Compare OTFs

[davelab6]

We only compare TTFs for now, not OTFs.

• [ ] Make the documentation clearer on this
• [ ] Add a filetype catcher to cleanly reject fonts (eg when there is no glyf table (even a filename catcher on .otf would be better than today)
• [ ] Support comparing OTFs

Example error when passing in OTFs:

$ diffenator ~/Downloads/NotoSansGrantha-Regular.otf ~/Downloads/mNotoSansGrantha-Regular.otf 

'diff_kerning'  8.96 ms

Traceback (most recent call last):

  File "/usr/local/bin/diffenator", line 11, in <module>

    load_entry_point('fontdiffenator==0.3.1', 'console_scripts', 'diffenator')()

  File "/usr/local/lib/python2.7/site-packages/diffenator/__main__.py", line 54, in main

    font_b,

  File "/usr/local/lib/python2.7/site-packages/diffenator/diff.py", line 126, in diff_fonts

    ('metrics', diff_metrics(font_a, font_b)),

  File "/usr/local/lib/python2.7/site-packages/diffenator/diff.py", line 101, in timed

    result = method(*args, **kw)

  File "/usr/local/lib/python2.7/site-packages/diffenator/diff.py", line 264, in diff_metrics

    metrics_a = dump_glyph_metrics(font_a)

  File "/usr/local/lib/python2.7/site-packages/diffenator/metrics.py", line 23, in dump_glyph_metrics

    return [glyph_metrics(font, g) for g in glyphs]

  File "/usr/local/lib/python2.7/site-packages/diffenator/metrics.py", line 8, in glyph_metrics

    lsb = _glyph_lsb(font, glyph)

  File "/usr/local/lib/python2.7/site-packages/diffenator/metrics.py", line 35, in _glyph_lsb

    return font['glyf'][glyph].xMin

  File "/usr/local/lib/python2.7/site-packages/fontTools/ttLib/ttFont.py", line 382, in __getitem__

    data = self.reader[tag]

  File "/usr/local/lib/python2.7/site-packages/fontTools/ttLib/sfnt.py", line 104, in __getitem__

    entry = self.tables[Tag(tag)]

KeyError: 'glyf'

[marekjez86]

@davelab6 , @m4rc1e : I got the same error. Now I know why

[anthrotype]

the hmtx table has advance widths and left side bearings, works for both TTF and CFF-flavored OpenType. The _TTGlyph objects that you get with TTFont.getGlyphSet() already expose width and lsb attributes (from htmx) for both the OT flavors. https://github.com/fonttools/fonttools/blob/7d59b729ab996af6fe08411267eaad0bec1a2615/Lib/fontTools/ttLib/ttFont.py#L732

The right side bearings can be inferred from the advance width and the bounding box xMax coordinate (from glyf table). For CFF you need to compute the bounds, e.g. see https://github.com/fonttools/fonttools/blob/7d59b729ab996af6fe08411267eaad0bec1a2615/Lib/fontTools/ttLib/tables/_h_h_e_a.py#L66-L70

[m4rc1e]

Thanks for the pointers @anthrotype!

[rsheeter]

What needs to be done to support comparing OTF? Are we able to enumerate the tasks or do we currently have no idea?

[m4rc1e]

Off the top of my head, I can only think of:

• Write CFF checks which are similar to our glyf checks.
• Uncouple ttf logic.

I originally wrote this tool only with ttfs in mind. If we want otf support, I'll need to refactor some of the logic so it's more flexible. Luckily, most of it is kept inside the diffenator.font module.

I also don't know how well hb, freetype and fonttools.varLib support otf fonts. I need to check this.

When will otf support arrive in Google Fonts?

[rsheeter]

We support and ship OTFs today.

[m4rc1e]

We can now diff and dump otfs.

Still not sure how well it will work for CFF VFs so I'm going to leave this open.

[marekjez86]

great news. @m4rc1e : thanks