[cairo] Alpha masks

Owen Taylor otaylor at redhat.com
Sun Sep 5 11:53:03 PDT 2004

I was trying to figure out how with Cairo to do the following
simple operation:
 Given a single channel (A8) surface, draw a solid color
 with the surface as the mask.

What I came up with was:
  image_pattern = cairo_pattern_create_for_surface (image_surface);
  tmp_surface = cairo_surface_create_similar (cairo_current_target_surface (cr),
                                              CAIRO_FORMAT_RGB24, 1, 1);
  cairo_surface_set_repeat (tmp_surface, 1);
  cairo_save (cr);
  cairo_set_target_surface (cr, tmp_surface);
  cairo_rectangle (context, 0, 0, 1, 1);
  cairo_fill (cr);
  cairo_restore (cr);
  cairo_set_pattern (cr, image_pattern);
  cairo_show_surface (cr, tmp_surface, width, height);

  cairo_restore (cr);
  cairo_pattern_destroy (image_pattern);

Which more or less works (I think there are some bugs in the cairo_gstate_show_surface()
transform fun), but feels clumsy to me, and not particularly natural either. Why is the 
pattern being used as the *mask*? The prototypical pattern doesn't have a alpha
channel at all.

My expectation would be that showing a RGB or RGBA surface would ignore the pattern,
showing a A-only surface would use the surface as a mask for the pattern. Does this
make sense to other people?

I suppose the utility of the current interface is that you can easily use
gradient-with-alpha to blend an image, but that operation seems rare by comparison.

(You *could* make A-only images use the image as the mask and RGB-images use the
pattern as the mask, but that would be a rather peculiar interface.)


(Currently showing a A-only surface bad-matches in the Xlib backend, but the code
isn't going to do anything useful in any case.)

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