tottfgpos: Fix needless warning about 16-bit field #5136
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When a single substitution table (format 1) is to be written and the substitution offset is very big and backwards (i.e. to lower numbers) a warning is issued although the encoded font is correct. For example codepoint 65342 is to be replaced by 1683. The offset is -63659 in this case (old codepoint + offset = new codepoint). The warning in putshort() triggers: if(sval < -32768 || sval > 65535) ... [how] The offset is calculated with signed ints, so we have the result above. The actual calculations 'in the font' are done with uint16_t. According to specs "Addition of deltaGlyphID is modulo 65536" [1]. So we simply need to drop all the bits outside uint16_t and still have the correct calculation. With the new offset value of -63659 & 0xFFFF = 1877 the calculation is 65342 + 1877 = 67219 == 1683 (assuming unit16_t arithmetic) [1] https://learn.microsoft.com/en-us/typography/opentype/spec/gsub#11-single-substitution-format-1 Fixes: fontforge#3009 Reported-by: Thayne McCombs <tmccombs> Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>
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[why]
When a single substitution table (format 1) is to be written and the substitution offset is very big and backwards (i.e. to lower numbers) a warning is issued although the encoded font is correct.
For example codepoint 65342 is to be replaced by 1683. The offset is -63659 in this case (old codepoint + offset = new codepoint).
The warning in putshort() triggers:
if(sval < -32768 || sval > 65535) ...[how]
The offset is calculated with signed ints, so we have the result above. The actual calculations 'in the font' are done with uint16_t. According to specs "Addition of deltaGlyphID is modulo 65536" [1]. So we simply need to drop all the bits outside uint16_t and still have the correct calculation.
With the new offset value of -63659 & 0xFFFF = 1877 the calculation is
65342 + 1877 = 67219 == 1683 (assuming unit16_t arithmetic)
[1] https://learn.microsoft.com/en-us/typography/opentype/spec/gsub#11-single-substitution-format-1
Fixes: #3009
Reported-by: Thayne McCombs @tmccombs
Signed-off-by: Fini Jastrow ulf.fini.jastrow@desy.de
Type of change
Reproduction / Test
Font to try:
Repo link https://github.com/googlefonts/noto-cjk/releases/tag/Sans2.004
Zip link https://github.com/googlefonts/noto-cjk/releases/download/Sans2.004/06_NotoSansCJKjp.zip
Font file in zip
NotoSansCJKjp-Bold.otfAffected lookup: CID -> CID font info -> Lookups -> 31 ('hist')
Just open and immediately re-generate new CID font.
See comments in #3009.