[read-fonts] experimental Font API: add GlobalMetrics - #2111
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looks good, a few ignorable questions inline
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| #[derive(Clone)] | ||
| enum Repr { | ||
| Default(Font), |
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Is this only used for variable fonts, or also statics?
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Exactly the right question, and the motivation for this change. This allows future code to accept only FontInstance while providing a free (sans atomic counter bump) conversion from Font for both statics and variable fonts at default location.
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| impl From<&Font> for FontInstance { | ||
| /// A font is an instance of itself, at the default location. | ||
| fn from(font: &Font) -> Self { | ||
| Self(Repr::Default(font.clone())) | ||
| } | ||
| } |
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Good catch. Both conversions make sense here.
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| fn widen(value: Fixed) -> F48Dot16 { | ||
| F48Dot16::from_bits(value.to_bits() as i64) | ||
| } |
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should this be a method on Fixed? (maybe not, if it's only internal)
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and it already is! Fixed (pun intended?)
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…tion GlobalMetrics reads head, maxp, hhea, vhea and OS/2, and applies MVAR at a location. A font states its line metrics three times and the three disagree, so all three are reported and choosing between them is left to the caller. Measurements are exact, since a location can move one off a whole design unit and rounding here would decide for every caller how that fraction is spent. FontInstance drops its size. Size belongs to whatever rasterizes, not to a point in the design space, and nothing read it. What remains is a font and a location, so an instance at the default location is now held as nothing more than its font and building one allocates nothing. Anywhere else it owns what the location implies, behind an Arc so that clones share a single resolution rather than repeating it. An instance at the default location now selects no feature variations. A variable font is meant to keep working for a client that knows nothing of variations, and such a client reaches the default instance without ever consulting that table; selecting there would render the font one way for it and another for everyone else. This changes what a font stating a universal condition set reports at the default location, which previously matched because conditions were evaluated against implicit zeros. Font gains num_glyphs, units_per_em and default_instance.
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Adds font-wide metrics to
FontandFontInstance. The list of metrics is slightly different here than inskrifa::metrics::Metricsbecause this slices the set to the values that don't require theposttable (those will follow later inStyleMetrics). The reasoning is that none of those are required for shaping so we don't want to pull largepost2.0 tables unnecessarily.Also removes any notion of size from
FontInstance(that will come later as someScalertype), soFontInstancebecomes internallyArc'd and we can now create an instance at the default location for free.Requires #2110