>> Correct font rendering with linear alpha blending and gamma correction >> can make text look pale like reported in >> https://bugreports.qt.io/browse/QTBUG-41590 due to the way the math >> behind it works out. Stem darkening compensates for the paleness. > > Any idea why exactly?
I'm not sure what you're asking. Do you mean why LAB+GC results in pale(r) text? The paleness is an expected side-effect of gamma correction on low-DPI screens. Have a look at https://www.freetype.org/image/BlendingExamples.png. All (grayscale) examples are composed onto the surface using linear alpha blending. Column 1 has no gamma correction, column 2 uses a gamma correction factor of 1.8. The result in column 2 is clean but paler text. The glyphs that FreeType outputs are really 8-bit alpha maps in linear space: a pixel that has zero coverage of a glyph is white (255), one with 100% coverage is black (0), one with 50% coverage is set to a value of 127, or a middle shade of gray. Since most monitors out there do not have a linear gamma curve, you need to bend your alpha map after you have composited onto a surface. The 50% coverage or 127 value in linear space maps to a value of around 180 or so on a standard sRGB surface, 127 would be much darker. So now you go and bump that 127 up to 180 -- tada, the pixel and by extension the entire glyph is now lighter, or paler. But also nice and clean. You can also have a look at ftview from the freetype2-demos package. It uses LAB+GC by default. Using LCD filtered rendering actually makes little difference to my own surprise (press 'A' and 'C' to switch between grayscale and LCD filtered rendering, respectively). Since 1) a pixel's chance of being 100% covered is much higher on HiDPI screens and 2) black is the same thing in linear space and all other gamma curves, this is primarily an issue on LoDPI screens, so 99% of screens out there. > Anyway from an API point of view, the easiest implementation would be if Qt > could read on initialization if freetype can do stem darkening on all outline > fonts. Since it can't do that currently, an alternative would be to be able > to > query it per face, and then carry the information on. If the new FT_Face_Option() is present, FT can do stem darkening on (almost entirely) all outline fonts :) The toolkit just has to invoke the function on all FT_Faces to be rendered. _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
