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Update CJK vertical metrics #135

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Update CJK vertical metrics #135

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0f355cb
Update metrics.md for CJK
NightFurySL2001 Mar 18, 2024
70d4225
notes on negative
NightFurySL2001 Oct 9, 2024
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17 changes: 13 additions & 4 deletions gf-guide/metrics.md
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Expand Up @@ -241,17 +241,26 @@ If the font is already hosted on [fonts.google.com](http://fonts.google.com/), y

## CJK Vertical Metrics

CJK vertical metrics are based on Source Han Sans and Noto CJK fonts.
*The following CJK vertical metrics are based on Source Han and Noto CJK fonts.*

The following vertical metric values must be applied to all CJK fonts
The values for CJK vertical metrics are based on a few core concepts:

1. The OS/2.sTypoAscender and OS/2.sTypoDescender values defines the ideographic em-box, such as CJK ideographs and full-width symbols. Thus, the value of OS/2.sTypoAscender - OS/2.sTypoDescender must equals UPM value, or in other words, the difference of the UPM factors for OS/2.sTypoAscender and OS/2.sTypoDescender should equals 1.
> Source Han/Noto CJK uses 880/-120 or 0.88/-0.12. Other usual pairs are 0.85/-0.15 and 0.8/-0.2. The factors may be modified to match the Latin part of the font.
2. Line gaps should be set to 0 matching Latin font practice. See above [Requirement point 8](#8-linegap-values-must-be-0).
3. hhea values should be set to look harmonized (excluding extreme vertical height symbols only used in vertical typesetting such as U+3031 and U+3032), including Latin parts (such as hanyu pinyin Ǚ). See above [Requirement point 10](#10-typohheaascender-value-should-leave-open-room-for-stacked-diacritics).
> Source Han Sans/Noto Sans CJK uses 1160/-288.
4. Disable OS/2.fsSelection bit 7 (Use_Typo_Metrics) as both sType* and usWin* are same.

The following vertical metric values may be applied to CJK fonts.

| Attrib | Value | Example using 1000upm font |
|-------------------------------------------|------------------------------------------|----------------------------|
| OS/2.sTypoAscender | 0.88 \* font upm | 880 |
| OS/2.sTypoDescender | -0.12 \* font upm | -120 |
| OS/2.sTypoDescender | -0.12 \* font upm （should be negative） | -120 |
| OS/2.sTypoLineGap | 0 | 0 |
| hhea.ascender | Set to look comfortable (\~1.16 \* upm) | 1160 |
| hhea.descender | Set to look comfortable (\~0.288 \* upm) | -288 |
| hhea.descender | Set to look comfortable (\~-0.288 \* upm, should be negative) | -288 |
| hhea.lineGap | 0 | 0 |
| OS/2.usWinAscent | Same as hhea.ascent | 1160 |
| OS/2.usWinDescent | abs(value) of hhea.descent | 288 |
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