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Handle fonts with missing or invalid OS/2 tables (#2985)

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It turns out this happens more than I thought. Most notably, apple's
fallback font for a bunch of symbols, `Apple Gothic`, has no OS/2 table
🙃. I've gone ahead and fixed it so that both CoreText and FreeType
handle this gracefully.

Speaking of graceful handling, this problem came to attention due to our
very *un*-graceful handling of when a font fails to load due to a
metrics calculation error. It results in a bunch of blanked out text,
very bad, we should fix that.

*At some point I'll try to collect a group of test fonts with all sorts
of weirdnesses to exercise the weird edge cases in our handling.*
This commit is contained in:
Mitchell Hashimoto
2024-12-16 12:43:59 -08:00
committed by GitHub
parent bd48dcc748 2b78ac4382
commit 56f285cbc2
3 changed files with 180 additions and 146 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
pkg/macos/text/font.zig
View File

@ -217,6 +217,10 @@ pub const FontOrientation = enum(c_uint) {
pub const FontTableTag = enum(u32) {
svg = c.kCTFontTableSVG,
os2 = c.kCTFontTableOS2,
head = c.kCTFontTableHead,
hhea = c.kCTFontTableHhea,
post = c.kCTFontTablePost,
_,
pub fn init(v: *const [4]u8) FontTableTag {

75
src/font/face/coretext.zig
View File

@ -534,8 +534,6 @@ pub const Face = struct {
CopyTableError,
InvalidHeadTable,
InvalidPostTable,
InvalidOS2Table,
OS2VersionNotSupported,
InvalidHheaTable,
};
@ -569,18 +567,16 @@ pub const Face = struct {
};
};
// Read the 'OS/2' table out of the font data.
const os2: opentype.OS2 = os2: {
// Read the 'OS/2' table out of the font data if it's available.
const os2_: ?opentype.OS2 = os2: {
const tag = macos.text.FontTableTag.init("OS/2");
const data = ct_font.copyTable(tag) orelse return error.CopyTableError;
const data = ct_font.copyTable(tag) orelse break :os2 null;
defer data.release();
const ptr = data.getPointer();
const len = data.getLength();
break :os2 opentype.OS2.init(ptr[0..len]) catch |err| {
return switch (err) {
error.EndOfStream => error.InvalidOS2Table,
error.OS2VersionNotSupported => error.OS2VersionNotSupported,
};
log.warn("error parsing OS/2 table: {}", .{err});
break :os2 null;
};
};
@ -603,18 +599,21 @@ pub const Face = struct {
const px_per_unit: f64 = px_per_em / units_per_em;
const ascent: f64, const descent: f64, const line_gap: f64 = vertical_metrics: {
const hhea_ascent: f64 = @floatFromInt(hhea.ascender);
const hhea_descent: f64 = @floatFromInt(hhea.descender);
const hhea_line_gap: f64 = @floatFromInt(hhea.lineGap);
if (os2_) |os2| {
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 .{
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
@ -624,24 +623,17 @@ pub const Face = struct {
// 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 .{
if (hhea.ascender != 0 or hhea.descender != 0) 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 .{
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);
@ -652,6 +644,15 @@ pub const Face = struct {
-win_descent * px_per_unit,
0.0,
};
}
// If our font has no OS/2 table, then we just
// blindly use the metrics from the hhea table.
break :vertical_metrics .{
hhea_ascent * px_per_unit,
hhea_descent * px_per_unit,
hhea_line_gap * px_per_unit,
};
};
// Some fonts have degenerate 'post' tables where the underline
@ -672,30 +673,44 @@ pub const Face = struct {
@as(f64, @floatFromInt(post.underlineThickness)) * px_per_unit;
// Similar logic to the underline above.
const strikethrough_position, const strikethrough_thickness = st: {
const os2 = os2_ orelse break :st .{ null, null };
const has_broken_strikethrough = os2.yStrikeoutSize == 0;
const strikethrough_position: ?f64 = if (has_broken_strikethrough and os2.yStrikeoutPosition == 0)
const pos: ?f64 = if (has_broken_strikethrough and os2.yStrikeoutPosition == 0)
null
else
@as(f64, @floatFromInt(os2.yStrikeoutPosition)) * px_per_unit;
const strikethrough_thickness: ?f64 = if (has_broken_strikethrough)
const thick: ?f64 = if (has_broken_strikethrough)
null
else
@as(f64, @floatFromInt(os2.yStrikeoutSize)) * px_per_unit;
// We fall back to whatever CoreText does if
// the OS/2 table doesn't specify a cap height.
const cap_height: f64 = if (os2.sCapHeight) |sCapHeight|
break :st .{ pos, thick };
};
// We fall back to whatever CoreText does if the
// OS/2 table doesn't specify a cap or ex height.
const cap_height: f64, const ex_height: f64 = heights: {
const os2 = os2_ orelse break :heights .{
ct_font.getCapHeight(),
ct_font.getXHeight(),
};
break :heights .{
if (os2.sCapHeight) |sCapHeight|
@as(f64, @floatFromInt(sCapHeight)) * px_per_unit
else
ct_font.getCapHeight();
ct_font.getCapHeight(),
// Ditto for ex height.
const ex_height: f64 = if (os2.sxHeight) |sxHeight|
if (os2.sxHeight) |sxHeight|
@as(f64, @floatFromInt(sxHeight)) * px_per_unit
else
ct_font.getXHeight();
ct_font.getXHeight(),
};
};
// Cell width is calculated by calculating the widest width of the
// visible ASCII characters. Usually 'M' is widest but we just take

83
src/font/face/freetype.zig
View File

@ -600,7 +600,6 @@ pub const Face = struct {
const CalcMetricsError = error{
CopyTableError,
MissingOS2Table,
};
/// Calculate the metrics associated with a face. This is not public because
@ -629,21 +628,25 @@ pub const Face = struct {
const post = face.getSfntTable(.post) orelse return error.CopyTableError;
// Read the 'OS/2' table out of the font data.
const os2 = face.getSfntTable(.os2) orelse return error.CopyTableError;
const os2_: ?*freetype.c.TT_OS2 = os2: {
const os2 = face.getSfntTable(.os2) orelse break :os2 null;
if (os2.version == 0xFFFF) break :os2 null;
break :os2 os2;
};
// 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
if (os2.version == 0xFFFF) return error.MissingOS2Table;
const units_per_em = head.Units_Per_EM;
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: f64, const descent: f64, const line_gap: f64 = vertical_metrics: {
const hhea_ascent: f64 = @floatFromInt(hhea.Ascender);
const hhea_descent: f64 = @floatFromInt(hhea.Descender);
const hhea_line_gap: f64 = @floatFromInt(hhea.Line_Gap);
if (os2_) |os2| {
const os2_ascent: f64 = @floatFromInt(os2.sTypoAscender);
const os2_descent: f64 = @floatFromInt(os2.sTypoDescender);
const os2_line_gap: f64 = @floatFromInt(os2.sTypoLineGap);
@ -667,9 +670,6 @@ pub const Face = struct {
// 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,
@ -694,6 +694,15 @@ pub const Face = struct {
-win_descent * px_per_unit,
0.0,
};
}
// If our font has no OS/2 table, then we just
// blindly use the metrics from the hhea table.
break :vertical_metrics .{
hhea_ascent * px_per_unit,
hhea_descent * px_per_unit,
hhea_line_gap * px_per_unit,
};
};
// Some fonts have degenerate 'post' tables where the underline
@ -714,18 +723,24 @@ pub const Face = struct {
@as(f64, @floatFromInt(post.underlineThickness)) * px_per_unit;
// Similar logic to the underline above.
const strikethrough_position, const strikethrough_thickness = st: {
const os2 = os2_ orelse break :st .{ null, null };
const has_broken_strikethrough = os2.yStrikeoutSize == 0;
const strikethrough_position = if (has_broken_strikethrough and os2.yStrikeoutPosition == 0)
const pos: ?f64 = if (has_broken_strikethrough and os2.yStrikeoutPosition == 0)
null
else
@as(f64, @floatFromInt(os2.yStrikeoutPosition)) * px_per_unit;
const strikethrough_thickness = if (has_broken_strikethrough)
const thick: ?f64 = if (has_broken_strikethrough)
null
else
@as(f64, @floatFromInt(os2.yStrikeoutSize)) * px_per_unit;
break :st .{ pos, thick };
};
// Cell width is calculated by calculating the widest width of the
// visible ASCII characters. Usually 'M' is widest but we just take
// whatever is widest.
@ -754,37 +769,37 @@ pub const Face = struct {
break :cell_width max;
};
// The OS/2 table does not include sCapHeight or sxHeight in version 1.
const has_os2_height_metrics = os2.version >= 2;
// We use the cap height specified by the font if it's
// available, otherwise we try to measure the `H` glyph.
const cap_height: ?f64 = cap_height: {
if (has_os2_height_metrics) {
break :cap_height @as(f64, @floatFromInt(os2.sCapHeight)) * px_per_unit;
// We use the cap and ex heights specified by the font if they're
// available, otherwise we try to measure the `H` and `x` glyphs.
const cap_height: ?f64, const ex_height: ?f64 = heights: {
if (os2_) |os2| {
// The OS/2 table does not include these metrics in version 1.
if (os2.version >= 2) {
break :heights .{
@as(f64, @floatFromInt(os2.sCapHeight)) * px_per_unit,
@as(f64, @floatFromInt(os2.sxHeight)) * px_per_unit,
};
}
}
break :heights .{
cap: {
if (face.getCharIndex('H')) |glyph_index| {
if (face.loadGlyph(glyph_index, .{ .render = true })) {
break :cap_height f26dot6ToF64(face.handle.*.glyph.*.metrics.height);
break :cap f26dot6ToF64(face.handle.*.glyph.*.metrics.height);
} else |_| {}
}
break :cap_height null;
};
// We use the ex height specified by the font if it's
// available, otherwise we try to measure the `x` glyph.
const ex_height: ?f64 = ex_height: {
if (has_os2_height_metrics) {
break :ex_height @as(f64, @floatFromInt(os2.sxHeight)) * px_per_unit;
}
break :cap null;
},
ex: {
if (face.getCharIndex('x')) |glyph_index| {
if (face.loadGlyph(glyph_index, .{ .render = true })) {
break :ex_height f26dot6ToF64(face.handle.*.glyph.*.metrics.height);
break :ex f26dot6ToF64(face.handle.*.glyph.*.metrics.height);
} else |_| {}
}
break :ex_height null;
break :ex null;
},
};
};
var result = font.face.Metrics.calc(.{