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[bug #57335] PNG writing support appears to be broken

From: Fred Kiefer
Subject: [bug #57335] PNG writing support appears to be broken
Date: Sun, 1 Dec 2019 17:00:10 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0

Update of bug #57335 (project gnustep):

                  Status:                    None => Confirmed              
             Assigned to:                    None => FredKiefer             


Follow-up Comment #1:

The test code you provides tries to convert a tiff file into a PNG image. This
should be possible with GNUstep. I tried to convert the GNUstep logo from tiff
to PNG and ended up with an image where the white bits where shown as black.
Is this the bug you are reporting?

What the code here does is to follow the PNG specification. PNG stores values
with non pre-multiplied alpha. GNUstep internally uses pre-multiplied alpha as
this allows for faster drawing. When exporting to PNG we need to make sure we
output the data non pre-multiplied and this code here tries to do the

What you did is switch off this conversion. And for simple data where alpha is
either 1 or 0 this results in correct images. But this of course does not
solve the issue why the conversion is wrong. Sorry, I will need some more time
to look into this.


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