[cairo] Bug drawing PNG images with transparency

Franz Schmid Franz.Schmid at altmuehlnet.de
Tue Aug 1 14:03:46 PDT 2006

Carl Worth schrieb:
> On Tue, 01 Aug 2006 13:43:23 +0200, Franz Schmid wrote:
>>Hi all,
>>during the development of Scribus i've encountered a strange bug
>>with the drawing of images with an alpha channel.
> Hi Franz,
> Thanks for the report. I am having a bit of trouble figuring out what
> bug you are trying to report here. Perhaps you can give me a few more
> details.
> For starters, which backend are you using?
I'm using the image backend.

>>The code i've used to draw the images is as following:
>>	cairo_set_fill_rule(m_cr, cairo_get_fill_rule(m_cr));
>>	cairo_surface_t *image2;
>>	image2  = cairo_image_surface_create_for_data ((uchar*)image->bits(), CAIRO_FORMAT_ARGB32, image->width(), image->height(), image->width()*4);
>>	cairo_scale(m_cr, m_zoomFactor, m_zoomFactor);
>>	cairo_set_source_surface (m_cr, image2, 0, 0);
>>	cairo_paint_with_alpha(m_cr, fill_trans);
>>	cairo_surface_destroy (image2);
>>I've attached four images to illustrate the problem:
>>drawbug2.png shows as it should be
> For "as it should be" is that with m_zoomFactor set to 1.0 and
> fill_trans set to 1.0 ? And is this image the result of correct
> behavior from cairo in this case? Or is this an independent
> representation of the result you want?
Correct, and this is the expected correct behaviour for a m_zoomFactor = 1.0
and fill_trans = 1.0

> And I assume the destination surface is solid yellow before the
> drawing code above?
>>drawbug1.png shows the display with m_zoomFactor set to 0.5
>>drawbug3.png shows the display with fill_trans set to 0.5
> So could you describe the bug(s) you are seeing here?
> Let me guess at what you might be seeing as problems: 1) Portions of
> the image that are entirely transparent are resulting in white in the
> final result. 2) Areas outside the image are being affected in the
> fill_trans case.
Case 1 is the bug, transparent areas of the image become white when
you change either the m_zoomFactor or fill_trans to any value != 1.0
Furthermore some colors inside the image are changing strangely when
you have a fill_trans != 1.0, and this is dependend of the value of

> One step that would clear up almost all of the above questions would
> be to write (or adapt) one of the tests in the cairo test suite to
> replicate your problem. A great place to start would be the
> paint-with-alpha test which looks very much like your test case. What
> happens if you do the following in a cairo source tree on your system:
> 	cd test
> 	make paint-with-alpha
> 	./paint-with-alpha
> If that test doesn't replicate your problem, can you identify some
> minimal change to the test that would cause it to replicate your
> problem?
Will try to do that tomorrow.

Anyway thanks for your time to look into that problem.

Franz Schmid

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