[cairo] Duplicate surface with different format

[Carlos López Camey]

Thank you Kyzystof, I wasn't clear to me that there were different
types of cairo_surface_t's so your response really helped me. I ended
getting the data from the corresponding pixbuf after all, for the
archives, here's how I did it:
http://stackoverflow.com/questions/6951878/getting-a-coordinates-pixel-color-from-a-gtkdrawingarea/7005803#7005803

Carlos.

2011/8/6 Krzysztof Kosiński <tweenk.pl at gmail.com>:
> 2011/8/5 Carlos López Camey <c.lopez at kmels.net>:
>> I've a GtkDrawingArea and its surface has format CAIRO_FORMAT_INVALID
>> by _default_, i.e. "no such format exists or is supported" for that
>> surface's data. Is there a way to create a copy of my GtkDrawingArea
>> surface with any format I choose? Then I'd be able to query the data
>> knowing its format, I'm trying to get a coordinate RGBA values.
>
> The surface of GtkDrawingArea is an X11 surface, not an image surface.
> You cannot access its pixels directly, because it's in a different
> process (though I it would be possible if Cairo used XShm). To access
> the pixels, you need to copy the data to an image surface, e.g.:
>
> // depends on GTK version - in 2.x it's gdk_cairo_create(widget->window);
> cairo_t *widget_ct = ...
> // w and h can be had from widget's allocation
> cairo_surface_t *area_copy = cairo_image_surface_create(w, h,
> CAIRO_FORMAT_ARGB32); // get w and h
> cairo_t *ct = cairo_create(area_copy);
> cairo_set_source_surface(ct, cairo_get_target(widget_ct), 0, 0);
> cairo_set_operator(ct, CAIRO_OPERATOR_SOURCE);
> cairo_paint(ct);
> cairo_destroy(ct);
> unsigned char *pixels = cairo_image_surface_get_data(area_copy);
> // ...do something with pixels
>
> (Aside: I know that internally there is a backing client-side image
> surface for every X11 surface, which handles many operations not
> accelerated by the X server. I wonder why this backing surface is not
> exposed in Cairo API, even though it appears pixel access to X11
> surfaces would be possible in almost all scenarios - either through
> reusing the backing surface, or by giving away a pointer directly into
> X server's address space through XShm)
>
> Regards, Krzysztof
>