[cairo] Bug drawing PNG images with transparency

Franz Schmid Franz.Schmid at altmuehlnet.de
Fri Aug 4 14:22:59 PDT 2006

Franz Schmid schrieb:

>> For starters, which backend are you using?
> I'm using the image backend.
Made some more tests with Scribus and other small test
applications on that issue. It comes out that the
image surface is responsible for the vanishing transparency,
if i use the xlib backend everything works as expected.

Sadly i can't use the xlib backend in Scribus as i need to
get access to the image data for implementing our special

Drawing an image with transparent areas onto a cairo_image_surface
fails with the transparency information lost in the following cases:
1) a cairo_scale operator != 1 is applied to the surface
2) the image is painted with cairo_paint_with_alpha with an alpha
    value != 1, in this case the former transparent area has some
    weired color shifts
3) a cairo_rotate operation is applied to the surface
4) a clipping paths is set

I hope that these informations help debugging that a bit.

Franz Schmid

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