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font/coretext: properly resolve metrics for bitmap-only fonts

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macOS bitmap-only fonts are a poorly documented format, which are often
distributed as `.dfont` or `.dfon` files. They use a 'bhed' table in
place of the usual 'head', but the table format is byte-identical, so
enabling the use of bitmap-only fonts only requires us to properly fetch
this table while calculating metrics.

ref: https://fontforge.org/docs/techref/bitmaponlysfnt.html
This commit is contained in:
Qwerasd
2024-12-30 14:44:30 -05:00
committed by Jonathan Lopez
parent 97a5e463f3
commit a300c0c153
1 changed files with 11 additions and 2 deletions
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13
src/font/face/coretext.zig
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@ -515,8 +515,17 @@ pub const Face = struct {
fn calcMetrics(ct_font: *macos.text.Font) CalcMetricsError!font.face.Metrics { fn calcMetrics(ct_font: *macos.text.Font) CalcMetricsError!font.face.Metrics {
// Read the 'head' table out of the font data. // Read the 'head' table out of the font data.
const head: opentype.Head = head: { const head: opentype.Head = head: {
const tag = macos.text.FontTableTag.init("head"); // macOS bitmap-only fonts use a 'bhed' tag rather than 'head', but
const data = ct_font.copyTable(tag) orelse return error.CopyTableError; // the table format is byte-identical to the 'head' table, so if we
// can't find 'head' we try 'bhed' instead before failing.
//
// ref: https://fontforge.org/docs/techref/bitmaponlysfnt.html
const head_tag = macos.text.FontTableTag.init("head");
const bhed_tag = macos.text.FontTableTag.init("bhed");
const data =
ct_font.copyTable(head_tag) orelse
ct_font.copyTable(bhed_tag) orelse
return error.CopyTableError;
defer data.release(); defer data.release();
const ptr = data.getPointer(); const ptr = data.getPointer();
const len = data.getLength(); const len = data.getLength();