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LibPDF: Fix sign (and fixed point) in glyph decoding opcode 24

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Two bugs:

1. We decoded a u32, not an i32 as the spec wants
2. (minor) Our fixed-point divisor was off by one

Fixes text rendering in Bakke2010a.pdf in pdffiles, and rendering of
other fonts with negative width adjustments from optcode 255.
That PDF was produced by "Apple pstopdf" and uses font SFBX1200,
which is apparently a variant of Computer Modern. So maybe this
helps with lots of PDFs produced from TeX files, but I haven't
checked that.
This commit is contained in:
Nico Weber 2023-10-15 09:44:40 -04:00 committed by Andreas Kling
parent 96a4936567
commit 007d7cdd53
Notes: sideshowbarker 2024-07-16 21:51:02 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/007d7cdd53
Pull-request: https://github.com/SerenityOS/serenity/pull/21453
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/gmta
1 changed files with 13 additions and 9 deletions
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22
Userland/Libraries/LibPDF/Fonts/Type1FontProgram.cpp
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@ -135,6 +135,9 @@ void Type1FontProgram::consolidate_glyphs()
PDFErrorOr<Type1FontProgram::Glyph> Type1FontProgram::parse_glyph(ReadonlyBytes const& data, Vector<ByteBuffer> const& subroutines, GlyphParserState& state, bool is_type2)
{
// Type 1 Font Format: https://adobe-type-tools.github.io/font-tech-notes/pdfs/T1_SPEC.pdf (Chapter 6: CharStrings dictionary)
// Type 2 Charstring Format: https://adobe-type-tools.github.io/font-tech-notes/pdfs/5177.Type2.pdf
auto push = [&](float value) -> PDFErrorOr<void> {
if (state.sp >= state.stack.size())
return error("Operand stack overflow");
@ -250,16 +253,17 @@ PDFErrorOr<Type1FontProgram::Glyph> Type1FontProgram::parse_glyph(ReadonlyBytes
int v = data[i];
if (v == 255) {
TRY(require(4));
int a = data[++i];
int b = data[++i];
int c = data[++i];
int d = data[++i];
// Both Type 1 and Type 2 spec:
// "If the charstring byte contains the value 255, the next four bytes indicate a two’s complement signed number.
// The first of these four bytes contains the highest order bits [...]"
i32 a = static_cast<i32>((data[i + 1] << 24) | (data[i + 2] << 16) | (data[i + 3] << 8) | data[i + 4]);
i += 4;
if (is_type2) {
auto integer = float((a << 8) | b);
auto fraction = float((c << 8) | d) / AK::NumericLimits<u16>::max();
TRY(push(integer + fraction));
} else
TRY(push((a << 24) + (b << 16) + (c << 8) + d));
// Just in the Type 2 spec: "This number is interpreted as a Fixed; that is, a signed number with 16 bits of fraction."
TRY(push(a / (float)0x1'0000));
} else {
TRY(push(a));
}
} else if (v >= 251) {
TRY(require(1));
auto w = data[++i];