[cairo] how to make existing color transparent

Pablo Romero romero619 at hotmail.com
Thu Feb 5 18:02:19 PST 2009

Is it possible to set the alpha transparency on an existing color within a surface, and cause that color & anything beneath it to have 0 alpha, completely transparent???
I want to draw a purple rectangle on a surface, over existing image data.
Next, I want to make that rectangle completely transparent in the resulting png file (i.e., 'set any/all areas that are purple to 100% transparent on cairo_surface_t *surface'). I dont want any existing colors/graphics that were below the rectangle to show up.
I tried using "set_source_rgba" and setting the alpha to '0.0', but this doesnt cause the underlying colors/images to become transparent...
How can I achieve this effect? similar to gd's "gdImageColorTransparent" function...
Please help...

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