IBM Plex Sans JP: the .ttf and .otf have different line metrics (hhea and OS/2 lineGap)
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调研方向
The measurements are in packages/plex-sans-jp/fonts/complete, comparing ttf/hinted and otf/hinted Regular files. Dump the hhea, OS/2 and typo metrics of each edition with a font inspection tool such as fontTools, to confirm the reported 1060/-440/0 and 880/-120/1000 values. Done means the maintainers have chosen the intended metrics and both editions of Plex Sans JP report the same hhea and typo values, but the target values need a maintainer decision first.
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描述
Hello,
In IBM Plex Sans JP, the TrueType and the OpenType (CFF) editions of the
same release have different vertical metrics, so the same text is set with
different line spacing depending on which file is used. The other CJK
families agree between their two editions, so this looks unintended, but
I may be missing a reason.
Measured in packages/plex-sans-jp/fonts/complete (all files Version
1.004, unitsPerEm 1000):
| File | hhea ascender | hhea descender | hhea lineGap | OS/2 typoLineGap | Line height |
|---|---|---|---|---|---|
ttf/hinted/IBMPlexSansJP-Regular.ttf |
1060 | -440 | 0 | 500 | 1.5 em |
ttf/unhinted/IBMPlexSansJP-Regular.ttf |
1060 | -440 | 0 | 500 | 1.5 em |
otf/hinted/IBMPlexSansJP-Regular.otf |
880 | -120 | 1000 | 1000 | 2.0 em |
otf/unhinted/IBMPlexSansJP-Regular.otf |
880 | -120 | 1000 | 1000 | 2.0 em |
The OS/2 typoAscender and typoDescender (880, -120) and the usWinAscent
and usWinDescent (1060, 440) are the same in both editions; the hhea
values and the typoLineGap differ. Every weight of the .ttf has the
1060 / -440 / 0 values.
For comparison, both editions agree in the other families:
| Family | hhea (both editions) | OS/2 typo (both editions) |
|---|---|---|
| Plex Sans SC | 880 / -120 / 1000 | 880 / -120 / 1000 |
| Plex Sans TC | 880 / -120 / 1000 | 880 / -120 / 1000 |
| Plex Sans KR | 1085 / -415 / 0 | 780 / -220 / 500 |
Applications that take the line height from hhea (most do on macOS and
Linux, and PDF libraries generally do) set Japanese text 1.5 em apart with
the .ttf and 2.0 em apart with the .otf. We noticed it in PDFjet
(https://github.com/edragoev1/pdfjet), a PDF library that embeds IBM Plex:
the same page of Japanese changed its line spacing when we switched files.
Could you confirm which metrics are intended for Plex Sans JP, and align the
other edition in a future release? Either is fine for us; we only want the
two files to agree.
Thank you for IBM Plex.
Evgeni Dragoev
PDFjet Software
https://pdfjet.com
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