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List:       kde-bugs-dist
Subject:    [Bug 299069] New: color model based layer blending modes produce unexpected results over transparent
From:       T Hall <tahall256 () gmail ! com>
Date:       2012-04-29 23:20:45
Message-ID: bug-299069-17878 () http ! bugs ! kde ! org/
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https://bugs.kde.org/show_bug.cgi?id=299069

            Bug ID: 299069
          Severity: normal
           Version: git master
          Priority: NOR
          Assignee: krita-bugs-null@kde.org
           Summary: color model based layer blending modes produce
                    unexpected results over transparent (alpha<1) stuff
    Classification: Unclassified
                OS: Linux
          Reporter: tahall256@gmail.com
          Hardware: Compiled Sources
            Status: UNCONFIRMED
         Component: Color models
           Product: krita

Consider the attached .kra file. The top layer is set to hue. The middle layer
is mostly transparent. The background is opaque. The result is as expected:
Where the top layer has blue, the red of the middle layer changes hue to blue,
and the white has changed hue to blue, but since it's white, it's still white.
Neither the luminance nor the saturation of any pixel is affected.

Now toggle the visibility of the white background. The red pixels are
transformed as before. However, where before the white remained white, now the
transparent pixels have become blue! The hue layer blending mode keeps the
luminance and saturation constant. Why has it changed the alpha value?

In my mind, the simplest solution is simply to keep the original alpha value.
Perform the relevant colour transformation on the *color* data, R, G and B, (or
whatever color model the layers are using) and leave the alpha blending out of
it.

Without having a transparent background, the effect also shows up if the top
layer in the example file is merged with the layer below it (with the white
background layer visible). This results in a dramatic change to the appearance
of the image, which is out of keeping with most cases whereby merging layers
does not alter the overall appearance of the image.

Another way to make the effect appear is to put the top two layers into a layer
group so that the hue mode operates on the transparent bits before they are
overlaid on the white (like merging the layers, but without merging them).

Vaguely similar to bug 294122 (filter layers over transparent).

Reproducible: Always

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