to put that back soon.

But the recommendation of "(atend)" is even better. That means I can
fix this much sooner.

>      2. The header comments indicate ASCII (Clean7Bit) data, but
>         you are embedding binary data.
>      3. There is no %%EndPage comment defined by the DSC.

Thanks for these corrections. I'll fix those.

>      4. Presumably you'll want a %%Pages comment after the
>         %%Trailer...

Yes, easy to do.

>      5. Unless you are really using Level 3 language elements,
>         the %%LanguageLevel comment should specify version 2.
>         (this one is not really a DSC problem, but a general nit)

The comment we have in cairo for this one is:

    /* The "/FlateDecode filter" currently used is a feature of
     * LanguageLevel 3 */

I'll have to look closer to see if that's still appropriate.

> Once I have finished up CUPS 1.2 (probably sometime in March), I'll
> be happy to work with you on this part of Cairo...

We'll definitely look forward to your contributions!

Thanks Michael,

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