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cmap 4 idDelta is a signed short

See original GitHub issue

I just got sent a font file which fails MS fontval validation due to problems with the cmap table. It’s a CJK font and has lots of glyphs scattered around lots of codepoints. Debugging this, I noticed one immediate problem:

https://docs.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values :

int16 | idDelta[segCount] | Delta for all character codes in segment.

https://docs.python.org/3/library/struct.html:

Format	C Type	Python type	Standard size	Notes
h	short	integer	2	(2)
H	unsigned short	integer	2	(2)

https://github.com/fonttools/fonttools/blob/cace698bb029d58cdae00603140438fe4993bfa6/Lib/fontTools/ttLib/tables/_c_m_a_p.py#L907

That should be “h”, right?

Issue Analytics

State:
Created a year ago
Comments:13 (3 by maintainers)

Top GitHub Comments

1reaction

simoncozenscommented, Apr 5, 2022

Correct - I tried that.

1reaction

justvanrossumcommented, Apr 5, 2022

The Apple specification lists it as UInt16, so MS seems to have changed this.

https://docs.microsoft.com/en-us/typography/opentype/spec/cmap

Both specs say “The idDelta arithmetic is modulo 65536”. Not sure how this affects the issue.

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