sfnt: read the format 4 glyph id array from its real start - #54
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The glyph id array of a format 4 cmap begins after the idRangeOffset array, at 16+4*segCountX2. The reader sliced it at 16+3*segCountX2 - the start of the offset array - while lookup measured from its end, so every code point in a range-offset segment read the id segCountX2 bytes early, a neighbour's glyph or none. Delta segments were unaffected, which is all the test font had. In Noto Sans that drew "•" as "," in every embedded PDF and left 753 BMP code points (Romanian Ș/ț, much of Cyrillic Extended, the combining marks) without a glyph. The PDF's ToUnicode map came from the rune, so copied text still read correctly and hid it. Against x/image/font/sfnt the lookup now agrees on every BMP code point of Noto Sans Regular, Bold and Italic. sfnttest.Cmap4Ranged is a format 4 map with two range-offset segments, so the offset arithmetic is checked at two different segment indices. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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The glyph id array of a format 4 cmap begins after the idRangeOffset array (
16+4*segCountX2).read4sliced it at16+3*segCountX2— the start of the offset array — whilelookupmeasured from its end, so every code point in a range-offset segment read the idsegCountX2bytes early: a neighbour's glyph, or none. Delta segments were unaffected, and the test font only had those.Impact with Noto Sans (the Fyne UI font):
•drew as,in every embedded PDF, and 753 BMP code points (Romanian Ș/ț, much of Cyrillic Extended, combining marks) had no glyph. ToUnicode is built from the rune, so copied text still read correctly and hid it.c.glyphIDs = t[16+4*c.segX2:]sfnttest.Cmap4Ranged: a format 4 map with two range-offset segments, so the offset arithmetic is checked at two segment indices;TestTheCmapFormats/format 4 with range offsetsis red before, green after.golang.org/x/image/font/sfnt: every BMP code point of Noto Sans Regular, Bold and Italic now maps identically (was 753 mismatches).🤖 Generated with Claude Code