[cairo] How to export image_surface pixel data?

Carl Worth cworth at redhat.com
Tue Jan 17 15:55:31 PST 2006

On Tue, 17 Jan 2006 18:38:24 -0500, Owen Taylor wrote:
> Multiple return values are distinctly language binding unfriendly.

Thanks. The choice was feeling arbitrary, so it's good to get
guidance, (as well as a reminder about what led to separate get_width
and get_height originally).

> But even then you have to know the order. (C has the ordering problem
> too, actually..)

Oh, good point. I'll definitely take more type safety over less typing.

> From the GdkPixbuf experience I'd caution thinking ahead of time about
> what the above means for modifying data behind cairo's back. Are
> the cairo_surface_mark_dirty() and cairo_surface_flush() sufficient?
> Can you avoid copying the entire contents in:
>  Create image surface
>  Draw to it
>  Draw to PS surface metafile
> ? For GdkPixbuf I've regretted not having a lock_data(), unlock_data()
> instead,

There is a good point to be made that mark_dirty only provides the
second half of a lock_data/unlock_data() pair. We could still add the
first half of that pair to provide more efficient behavior when used,
(and do the more pessimistic copying in your meta-file example

As it turns out, we're doing copying all over the place in the
meta-surface paths right now (waiting for an implementation of
copy-on-write for surfaces).

Any suggestions on what a "lock" function to pair with mark_dirty
might be named? As you mention below, get_data can't serve this
purpose since we also need locking for the create_from_data case.

>         though that particular Rubicon may already have been crossed
> with the current ability to create an image surface from user data.

This is the train of thought that has led me not to feel hesitant
about adding get_data, (even on the eve of a major release).

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