[cairo] On recovery from errors in cairo

[Bill Spitzak]

Owen Taylor wrote:

> What should you get from:
> 
>  cairo_scale(cr, 0, 0);
>  cairo_set_source_image(cr, my_pattern, 0, 0);
>  cairo_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
>  cairo_paint();
> 
> ? I guess you could argue that it should be a solid fill with the
> average pixel color of the pattern...

If this is the one that replicates the edge pixels, what it actually 
should draw is four quadrants of solid color, each is the color of one 
of the corner pixels of the source image. The intersection of the lines 
separating the quadrants is at the translation of the transform. The 
angles of the lines depend on the rotation and skew of the (now 
degenerate) transform.

If, as I suspect, figuring out these angles is really difficult or 
ambiguous, it could fail. But Cairo should only fail on things it cannot 
draw, ONLY that should produce an error. The scale(0,0) should NOT 
produce an error. The set_source should not produce an error. No error 
should be produced until the paint() operation. Even that should not 
produce an error if the EXTEND_REPEAT is not on, or if the destination 
area is empty (probably due to the path being constructed in the 
degenerate ctm), and it even should check if the 4 corner pixels are 
equal and avoid an error then, too!