Roundtripping NotoColorEmoji-Regular.ttf makes COLR table grow

[behdad]

COLR table of original font is 1058175 bytes. Just roundtripping it through TTX makes it grow by 20kb to 1079817.

I have no explanation for this. COLRv1 uses all 24 or larger offsets types, so all object-sharing should happen all the same. This is very unsettling to me currently. :-)

Font file downloaded from https://fonts.google.com/noto/specimen/Noto+Color+Emoji (Click on "Download family").

[behdad]

Cosimo and I tracked this down to hb-subset vs fonttools packer difference. The smaller font is packed by hb-subset. The TTX of the two is the same, and there's no overflows. So this suggests a bug in the fonttools packer.

[behdad]

I checked how many objects each packer is producing, and hb-subset's was about 3000 fewer.

I think I see what's going on. In FontTools packer, each OTTableWriter has a member offsetSize, which is the width of the offset to this table. So FontTools won't share an object that is reached to by different offset-sizes. The COLR table uses a mix of 32bit and 24bit offsets. I believe that's what's going on here.