stb_truetype.h: Support Font Variations

[kphillisjr]

I figure this is probably goes along with adding support for hinting. However there is some fonts where a lot of data is missing because of the lack of support for Font Variations. The two major variations for this are as follows:

Variation 1: Apple Advanced Typography (AAT) - This is Apple's Variation on TrueType Fonts, and has a slightly different signature than OpenType. This is still a good reference since a lot of the behavior is the same in this, and OpenType. The only difference may be the table name is changed.

Variation 2: OpenType Version 1.8.2 or newer - This includes most of the tables used by Apple, and adds a few mores. Also, Collections for OpenType use the OTC file extension.

To help with implementation, I have a list of the tables that need implementing to add this feature, and you can find the list in the table below...

Table Name	Description	AAT Support	OpenType Support
avar	Axis Variations Table	Y	Y
cvar	Control Value Variations table.	Y	Y
fvar	Font Variation Table	Y	Y
gvar	Glyph Variation Table	Y	Y
name	Name Table	Y	Y
HVAR	Horizontal Metrics Variations	N	Y
MVAR	Metrics Variations Table	N	Y
VVAR	Vertical Metrics Variations	N	Y
ttcf	TrueType Collections Table	N	Y

Also, Since there is a need for fonts to test to make sure this works, you can find the fonts in the following list. In general these fonts have have two downloadable versions where one version includes the Font Variations, and the other version has the fonts variations as separate font files.

• Adobe Fonts: Note: This font set is currently the best option to test this functionality for users who are not using Chinese/Japanese/Korean (aka CJK).
  • Adobe Source Code Pro - https://github.com/adobe-fonts/source-code-pro
  • Adobe Source Sans Pro - https://github.com/adobe-fonts/source-sans-pro
  • Adobe Source Serif Pro - https://github.com/adobe-fonts/source-serif-pro
• Google's Noto Fonts - This has a lot of fonts, and variations. I recommend just looking at the "Super OTC" version since this is most likely going to be the font which can generate problems since a single font can contain 30+ fonts in this case.
  • Noto Sans CJK
  • Noto Sans Mono CJK- This font is merged as a sub-font of the Noto Sans CJK Super OTC Font.
  • Noto Serif CJK

[nothings]

I am unlikely to tackle this problem anytime soon. as variation support looks non-trivial.

(But FWIW stb_truetype already supports the 'ttcf' table with "stbtt_GetFontOffsetForIndex()", and has some (possibly useless) support for the 'name' table.)

[kphillisjr]

After seeing that this is probably a non-trivial change, I went ahead and saw what changes would be required to see how Implementing support for this feature request would work. For the most part this feature request entails fixing the remaining features found in OpenType Version 1.7, and then going about implementing features found in OpenType version 1.8.x.

Stage 1: add some of the missing features found OpenType Version 1.7...

• [ ] Implement support for the F2DOT14 format.
• [ ] Fix the Glyph Data Table (glyf) implementation. This table also contains information on Font Scaling and uses the F2DOT14 format to store this information.
• [ ] Implement Support for Vertical Font Metrics.
  • [ ] Vertical Header Table (vhea)
  • [ ] Vertical Metrics Table(vmtx)
• [ ] Optional Implement Baseline Table (BASE) Support. This table is used to help with text flow when fonts of various sizes, and styles are mixed together.
• [ ] Optional: Implement Vertical Origin Table(VORG)
• [ ] Optional Implement support for the Glyph Positioning Table (GPOS).

Stage 2: Actually Implement Font Variations.

• [ ] Add Functionality to retrieve strings from the Naming Table (name). This is important since this table contains not only the font name, but the name of the font variations.
• [ ] Implement functions to query, and select font variations.
• [ ] Required: Font Variations Table (fvar) table. - This is required by all of the following tables.
• [ ] Required: Style Attributes Table ('STAT') - This the various Named axis.
• [ ] Fix Kerning Support for Variable Fonts. This is mostly improving on the Glyph Position Table.
• [ ] Implement Font Variations for TrueType Outlines
  • [ ] Implement Glyph Variations Table ('gvar') - This modifies the Glyph data outlined by 'glyf' Table.
  • [ ] CVT Variations Table ('cvar') - This modifies the Control Value Table data.
• [ ] Implement Various Font-Axis Table Support
  • [ ] Axis Variations Table (avar) - This controls how the Glyphs are interpolated.
  • [ ] Font Weight Axis ('wght')
  • [ ] Font Width Axis ('wdth')
  • [ ] Italic axis. ('ital')
  • [ ] Optical size Axis ('opsz')
  • [ ] Slant Axis ('slnt')
  • [ ] Implement Custom Axis Names - In general these are linked to the previous axis values.
• [ ] Implement Font Metric Modifications
  • [ ] Per-Font variation Font Metrics: Metrics Variations Table (MVAR)
  • [ ] Horizontal Font Metrics Modifications: Horizontal Metrics Variations Table(HVAR)
  • [ ] Vertical Font Metrics Modifications: Vertical Metrics Variations Table(VVAR) - This requires the Vertical Font metric support.
• [ ] Optional: Implement support for Compact Font Format (CFF) Version 2 (the CFF2 table)

Note: Currently very few fonts exist using the CFF2 table, and the best example I can find is the Adobe Variable Font Prototype

[coderwj]

When multi styles mixed in a single font file, How can I choose a style?
It has not been supported , Right? (I see your comments "Optional Implement Baseline Table (BASE) Support. This table is used to help with text flow when fonts of various sizes, and styles are mixed together.") Please help me~ thanks~

[abainbridge]

Note: Currently very few fonts exist using the CFF2 table, and the best example I can find is the [Adobe Variable Font Prototype]

The system fonts on Android 15 now use CFF2. Or at least the one my app loads seems to (/system/fonts/NotoSansCJK-Regular.ttc).

Here's what TTX has to say about the first font in that collection:

C:\Users\andy>ttx -y 0 -l c:\tmp\NotoSansCJK-Regular.ttc
Listing table info for "c:\tmp\NotoSansCJK-Regular.ttc":
    tag     checksum    length    offset
    ----  ----------  --------  --------
    BASE  0x6C1B8E98       278  29739416
    CFF2  0xA4D5711E  29217242       420
    GDEF  0x8002BFBB       144  29739696
    GPOS  0x32E1C03E     78320  29739840
    GSUB  0x8F893223    172968  29818160
    HVAR  0x78307A25    131120  29991128
    OS/2  0x92EC0EE3        96  29479892
    STAT  0x8B096E6B       116  30122248
    VORG  0x91052902       988  30122364
    VVAR  0xAE9C8E20     59450  30123352
    avar  0x3AC93334        30  30182888
    cmap  0xBA0B23CA    257183  29479988
    fvar  0x93AA699B        84  30182804
    gasp  0x00000010         8  29739408
    head  0x25B78687        54  29217664
    hhea  0x0C1B084F        36  29479856
    hmtx  0x2BEA5830    262134  29217720
    maxp  0xFFFF5000         6       412
    name  0xCA16F7FE      2195  29737180
    post  0xFF860032        32  29739376
    prep  0x68068C85         7  29737172
    vhea  0x0CBB1587        36  30444308
    vmtx  0x9D93F118    261386  30182920

BTW, thanks for making stb_truetype.h.

[j-kaltes]

Yes, because /system/fonts/NotoSansCJK-Regular.ttc became CFF2 under Android 15 and 16, OpenGL apps can only use NotoSerifCJK-Regular.ttc for Chinese or increase the size of their app by 20 megabyte.