Avoid "reloading" TTFonts in PostProcessor.__init__

[madig]

Going from https://github.com/fonttools/fonttools/issues/1095, quite some time is spent serializing/deserializing fonts somewhere inbetween compiling the final binary. This slows down the entire process. PostProcessor.__init__ seems to "reload" every font, maybe to clean up loose data? I think it should maybe just work with what it's given?

[simoncozens]

One thing the postprocessor does is rename the glyphs to production names. It's very hard to do this without a save/load, because internally to fonttools, glyphs are referred to by name, not by GID. If you change alef-ar to uni0627 in GlyphOrder, but font["GSUB"].table.LookupList.Lookups[5].Subtable[0].InputCoverage[0] still refers to alef-ar, your font will no longer save. So it's easier to save and reload than to go through every subtable everywhere trying to remap things.

[madig]

Argh. That's what I'm running into locally... So, what do?

[anthrotype]

yes, what Simon said. however note that for VFs I think we do the glyph renaming only on the final VF, not on each master TTF

[anthrotype]

maybe the postprocessor should only do the reloading if it's going to actually rename glyphs, not unconditionally like now

[madig]

There's a few other things that happen, I'll push a branch.

[behdad]

Renaming glyphs is such a misunderstood op. One can build a font, make sure all tables are compiled, then do the renaming glyphs, which would involve just loading and saving the post table...

[behdad]

Or should even be possible to provide the rename mapping at load time.

[behdad]

So, a lot of this would be a non-issue if we were dealing with immutable data types, such that any modification would involve copy-on-write duplication... Many functional languages work that way. In Python, I'm sure there's some dark black magic with metaclasses to get some of that, but there will be dragons. Also, there's no way to get compile-time errors for violations with Python.

Anyway, attrs2 on this: https://www.attrs.org/en/stable/examples.html#immutability

[madig]

I'd take some immutability, but the issues I'm seeing are data normalization (e.g. post.italicAngle of 0.0 does not get normalized to 0 like when reloading), data sorting (e.g. name table being in a different order) and glyph names staying as they are :neutral_face:

[behdad]

I see.

[madig]

So, what do? Ideally the compilation pass spits out something that we don't need to reload to make it mergable. Or we reload as little as possible? Any insights appreciated over at https://github.com/googlefonts/ufo2ft/pull/486.

[behdad]

So, what do? Ideally the compilation pass spits out something that we don't need to reload to make it mergable.

Yes, I'm happy to work on making that reality.

Or we reload as little as possible? Any insights appreciated over at #486.

Okay great. That gives me a point to start. Let's continue there.

[madig]

So, going from https://github.com/googlefonts/ufo2ft/pull/486, I stumbled over the following when reloading strictly only for renaming or post version changes:

• maxp table missing values
• various post attributes being filled with verbatim Python types rather than the final value
• the post extraNames list not being filled in
• the STAT table not having AxisValueCount filled in
• the name table being unsorted

Doing in the following in the middle of PostProcessor.process makes tests pass:

        # Sort the name table in case varLib added new entries.
        if "name" in self.otf:
            self.otf["name"].names.sort()
        if "maxp" in self.otf:
            self.otf["maxp"].compile(self.otf)
        if "STAT" in self.otf:
            data = self.otf["STAT"].compile(self.otf)
            self.otf["STAT"].decompile(data, self.otf)
        if "post" in self.otf and self.otf["post"].formatType == 2.0:
            self.otf["post"].extraNames = []
            self.otf["post"].compile(self.otf)

[behdad]

• the post extraNames list not being filled in

We should completely remove extraNames. It serves no functional purpose.

[behdad]

• the STAT table not having AxisValueCount filled in

Just make the STAT builder set it. The Count values in all of otData are also redundant and could be removed if we are willing to take the breakage and update code depending on it.

[khaledhosny]

The Count values in all of otData are also redundant and could be removed

Yes, please! It always seemed odd to have a separate Count field when the data is stored in a Python list anyway.

[behdad]

I feel like a fonttools API break is gaining support...

[madig]

Would this conflict with fT's ability to carry and serialize meaningless/wrong input data in loaded fonts or is it sufficiently high level that that makes little sense? Is that even a design goal?

[khaledhosny]

These *Counts are written as comments in TTX, so probably can be kept regardless of the API change. In Python, LookupCount would be replaced by len(Lookup) and so on. May be the first step would to stop using these *Counts internally but set them when decompiling, so code building tables from scratch don’t have to set them, and later be removed entirely.

[behdad]

Would this conflict with fT's ability to carry and serialize meaningless/wrong input data in loaded fonts or is it sufficiently high level that that makes little sense?

The count values are always ignored when compiling the font.

Is that even a design goal?

Nope. We don't even pretend.

These *Counts are written as comments in TTX, so probably can be kept regardless of the API change. In Python, LookupCount would be replaced by len(Lookup) and so on. May be the first step would to stop using these *Counts internally but set them when decompiling, so code building tables from scratch don’t have to set them, and later be removed entirely.

Or replace them with properties that warn when accessed; set a deadline for removing them, and advise code to switch. The replacement code can even be offered in the warning.