Uneven font rendering with artifacts on Firefox on Android

[MarcoBrieden]

The text rendering of my buttons is very uneven in Firefox on my Android phone, meaning e.g. the right vertical line of an "N" is 4x as big as the left one. There are also artifacts, like vertical lines next to some letters.

Here is a magnified screenshot:

The same page renders correctly in Chrome on the same phone.

I tested this both in the latest release (0.27.1) and the latest master (master#a541e021) with the same results.

The Firefox version is 124.1.0 (Build #2016010543) on Android 14 (March 5, 2024 update) on a Pixel 8 Pro.

Please let me know if I can provide additional info.

[wareya]

This seems to also be a problem for native Android builds, too, at certain font sizes. Whatever's responsible for generating verts and UVs for text glyphs probably has rounding errors in it somewhere (maybe from fp16 rounding?). Should also double check whether it generates the necessary 1px of empty space between glyphs when packing them into whatever glyph atlas it uses, but not doing so wouldn't cause these problems.

[lucasmerlin]

This is also happening on rerun:

[lucasmerlin]

Maybe it'll be magically fixed by #5784

[wareya]

I made a fork of the egui repo and changed the precision mediump float; instances in the egui_glow shaders to use highp instead and the problem went away! Then I undid that and the problem came back. The problem is indeed fp16 rounding.