WIP: Compact Font Format (CFF) Version 2

[brianpopow]

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• I have provided test coverage for my change (where applicable)

Description

This PR aims to add support for CFF2 format. Its still a work in progress.

The Specification can be found here.

Introduction to open type variable fonts: introducing-opentype-variable

Relevant tables for the font variations are:

• fvar
• gvar
• avar
• hvar

Example font for CFF2:
RobotoFlex.zip

To see how the different variations settings affect the font the website wakamaifondue is usefull, just drop the example font there and go to the variations tab to experiment with different settings.

TODO:

• The parsing of the ItemVariations data does not match what fontkit parses, see comment
• We need some method to create a new font based on the variation settings a user chooses. For example: font.GetVariation(variationOptions). The equivalent in fontkit would be font.getVariation({wght: 300})
• Font class needs a method to get the available variation axes that this font supports. Note: FontMetrics class already has a TryGetVariationAxes
• Font class needs a method to get the the named variation instances that the font designer has specified.

[codecov]

Codecov Report

Attention: Patch coverage is 60.91549% with 333 lines in your changes missing coverage. Please review.

Project coverage is 85%. Comparing base (977001a) to head (606f16b).
Report is 34 commits behind head on main.

❗ Current head 606f16b differs from pull request most recent head ff606eb

Please upload reports for the commit ff606eb to get more accurate results.

Files	Patch %	Lines
src/SixLabors.Fonts/Tables/Cff/Cff2Parser.cs	0%	101 Missing ⚠️
...dTypographic/Variations/GlyphVariationProcessor.cs	31%	59 Missing and 1 partial ⚠️
src/SixLabors.Fonts/Tables/Cff/CffParserBase.cs	71%	31 Missing and 13 partials ⚠️
.../SixLabors.Fonts/Tables/Cff/CffEvaluationEngine.cs	17%	21 Missing and 2 partials ⚠️
src/SixLabors.Fonts/Tables/Cff/Cff2Table.cs	0%	17 Missing ⚠️
...s/AdvancedTypographic/Variations/TupleVariation.cs	61%	11 Missing and 2 partials ⚠️
...AdvancedTypographic/Variations/DeltaSetIndexMap.cs	57%	10 Missing and 2 partials ⚠️
...ancedTypographic/Variations/VariationRegionList.cs	68%	7 Missing and 3 partials ⚠️
...vancedTypographic/Variations/ItemVariationStore.cs	69%	5 Missing and 3 partials ⚠️
src/SixLabors.Fonts/StreamFontMetrics.cs	72%	2 Missing and 4 partials ⚠️
... and 14 more

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #342    +/-   ##
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- Coverage     86%     85%    -2%     
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  Files        238     259    +21     
  Lines      15184   15865   +681     
  Branches    2101    2205   +104     
======================================
+ Hits       13088   13488   +400     
- Misses      1613    1871   +258     
- Partials     483     506    +23     

Flag	Coverage Δ
unittests	85% <60%> (-2%)	⬇️

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[JimBobSquarePants]

@brianpopow I had a quick look at the ItemVariationData and spotted an issue with the longwords check however I'm not sure how you tested the values here. How did you compare the values?

[brianpopow]

@brianpopow I had a quick look at the ItemVariationData and spotted an issue with the longwords check however I'm not sure how you tested the values here. How did you compare the values?

I use the tagged version 1.8.0 of fontkit and the CFF2 font RobotoFlex.ttf linked above.
To debug fontkit, I use a unit test. As IDE, I do use webstorm. For example create a new file example.js in test folder of fontkit with the following code:

import fontkit from '../src';
import assert from 'assert';
import fs from 'fs';

describe('metadata', function() {
    it('fontkit test', function() {
        let font = fontkit.openSync(__dirname + '/data/RobotoFlex.ttf');
        var variationFont = font.getVariation({wght: 300})
        var run = font.layout("hello");
    });
});

This creates a new font variation with the weight 300 as an option. Then add a breakpoint, when the font is loaded and inspect font or variationFont. Look into the HVAR table and then into itemVariationStore -> itemVariationData.
For easier reading I usually do right click and Copy Json and past into into a text file.

[brianpopow]

@JimBobSquarePants I think the ItemVariationsData now looks equal to fontkit, maybe you could double check, if I have missed something.

[JimBobSquarePants]

Nice work. Looks like our indexes are arranged in a different order though?

[JimBobSquarePants]

OK. Figured that out. The two arrays were being concatenated in the wrong order.

I'll have to do a fair bit of code reading to be able to be more useful. Will start asap.

[JimBobSquarePants]

@brianpopow I've gone through and updated the PR to allow merging with the changes introduced during the submodule and target framework updates. The file-scoped namespace changes killed the ability to auto-merge. It looks like I've done everything correctly.

[avisra]

did this get dropped or still being worked on

[JimBobSquarePants]

Needs someone to finish it off. It’s a lot of work and we have very few contributors