Say which glyphs the built-in font actually has - #462
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A rune with no glyph still ADVANCES -- deliberately, so that a column of text stays lined up -- so measuring a string says nothing about whether it can be read. Until now the only way to know was to read the table, and the table was unexported, so the answer was folklore. The folklore was wrong in both directions. go-xrkit/desk carries a hand-written rule forbidding apostrophes and dashes in its settings window, arrived at after "the glasses' own menu bar" came out with a hole in it. But the plain hyphen draws fine (5 pixels), and that same window ships "Turn this Mac's screen off" -- an apostrophe, which does not. BitmapCovers(r) and BitmapMissing(s) answer it. A consumer can now assert its own strings instead of remembering a rule, which is the difference between a rule and a guard. ⚠ For the BUILT-IN font only. A TrueType face has far wider coverage; this is the fallback, and the fallback is what a platform with no system face falls back TO. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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A rune with no glyph still advances — deliberately, so that a column of text stays lined up — so measuring a string says nothing about whether it can be read. Until now the only way to know was to read the table, and the table was unexported, so the answer was folklore.
The folklore was wrong in both directions
go-xrkit/deskcarries a hand-written rule forbidding apostrophes and dashes in its settings window, arrived at after "the glasses' own menu bar" came out with a hole in it.Measured:
'and’-—éAnd that same window ships
Turn this Mac's screen off— an apostrophe, which does not draw. The rule is both incomplete and unenforced.What this adds
BitmapCovers(r)andBitmapMissing(s). A consumer can now assert its own strings instead of remembering a rule — which is the difference between a rule and a guard.The test also pins the property that makes this necessary: a covered and an uncovered rune measure the same. If they ever differed, callers could have used width to detect coverage, and this API would be redundant.
⚠ For the built-in font only. A TrueType face has far wider coverage; this is the fallback, and the fallback is what a platform with no system face falls back to.
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