8000 feaLib shouldn’t ignore subtable statements · Issue #1279 · fonttools/fonttools · GitHub
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feaLib shouldn’t ignore subtable statements #1279

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moyogo opened this issue Jun 21, 2018 · 4 comments
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feaLib shouldn’t ignore subtable statements #1279

moyogo opened this issue Jun 21, 2018 · 4 comments
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@moyogo
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moyogo commented Jun 21, 2018

The AFDKO syntax spec says:

4.g. subtable
The feature file implementation must insert subtable breaks among the rules for a particular lookup if needed. For example, if a set of alternate substitution rules specified in the feature file exceeds the subtable size limit, several subtables must be automatically created.

The "subtable" statement may be used as follows:
subtable;
to explicitly force a subtable break after the previous rule.

This is intended for use with only Pair Adjustment Positioning Format 2 (i.e. pair class kerning). See 6.b.iii for details. This hint to the compiler is required for class kerning, as it is difficult for a compiler to determine where to best place a subtable break in this lookup type, in order to reduce lookup size.

I tested writing a kern feature lookup with inserted subtable statements. As is, fonttools generates GPOS that is 94114 bytes for one of our fonts. If it was applying the subtable statements the GPOS would be 55712 bytes.
@miguelsousa
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Judging from comments that Behdad has made about the results of otf-making-benchmark I don't think it's the ignoring of subtable that's causing the significant size difference; makeotf uses some tricks to reduce the size of GPOS that are not implemented in feaLib.

@miguelsousa
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@moyogo see adobe-type-tools/otf-making-benchmark#1
I haven't had any time to dedicate to this project unfortunately.

@moyogo
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moyogo commented Jun 25, 2018

I think these are two separate issues. While a lookup without any subtable statements should be compiled as efficiently as possible, a lookup with subtable statements should be compiled as specified.

@anthrotype
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it's in the spec, so I guess we should support manual subtable breaks.

anthrotype added a commit to anthrotype/fonttools that referenced this issue Jul 24, 2018
@anthrotype
feaLib: support explicit 'subtable' statements in PairPos lookups
9456ae9 
Fixes fonttools#1279
@anthrotype anthrotype mentioned this issue Jul 24, 2018
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