CJK vertical metrics suggestion

[NightFurySL2001]

According to https://googlefonts.github.io/gf-guide/metrics.html#cjk-vertical-metrics, it is suggested that the OS/2.sTypoAscender is set to 0.88 * font UPM and the OS/2.sTypoDescender is set to -0.12 * font UPM. The table is attached below:

Attrib	Value	Example using 1000upm font
OS/2.sTypoAscender	0.88 * font upm	880
OS/2.sTypoDescender	-0.12 * font upm	-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.lineGap	0	0
OS/2.usWinAscent	Same as hhea.ascent	1160
OS/2.usWinDescent	abs(value) of hhea.descent	288
OS/2.fsSelection bit 7 (Use_Typo_Metrics)	Do not set / disabled	0

Our decision to follow the Adobe schema was based on Dr. Ken Lunde’s comments and his release notes on Source Han Sans:

• Line spacing/vertical metrics approach discussion thread source-foundry/font-line#2
• SourceHanSansReadMe.pdf

However, these metrics are only used in Japanese fonts. In Korean font (such as IBM Plex KR developed by Sandoll Inc), the ratio used is 0.8:0.2, which some Chinese font foundries use too. Another typical Chinese font foundry used is 0.85:0.15, as this allows more descender space compared to 0.88:0.12. There are some other combination of values which is used for individual creators too.

The referenced issue provided to justify these values (source-foundry/font-line#2) only explicitly mention this (quoted from @kenlunde):

To add to the discussion with regard to the sTypoAscender and sTypoDescender values in the 'OS/2' table, while I cannot speak with any authority to non-CJK fonts, at least for CJK fonts I feel somewhat strongly that these values 1) add up to equal the UPM and 2) correspond to the top and bottom of the ideographic em-box in which CJK glyphs are optically centered. For the other vertical metrics values, I see little or no disagreement.

Thus, I would like to suggest the guide to change the guidelines to allow any ratio for the OS/2 sTypo values as long as:

• OS/2.sTypoAscender - OS/2.sTypoDescender = UPM (In the sample it is 880 - (-120) = 1000)
• OS/2.sTypoAscender and OS/2.sTypoDescender reflects the CJK ideographic bounding box only, not any other values.

[RosaWagner]

That is good info, thanks.

@davelab6 @chrissimpkins should we update the vertical metric policy for CJK fonts accordingly?

cc @vv-monsalve @m4rc1e @aaronbell

[NightFurySL2001]

This would also fix the issue of IBM Plex KR is different from its source file as the original KR font uses 780/-220. (Issue reference)

[yisibl]

Why is it recommended to disable Use_Typo_Metrics here?

[NightFurySL2001]

OS/2.sTypoAscender and OS/2.sTypoDescender are used for CJK bounding box. Setting Use_Typo_Metrics will make the font look shorter (usually).
Source quote:

Else If this is a CJK font:

ideoEmboxBottom = OS/2.sTypoDescender
ideoEmboxTop = OS/2.sTypoAscender
ideoEmboxRight = head.unitsPerEm

[NightFurySL2001]

Any updates on changing the guidelines?

[vv-monsalve]

@aaronbell, this proposal requires CJK's expertise, could you please help us to review it?

[aaronbell]

The suggestion seems reasonable to me. Ken's method of 880 / -120 works well for standard CJK use, but I can see other ratios being employed depending on the requirements of the specific project. As long as the CJK bounding box aligns with the UPM, things should be alright.

[vv-monsalve]

Thanks @aaronbell. This would require a consensus before making the change here. Would you like to bring the subject to the next meeting as part of your current job in the CJK fonts?

[NightFurySL2001]

Quick notes:

1. hhea.descender should be negative, so (~0.288 * upm) should change to (~-0.288 * upm) with negative value. It seems that this can be easily confused (see nooriro/nooriro.github.io@2787e25). I have updated this in Update CJK vertical metrics requirements #135 .
2. Fontbakery check should be updated to disable typoascender_exceeds_Agrave when cjk_vertical_metrics is triggered. Errors could be raised with current setting (see Have reviewed the output with fontbakery? line/seed#4).

Checking that the typoAscender exceeds the yMax of the /Agrave.

Check ID: FontBakeryCheck:com.arrowtype.fonts/check/typoascender_exceeds_Agrave

🔥 FAIL OS/2.sTypoAscender value should be greater than 1008, but got 880 instead

[307587]

hi, coming from my issue above, may i suggest: add a 'short height variant' for the CJK family, like how 新一細明體 (2nd example font in the issue) is the 'short height variant' of 一點明體.