What steps will reproduce the problem?

In Chrome on Linux, visit https://rtsh.es/test/blur.html and FontConfig configured to produce requests for subpixel antialiasing or

FontRenderParams::SubpixelRendering ConvertFontconfigRgba(int rgba) {
  return FontRenderParams::SUBPIXEL_RENDERING_RGB;
}

in fontconfig_util.cc.

The test page has sample text in Roboto and Inter, on a will-change:transform transformed layer, which causes the layer to be composited.

What is the expected output? What do you see instead?

Output rendering no less crisp than in FreeType backend.

Analysis:

FreeType Case:

In the FreeType backend, to produce glyph rendering, the code reaches SkScalerContextFTUtils::generateGlyphImage with mask format being set to SkMask::kA8_Format due to the kUnknown_SkPixelGeometry case in SkScalerContext::MakeRecAndEffects, then the code proceeds to producing an actual grayscale rendering in:

                target.width = glyph.width();
                target.rows = glyph.height();
                target.pitch = glyph.rowBytes();
                target.buffer = static_cast<uint8_t*>(imageBuffer);
                target.pixel_mode = compute_pixel_mode(glyph.maskFormat());
                target.num_grays = 256;

                FT_Outline_Get_Bitmap(face->glyph->library, outline, &target);

Fontations case:

The equivalent Fontations code path is in Fontations SkScalerContext_Fontations::generateImage, which is modelled after the DirectWrite backend. It does the following:

            const bool doBGR = SkToBool(fRec.fFlags & SkScalerContext::kLCD_BGROrder_Flag);
            const bool doVert = SkToBool(fRec.fFlags & SkScalerContext::kLCD_Vertical_Flag);
            const bool a8LCD = SkToBool(fRec.fFlags & SkScalerContext::kGenA8FromLCD_Flag);
            const bool hairline = glyph.pathIsHairline();

            // Path offseting for subpixel positioning is not needed here,
            // as this is done in SkScalerContext::internalGetPath.
            GenerateImageFromPath(mask, *devPath, fPreBlend, doBGR, doVert, a8LCD, hairline);

This produces a less optimal quality rendering: We're anyway performing a from-path rendering here and intentionally degrading that by setting the a8LCD bool to true here, resulting in a more washed-out appereanced, as the RGB antialiasing is slightly wider, and then gets downsampled to grayscale.

Proposed Fix

Ignore the SkScalerContext::kGenA8FromLCD_Flag flag in the Fontations backend here, and render to antialiased grayscale without the genA8FromLCD suboptimal filtering. This might work here, as we do not need to match any RGB antialiased masks coming from any system rasteriser, for example the DirectWrite rasteriser.

Would there be any negative side effects with this approach?