Make NotoColorEmoji actually work and implement IndexSubtableFormat{0,2} on CBLC

[ebraminio]

Steps to repro:

1. Download NotoColorEmoji.ttf
2. make -Cbuild/src dump-emoji
3. rmdir out || true; mkdir out
4. build/src/dumo-emoji NotoColorEmoji.ttf

Actual:
Not all emojis can be dumped on out/ folder.

Expected:
All the glyphs be dumped correctly.

[ebraminio]

There is no IndexSubTable0 on https://docs.microsoft.com/en-us/typography/opentype/spec/eblc#indexsubtable1-variable-metrics-glyphs-with-4-byte-offsets how must of them are 0 on Noto? (put printf ("%d\n", (int) u.header.indexFormat); on top of IndexSubtable's sanitization or get_image_data to see yourself)

[ebraminio]

I wonder why this is implemented at the first place (probably for CLBC on Android) and how now that is broken no body cares

[khaledhosny]

Looks like his was introduced to get glyph extents since fonts with CBDT/CBLC tables must not have glyph outlines tables, see 8318525.

[ebraminio]

I know, and I feel it is implemented somewhere else so now nobody cares about harfbuzz implementation, but am thinking about why it is introduced first here and moved afterward.

[behdad]

I'll check this today.

[behdad]

@nona-google

[behdad]

I can confirm this doesn't work:

$ ./hb-shape  NotoColorEmoji.ttf -u 1f98a --font-funcs ot --show-extents 
[gid1120=0+2550<0,0,0,0>]

[behdad]

That said, we have a color-fonts.tests that seem to work.

[behdad]

Test font has:

    <strikedata index="0"> 
      <cbdt_bitmap_format_17 name="u1F42F"> 
        <SmallGlyphMetrics> 
          <height value="128"/> 
          <width value="136"/> 
          <BearingX value="0"/> 
          <BearingY value="100"/> 
          <Advance value="136"/> 
        </SmallGlyphMetrics> 
        <rawimagedata> 

NotoColorEmoji has:

    <strikedata index="0"> 
      <cbdt_bitmap_format_17 name="glyph02253"> 
        <SmallGlyphMetrics> 
          <height value="128"/> 
          <width value="136"/> 
          <BearingX value="0"/> 
          <BearingY value="101"/> 
          <Advance value="136"/> 
        </SmallGlyphMetrics> 
        <rawimagedata> 

so not sure why it doesn't work.

[behdad]

That code is too complicated for me to wrap my head around of. :(

[behdad]

Or rather, that table...

[behdad]

IIRC @nona-google implemented this and I cleaned up and merged it, but he also sent a patch to FreeType to allow loading metrics without loading bitmaps, and that got merged and he ended up using that. That would explain why this was broken and we never noticed.

[nona-google]

I just debugged and somehow the image format is invalid number and leached here.
https://cs.chromium.org/chromium/src/third_party/harfbuzz-ng/src/src/hb-ot-color-cbdt-table.hh?rcl=54d332dd9b0263821376161cdffb60ffb3c7847f&l=450