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[bug #20304] PNG with alpha channel not properly supported

From: Quentin Mathé
Subject: [bug #20304] PNG with alpha channel not properly supported
Date: Sat, 30 Jun 2007 22:21:16 +0000
User-agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X; fr) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3


                 Summary: PNG with alpha channel not properly supported 
                 Project: GNUstep
            Submitted by: qmathe
            Submitted on: samedi 30.06.2007 à 22:21
                Category: Gui/AppKit
                Severity: 3 - Normal
              Item Group: Bug
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any



With -make 2.0.0, -base 0.14, -gui 0.12 and -back 0.12 on Ubuntu Feisty

According to my tests, PNG files with an alpha channel suffer from various
drawing artifacts in GNUstep. In my tests, they are loaded through NSImage
and drawn by method -compositeToPoint:operation:

The kind of drawing method which is used probably doesn't matter, the problem
rather lies in PNG format reading code.

As an example, I attached a screenshot of a folder icon saved as PNG by GIMP
and displayed by a GNUstep application.
Here is a link to original PNG file:
If I save the folder icon file as TIFF and uses it instead of PNG one in the
same application, the issue vanishes.

I initially thought the issue could be a result of additional embedded
metadatas in the file (as Photoshop does it sometimes). But after having
resaved the file in GIMP and tried many save options, I don't think this
explanation is valid.

Drawing artifacts can be as extreme as seen in attached screenshot, but most
of time they are visible only as jagged border or pixels drawn with incorrect
color value.
The incorrectly drawn area of the folder icon is surely mapped to a partially
transparent region of alpha channel.



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