https://bugs.documentfoundation.org/show_bug.cgi?id=166339
Bug ID: 166339
Summary: Skia renderer is not applying gamma correction on text
Product: LibreOffice
Version: 25.2.2.2 release
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: minor
Priority: medium
Component: graphics stack
Assignee: [email protected]
Reporter: [email protected]
Created attachment 200518
--> https://bugs.documentfoundation.org/attachment.cgi?id=200518&action=edit
Text in Writer with Skia-backend enabled
Using the Skia-backend on Windows and X11, text is rendered too bold on white
backgrounds and too light on dark backgrounds. There is also colour bleeding if
you choose colours with a low brightness contrast, like pure green and pure
red. Similar to this bug report:
https://github.com/AvaloniaUI/Avalonia/issues/5282
There are also pronounced colour fringes from the Cleartype-like rendering.
I suspect, the Skia renderer is not applying correct gamma blending, i.e. is
doing the linear blending in the sRGB colour-space. The sRGB colour-space has a
gamma function on its values, e.g. RGB 128,128,128 is *not* neutral grey, but
darker.
Some background reading about gamma correct font rendering:
> https://www.puredevsoftware.com/blog/2019/01/22/sub-pixel-gamma-correct-font-rendering/
> https://freetype.org/freetype2/docs/hinting/text-rendering-general.html
> https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/
Windows is using a gamma of 1.9 internally for font rendering with Cleartype,
one could change these settings with Cleartype-Tuner, but it has become merely
disfunctional.
> https://learn.microsoft.com/de-de/dotnet/desktop/wpf/advanced/cleartype-registry-settings?view=netframeworkdesktop-4.8
Qt, starting at some point in the version 5 life-cycle, is also using 1.9:
> https://mail.kde.org/pipermail/kde/2017-December/028694.html
Chromium used hard-coded values for its different platforms, but a large patch
from Microsoft changed that on Windows, modern Edge is using that for some time
now:
> https://developer.chrome.com/blog/better-text-rendering-in-chromium-based-browsers-on-windows
I couldn't find the lines in the codebase that handle the text rendering, I
would appreciate any help to find them, so I can understand what's going on.
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