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font: more robust extraction of vertical metrics from tables

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Previously always assuming the typo metrics were good caused some fonts
to have abnormally short cell heights.
This commit is contained in:
Qwerasd
2024-12-13 12:46:36 -05:00
parent 13dd4bd897
commit 8a5d484729
3 changed files with 119 additions and 7 deletions
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1
pkg/freetype/face.zig
View File

@ -217,6 +217,7 @@ pub const SfntTag = enum(c_int) {
.os2 => c.TT_OS2,
.head => c.TT_Header,
.post => c.TT_Postscript,
.hhea => c.TT_HoriHeader,
else => unreachable, // As-needed...
};
}

69
src/font/face/coretext.zig
View File

@ -536,6 +536,7 @@ pub const Face = struct {
InvalidPostTable,
InvalidOS2Table,
OS2VersionNotSupported,
InvalidHheaTable,
};
fn calcMetrics(ct_font: *macos.text.Font) CalcMetricsError!font.face.Metrics {
@ -563,7 +564,7 @@ pub const Face = struct {
const len = data.getLength();
break :post opentype.Post.init(ptr[0..len]) catch |err| {
return switch (err) {
error.EndOfStream => error.InvalidOS2Table,
error.EndOfStream => error.InvalidPostTable,
};
};
};
@ -583,13 +584,73 @@ pub const Face = struct {
};
};
// Read the 'hhea' table out of the font data.
const hhea: opentype.Hhea = hhea: {
const tag = macos.text.FontTableTag.init("hhea");
const data = ct_font.copyTable(tag) orelse return error.CopyTableError;
defer data.release();
const ptr = data.getPointer();
const len = data.getLength();
break :hhea opentype.Hhea.init(ptr[0..len]) catch |err| {
return switch (err) {
error.EndOfStream => error.InvalidHheaTable,
};
};
};
const units_per_em: f64 = @floatFromInt(head.unitsPerEm);
const px_per_em: f64 = ct_font.getSize();
const px_per_unit: f64 = px_per_em / units_per_em;
const ascent = @as(f64, @floatFromInt(os2.sTypoAscender)) * px_per_unit;
const descent = @as(f64, @floatFromInt(os2.sTypoDescender)) * px_per_unit;
const line_gap = @as(f64, @floatFromInt(os2.sTypoLineGap)) * px_per_unit;
const ascent: f64, const descent: f64, const line_gap: f64 = vertical_metrics: {
const os2_ascent: f64 = @floatFromInt(os2.sTypoAscender);
const os2_descent: f64 = @floatFromInt(os2.sTypoDescender);
const os2_line_gap: f64 = @floatFromInt(os2.sTypoLineGap);
// If the font says to use typo metrics, trust it.
if (os2.fsSelection.use_typo_metrics) {
break :vertical_metrics .{
os2_ascent * px_per_unit,
os2_descent * px_per_unit,
os2_line_gap * px_per_unit,
};
}
// Otherwise we prefer the height metrics from 'hhea' if they
// are available, or else OS/2 sTypo* metrics, and if all else
// fails then we use OS/2 usWin* metrics.
//
// This is not "standard" behavior, but it's our best bet to
// account for fonts being... just weird. It's pretty much what
// FreeType does to get its generic ascent and descent metrics.
if (hhea.ascender != 0 or hhea.descender != 0) {
const hhea_ascent: f64 = @floatFromInt(hhea.ascender);
const hhea_descent: f64 = @floatFromInt(hhea.descender);
const hhea_line_gap: f64 = @floatFromInt(hhea.lineGap);
break :vertical_metrics .{
hhea_ascent * px_per_unit,
hhea_descent * px_per_unit,
hhea_line_gap * px_per_unit,
};
}
if (os2_ascent != 0 or os2_descent != 0) {
break :vertical_metrics .{
os2_ascent * px_per_unit,
os2_descent * px_per_unit,
os2_line_gap * px_per_unit,
};
}
const win_ascent: f64 = @floatFromInt(os2.usWinAscent);
const win_descent: f64 = @floatFromInt(os2.usWinDescent);
break :vertical_metrics .{
win_ascent * px_per_unit,
win_descent * px_per_unit,
0.0,
};
};
// Some fonts have degenerate 'post' tables where the underline
// thickness (and often position) are 0. We consider them null

56
src/font/face/freetype.zig
View File

@ -631,6 +631,9 @@ pub const Face = struct {
// Read the 'OS/2' table out of the font data.
const os2 = face.getSfntTable(.os2) orelse return error.CopyTableError;
// Read the 'hhea' table out of the font data.
const hhea = face.getSfntTable(.hhea) orelse return error.CopyTableError;
// Some fonts don't actually have an OS/2 table, which
// we need in order to do the metrics calculations, in
// such cases FreeType sets the version to 0xFFFF
@ -640,9 +643,56 @@ pub const Face = struct {
const px_per_em: f64 = @floatFromInt(size_metrics.y_ppem);
const px_per_unit = px_per_em / @as(f64, @floatFromInt(units_per_em));
const ascent = @as(f64, @floatFromInt(os2.sTypoAscender)) * px_per_unit;
const descent = @as(f64, @floatFromInt(os2.sTypoDescender)) * px_per_unit;
const line_gap = @as(f64, @floatFromInt(os2.sTypoLineGap)) * px_per_unit;
const ascent: f64, const descent: f64, const line_gap: f64 = vertical_metrics: {
const os2_ascent: f64 = @floatFromInt(os2.sTypoAscender);
const os2_descent: f64 = @floatFromInt(os2.sTypoDescender);
const os2_line_gap: f64 = @floatFromInt(os2.sTypoLineGap);
// If the font says to use typo metrics, trust it.
// (The USE_TYPO_METRICS bit is bit 7)
if (os2.fsSelection & (1 << 7) != 0) {
break :vertical_metrics .{
os2_ascent * px_per_unit,
os2_descent * px_per_unit,
os2_line_gap * px_per_unit,
};
}
// Otherwise we prefer the height metrics from 'hhea' if they
// are available, or else OS/2 sTypo* metrics, and if all else
// fails then we use OS/2 usWin* metrics.
//
// This is not "standard" behavior, but it's our best bet to
// account for fonts being... just weird. It's pretty much what
// FreeType does to get its generic ascent and descent metrics.
if (hhea.Ascender != 0 or hhea.Descender != 0) {
const hhea_ascent: f64 = @floatFromInt(hhea.Ascender);
const hhea_descent: f64 = @floatFromInt(hhea.Descender);
const hhea_line_gap: f64 = @floatFromInt(hhea.Line_Gap);
break :vertical_metrics .{
hhea_ascent * px_per_unit,
hhea_descent * px_per_unit,
hhea_line_gap * px_per_unit,
};
}
if (os2_ascent != 0 or os2_descent != 0) {
break :vertical_metrics .{
os2_ascent * px_per_unit,
os2_descent * px_per_unit,
os2_line_gap * px_per_unit,
};
}
const win_ascent: f64 = @floatFromInt(os2.usWinAscent);
const win_descent: f64 = @floatFromInt(os2.usWinDescent);
break :vertical_metrics .{
win_ascent * px_per_unit,
win_descent * px_per_unit,
0.0,
};
};
// Some fonts have degenerate 'post' tables where the underline
// thickness (and often position) are 0. We consider them null