[krita] [Bug 356462] Krita opacity bug

[Boudewijn Rempt via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

--- Comment #8 from Boudewijn Rempt  ---
Oh, and just to note: mypaint does all its work internally in 16 bits
premultiplied, iirc.

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[krita] [Bug 356462] Krita opacity bug

[Boudewijn Rempt via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

Boudewijn Rempt  changed:

   What|Removed |Added

 CC||b...@valdyas.org

--- Comment #7 from Boudewijn Rempt  ---
We used to do this in integers, afair, and it's still possible to specialize
per color model/channel depth. I think we used to have just as many artefacts
in the blending brush back then, though, if not more.

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[krita] [Bug 356462] Krita opacity bug

[wolthera via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

--- Comment #6 from wolthera  ---
I can't tell you about GIMP, but MyPaint just does it's normal blending mode in
integers:

https://github.com/mypaint/mypaint/blob/master/lib/blending.hpp

(fix_15_t is a uint32_t and fix_15_t_short is a uint16_t)

https://github.com/mypaint/mypaint/blob/master/lib/fix15.hpp

I am not sure what benefit it has to us to use this method anywhere but in the
normal/over/general lerp, but for normal/over/lerp it might actually be very
useful. The real question I suposse is if this issue is problematic enough in
16bit to have it there as well. (the float spaces don't need it, because well,
they're floats)

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[krita] [Bug 356462] Krita opacity bug

[Dmitry Kazakov via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

Dmitry Kazakov  changed:

   What|Removed |Added

 CC||dimul...@gmail.com

--- Comment #5 from Dmitry Kazakov  ---
Short: 
The solution for the original problem: "just use 51% opacity to avoid rounding,
or use another composite op"

Long:

The formula for Normal alpha blending looks like that:

newColor = dstColor + alpha * (srcColor - dstColor)

In our case:
dstColor = 1
srcColor = 0
alpha = 0.5

We do all the calculations in a floating point numbers, so we get:

newColor = 1.0 + 0.5 * (0.0 - 0.5) = 0.5

Then, we convert the floating-point number back to integers using rounding:

newColorInt = round(0.5) = 1

We cannot skip rounding, because if we do the effects will be drastic. The
image pixels will flip to different sides in a random way, creating artifacts
on the merged image.


So, speaking truly, I don't know if we can (or should) fix this bug. How do the
other applications work with this case?

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[krita] [Bug 356462] Krita opacity bug

[Sven Langkamp via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

Sven Langkamp  changed:

   What|Removed |Added

 CC||sven.langk...@gmail.com

--- Comment #3 from Sven Langkamp  ---
It works here, but it depends a bit on how often you paint over the same place.
With 50% opacity I have to paint about 8 times over to fully cover.

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[krita] [Bug 356462] Krita opacity bug

[wolthera via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

--- Comment #2 from wolthera  ---
Actually, no, now I think about it, the problem happens in the transparency
code: Somehow we're not ending up with 100% transparency.

You can test this by comparing the difference between 'pick from merged image'
and 'pick from layer'. On 'pick from layer' we get full black. On 'pick from
merged image' integer spaces give an offset. So the error happens in the
transparancy-mixing code, and that it is unable to get to 100%.

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[krita] [Bug 356462] Krita opacity bug

[wolthera via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=356462

wolthera  changed:

   What|Removed |Added

  Component|Brush engine|Color models

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