Add variable-aware ValueRecord and Anchor types to fea-rs #598
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anthrotype
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Nov 24, 2023
fea-rs/src/compile/lookups.rs
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| // the var_store is only used in GPOS, but we pass it everywhere | ||
| fn build(self, var_store: &mut VariationStoreBuilder) -> Self::Output; |
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it's feels a bit strange that one has to pass down a VariationStoreBuilder that is only used in some cases and not others (e.g. if only building GSUB table). Can this be avoided e.g. make it optional or add a different method to construct a lookup builder that has a VariationStoreBuilder?
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agree this is annoying, and unfortunately Option<&mut T> is annoying to work with, because it doesn't implement Copy, which means you need to do a dance to get the inner reference anywhere you need it, so I found this to be the best compromise.
I agree that this is confusing, though, and I will expand the comment to explain what's going on.
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These types store their deltas inline, which means that we don't need to have access to an ItemVariationStore in orderto create them. This, in turn, lets us move all of the concern over deltas to the end of the compile pass, and also lets us provide a nicer feature writer API.
This also tweaks the visibility of some fields.
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This does the minimum to get us compiling again; there's a larger patch to write that skips the intermediate step and just creates these types directly.
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Previously we used the
ValueRecordandAnchorTabletypes in write-fonts. This had the complication that anytime you needed to include deltas you had to have access to aVariationStoreBuilder, which was a bottleneck for fontc.With this change, we now just store the deltas inline in the record/anchor type, and only go and turn them into actual
write-fontstypes at the very end, after merging anything we've received from feature writers.This patch makes the fewest changes outside fea-rs that were possible in order to get things working again; as follow up we can go and figure out how best to update the kerning/marks code to take advantage of this new model.
This requires googlefonts/fontations#719; this includes a commit that patches Cargo.toml to point at that for the time being.