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LibPDF: Implement support for callgsubr in CFF font programs

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Font programs are bytecode programs defining glyphs. If several glyphs
share a piece of outline, that opcode sequence can be put in a
subroutine ("subr") table and the definition of those glyphs can then
call that subroutine by number, to reduce file size.

CFF fonts can in theory contain multiple fonts, and so there's a global
subr table shared by all the fonts in one CFF, and a local per-fornt
subr table.  We used to only implement the local subr table, now we
implement both.

(We only support one font per CFF, and at least in PDF files, that's
all that's ever used. So a global subr table isn't very useful.
But the spec explicitly allows it -- "Global subroutines may be used in
a FontSet even if it only contains one font." -- and it happens in
practice.)
This commit is contained in:
Nico Weber 2023-10-18 09:27:58 -04:00 • committed by Tim Flynn
parent 185573c03f
commit 3907374621
5 changed files with 15 additions and 11 deletions
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8
Userland/Libraries/LibPDF/Fonts/CFF.cpp
View file

@ -221,11 +221,9 @@ PDFErrorOr<NonnullRefPtr<CFF>> CFF::create(ReadonlyBytes const& cff_bytes, RefPt
TRY(parse_index(reader, [&](ReadonlyBytes const& subroutine_bytes) -> PDFErrorOr<void> {
return TRY(global_subroutines.try_append(TRY(ByteBuffer::copy(subroutine_bytes))));
}));
if (!global_subroutines.is_empty())
dbgln("CFF data contains Global subrs, which aren't implemented yet"); // FIXME
// Create glyphs (now that we have the subroutines) and associate missing information to store them and their encoding
auto glyphs = TRY(parse_charstrings(Reader(cff_bytes.slice(charstrings_offset)), local_subroutines));
auto glyphs = TRY(parse_charstrings(Reader(cff_bytes.slice(charstrings_offset)), local_subroutines, global_subroutines));
// CFF spec, "Table 16 Encoding ID"
// FIXME: Only read this if the built-in encoding is actually needed? (ie. `if (!encoding)`)
@ -765,13 +763,13 @@ PDFErrorOr<Vector<DeprecatedFlyString>> CFF::parse_charset(Reader&& reader, size
return names;
}
PDFErrorOr<Vector<CFF::Glyph>> CFF::parse_charstrings(Reader&& reader, Vector<ByteBuffer> const& subroutines)
PDFErrorOr<Vector<CFF::Glyph>> CFF::parse_charstrings(Reader&& reader, Vector<ByteBuffer> const& local_subroutines, Vector<ByteBuffer> const& global_subroutines)
{
// CFF spec, "14 CharStrings INDEX"
Vector<Glyph> glyphs;
TRY(parse_index(reader, [&](ReadonlyBytes const& charstring_data) -> PDFErrorOr<void> {
GlyphParserState state;
auto glyph = TRY(parse_glyph(charstring_data, subroutines, state, true));
auto glyph = TRY(parse_glyph(charstring_data, local_subroutines, global_subroutines, state, true));
return TRY(glyphs.try_append(glyph));
}));
return glyphs;