[cairo] cairo_gl_surface_swapbuffers()

[Simon Elliott]

Hi all

We're developing an embedded system which shows graphics using cairo, 
opengles and wayland.

Basically we have some vector graphics we've drawn into several surfaces 
that we blit onto a device surface and then display the device surface 
with cairo_gl_surface_swapbuffers().

We've run into a problem under certain circumstances.

Our blit code for copying the drawn surfaces to the device surface looks 
like this:
cairo_t* cr = cairo_create(surface_);
cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
cairo_set_source_surface(cr, source_surface_, x, y);
cairo_rectangle(cr, x,y, w, h);
cairo_fill(cr);
cairo_destroy(cr);

The device surface is created like this
cairo_surface_t* surface = cairo_gl_surface_create_for_egl(cairo_device, 
egl_surface, width, height);

We display the device surface like this:
cairo_gl_surface_swapbuffers(surface);

This all works flawlessly if at least one of the surfaces we've blitted 
onto the device surface is the same size as the device surface.

If we just blit a single surface that's smaller than the device surface, 
then call cairo_gl_surface_swapbuffers(), we see an older image on the 
screen with the smaller surface on top of it. The older image is the 
surface that was shown via cairo_gl_surface_swapbuffers() two iterations 
ago.

We've checked with cairo_surface_status() and there are no errors on any 
of the surfaces at any time.

Does anyone have any suggestions for resolving this?

I can't find any documentation for cairo_gl_surface_swapbuffers(). Can 
anyone point me at this?

I'd like to look at the contents of the surfaces, but e.g. 
cairo-image-surface-get-width() returns zero. How can I show the width 
and height of the surfaces?

Thanks in advance for any suggestions!