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[skrifa] hint reliant font detection based on table length/checksum (#…
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…1306)

Adds support for detecting trickiness based on checksum/length of the cvt, fpgm and prep tables.

Also, to avoid redundancy, moves checksum computation code out of write-fonts into read-fonts and makes it public.
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@dfrg
dfrg authored Jan 8, 2025
1 parent ffbb038 commit 449dc4f
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Showing 4 changed files with 267 additions and 24 deletions.
4 changes: 2 additions & 2 deletions read-fonts/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -267,7 +267,7 @@ impl<'a> CollectionRef<'a> {
/// by a borrowed slice containing font data.
#[derive(Clone)]
pub struct FontRef<'a> {
data: FontData<'a>,
pub data: FontData<'a>,
pub table_directory: TableDirectory<'a>,
}

Expand Down Expand Up @@ -320,7 +320,7 @@ impl<'a> FontRef<'a> {
.and_then(|record| {
let start = Offset32::new(record.offset()).non_null()?;
let len = record.length() as usize;
self.data.slice(start..start + len)
self.data.slice(start..start.checked_add(len)?)
})
}

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23 changes: 23 additions & 0 deletions read-fonts/src/tables.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -48,3 +48,26 @@ pub mod vhea;
pub mod vmtx;
pub mod vorg;
pub mod vvar;

/// Computes the table checksum for the given data.
///
/// See the OpenType [specification](https://learn.microsoft.com/en-us/typography/opentype/spec/otff#calculating-checksums)
/// for details.
pub fn compute_checksum(table: &[u8]) -> u32 {
let mut sum = 0u32;
let mut iter = table.chunks_exact(4);
for quad in &mut iter {
// this can't fail, and we trust the compiler to avoid a branch
let array: [u8; 4] = quad.try_into().unwrap_or_default();
sum = sum.wrapping_add(u32::from_be_bytes(array));
}

let rem = match *iter.remainder() {
[a] => u32::from_be_bytes([a, 0, 0, 0]),
[a, b] => u32::from_be_bytes([a, b, 0, 0]),
[a, b, c] => u32::from_be_bytes([a, b, c, 0]),
_ => 0,
};

sum.wrapping_add(rem)
}
246 changes: 240 additions & 6 deletions skrifa/src/outline/hint_reliant.rs
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Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
//! Name detection for fonts that require hinting to be run for correct
//! contours (FreeType calls these "tricky" fonts).
use crate::{string::StringId, FontRef, MetadataProvider};
use crate::{string::StringId, FontRef, MetadataProvider, Tag};

pub(super) fn require_interpreter(font: &FontRef) -> bool {
is_hint_reliant_by_name(font)
is_hint_reliant_by_name(font) || matches_hint_reliant_id_list(FontId::from_font(font))
}

fn is_hint_reliant_by_name(font: &FontRef) -> bool {
Expand All @@ -21,15 +21,15 @@ fn is_hint_reliant_by_name(font: &FontRef) -> bool {
if chars.next().is_some() {
return false;
}
matches_hint_reliant_list(core::str::from_utf8(&buf[..len]).unwrap_or_default())
matches_hint_reliant_name_list(core::str::from_utf8(&buf[..len]).unwrap_or_default())
})
.unwrap_or_default()
}

/// Is this name on the list of fonts that require hinting?
///
/// <https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/truetype/ttobjs.c#L174>
fn matches_hint_reliant_list(name: &str) -> bool {
fn matches_hint_reliant_name_list(name: &str) -> bool {
let name = skip_pdf_random_tag(name);
HINT_RELIANT_NAMES
.iter()
Expand Down Expand Up @@ -94,6 +94,240 @@ const HINT_RELIANT_NAMES: &[&str] = &[

const MAX_HINT_RELIANT_NAME_LEN: usize = 18;

#[derive(Copy, Clone, PartialEq, Default, Debug)]
struct TableId {
checksum: u32,
len: u32,
}

impl TableId {
fn from_font_and_tag(font: &FontRef, tag: Tag) -> Option<Self> {
let data = font.table_data(tag)?;
Some(Self {
// Note: FreeType always just computes the checksum
// <https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/truetype/ttobjs.c#L281>
checksum: raw::tables::compute_checksum(data.as_bytes()),
len: data.len() as u32,
})
}
}

#[derive(Copy, Clone, PartialEq, Default, Debug)]
struct FontId {
cvt: TableId,
fpgm: TableId,
prep: TableId,
}

impl FontId {
fn from_font(font: &FontRef) -> Self {
Self {
cvt: TableId::from_font_and_tag(font, Tag::new(b"cvt ")).unwrap_or_default(),
fpgm: TableId::from_font_and_tag(font, Tag::new(b"fpgm")).unwrap_or_default(),
prep: TableId::from_font_and_tag(font, Tag::new(b"prep")).unwrap_or_default(),
}
}
}

/// Checks for fonts that require hinting based on the length and checksum of
/// the cvt, fpgm and prep tables.
///
/// Roughly equivalent to <https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/truetype/ttobjs.c#L309>
fn matches_hint_reliant_id_list(font_id: FontId) -> bool {
HINT_RELIANT_IDS.contains(&font_id)
}

/// <https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/truetype/ttobjs.c#L314>
#[rustfmt::skip]
const HINT_RELIANT_IDS: &[FontId] = &[
// MingLiU 1995
FontId {
cvt: TableId { checksum: 0x05BCF058, len: 0x000002E4 },
fpgm: TableId { checksum: 0x28233BF1, len: 0x000087C4 },
prep: TableId { checksum: 0xA344A1EA, len: 0x000001E1 },
},
// MingLiU 1996-
FontId {
cvt: TableId { checksum: 0x05BCF058, len: 0x000002E4 },
fpgm: TableId { checksum: 0x28233BF1, len: 0x000087C4 },
prep: TableId { checksum: 0xA344A1EB, len: 0x000001E1 },
},
// DFGothic-EB
FontId {
cvt: TableId { checksum: 0x12C3EBB2, len: 0x00000350 },
fpgm: TableId { checksum: 0xB680EE64, len: 0x000087A7 },
prep: TableId { checksum: 0xCE939563, len: 0x00000758 },
},
// DFGyoSho-Lt
FontId {
cvt: TableId { checksum: 0x11E5EAD4, len: 0x00000350 },
fpgm: TableId { checksum: 0xCE5956E9, len: 0x0000BC85 },
prep: TableId { checksum: 0x8272F416, len: 0x00000045 },
},
// DFHei-Md-HK-BF
FontId {
cvt: TableId { checksum: 0x1257EB46, len: 0x00000350 },
fpgm: TableId { checksum: 0xF699D160, len: 0x0000715F },
prep: TableId { checksum: 0xD222F568, len: 0x000003BC },
},
// DFHSGothic-W5
FontId {
cvt: TableId { checksum: 0x1262EB4E, len: 0x00000350 },
fpgm: TableId { checksum: 0xE86A5D64, len: 0x00007940 },
prep: TableId { checksum: 0x7850F729, len: 0x000005FF },
},
// DFHSMincho-W3
FontId {
cvt: TableId { checksum: 0x122DEB0A, len: 0x00000350 },
fpgm: TableId { checksum: 0x3D16328A, len: 0x0000859B },
prep: TableId { checksum: 0xA93FC33B, len: 0x000002CB },
},
// DFHSMincho-W7
FontId {
cvt: TableId { checksum: 0x125FEB26, len: 0x00000350 },
fpgm: TableId { checksum: 0xA5ACC982, len: 0x00007EE1 },
prep: TableId { checksum: 0x90999196, len: 0x0000041F },
},
// DFKaiShu
FontId {
cvt: TableId { checksum: 0x11E5EAD4, len: 0x00000350 },
fpgm: TableId { checksum: 0x5A30CA3B, len: 0x00009063 },
prep: TableId { checksum: 0x13A42602, len: 0x0000007E },
},
// DFKaiShu, variant
FontId {
cvt: TableId { checksum: 0x11E5EAD4, len: 0x00000350 },
fpgm: TableId { checksum: 0xA6E78C01, len: 0x00008998 },
prep: TableId { checksum: 0x13A42602, len: 0x0000007E },
},
// DFKaiShu-Md-HK-BF
FontId {
cvt: TableId { checksum: 0x11E5EAD4, len: 0x00000360 },
fpgm: TableId { checksum: 0x9DB282B2, len: 0x0000C06E },
prep: TableId { checksum: 0x53E6D7CA, len: 0x00000082 },
},
// DFMing-Bd-HK-BF
FontId {
cvt: TableId { checksum: 0x1243EB18, len: 0x00000350 },
fpgm: TableId { checksum: 0xBA0A8C30, len: 0x000074AD },
prep: TableId { checksum: 0xF3D83409, len: 0x0000037B },
},
// DLCLiShu
FontId {
cvt: TableId { checksum: 0x07DCF546, len: 0x00000308 },
fpgm: TableId { checksum: 0x40FE7C90, len: 0x00008E2A },
prep: TableId { checksum: 0x608174B5, len: 0x0000007A },
},
// DLCHayBold
FontId {
cvt: TableId { checksum: 0xEB891238, len: 0x00000308 },
fpgm: TableId { checksum: 0xD2E4DCD4, len: 0x0000676F },
prep: TableId { checksum: 0x8EA5F293, len: 0x000003B8 },
},
// HuaTianKaiTi
FontId {
cvt: TableId { checksum: 0xFFFBFFFC, len: 0x00000008 },
fpgm: TableId { checksum: 0x9C9E48B8, len: 0x0000BEA2 },
prep: TableId { checksum: 0x70020112, len: 0x00000008 },
},
// HuaTianSongTi
FontId {
cvt: TableId { checksum: 0xFFFBFFFC, len: 0x00000008 },
fpgm: TableId { checksum: 0x0A5A0483, len: 0x00017C39 },
prep: TableId { checksum: 0x70020112, len: 0x00000008 },
},
// NEC fadpop7.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x40C92555, len: 0x000000E5 },
prep: TableId { checksum: 0xA39B58E3, len: 0x0000117C },
},
// NEC fadrei5.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x33C41652, len: 0x000000E5 },
prep: TableId { checksum: 0x26D6C52A, len: 0x00000F6A },
},
// NEC fangot7.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x6DB1651D, len: 0x0000019D },
prep: TableId { checksum: 0x6C6E4B03, len: 0x00002492 },
},
// NEC fangyo5.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x40C92555, len: 0x000000E5 },
prep: TableId { checksum: 0xDE51FAD0, len: 0x0000117C },
},
// NEC fankyo5.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x85E47664, len: 0x000000E5 },
prep: TableId { checksum: 0xA6C62831, len: 0x00001CAA },
},
// NEC fanrgo5.ttf
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x2D891CFD, len: 0x0000019D },
prep: TableId { checksum: 0xA0604633, len: 0x00001DE8 },
},
// NEC fangot5.ttc
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x40AA774C, len: 0x000001CB },
prep: TableId { checksum: 0x9B5CAA96, len: 0x00001F9A },
},
// NEC fanmin3.ttc
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x0D3DE9CB, len: 0x00000141 },
prep: TableId { checksum: 0xD4127766, len: 0x00002280 },
},
// NEC FA-Gothic, 1996
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x4A692698, len: 0x000001F0 },
prep: TableId { checksum: 0x340D4346, len: 0x00001FCA },
},
// NEC FA-Minchou, 1996
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0xCD34C604, len: 0x00000166 },
prep: TableId { checksum: 0x6CF31046, len: 0x000022B0 },
},
// NEC FA-RoundGothicB, 1996
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0x5DA75315, len: 0x0000019D },
prep: TableId { checksum: 0x40745A5F, len: 0x000022E0 },
},
// NEC FA-RoundGothicM, 1996
FontId {
cvt: TableId { checksum: 0x00000000, len: 0x00000000 },
fpgm: TableId { checksum: 0xF055FC48, len: 0x000001C2 },
prep: TableId { checksum: 0x3900DED3, len: 0x00001E18 },
},
// MINGLI.TTF, 1992
FontId {
cvt: TableId { checksum: 0x00170003, len: 0x00000060 },
fpgm: TableId { checksum: 0xDBB4306E, len: 0x000058AA },
prep: TableId { checksum: 0xD643482A, len: 0x00000035 },
},
// DFHei-Bd-WIN-HK-BF, issue #1087
FontId {
cvt: TableId { checksum: 0x1269EB58, len: 0x00000350 },
fpgm: TableId { checksum: 0x5CD5957A, len: 0x00006A4E },
prep: TableId { checksum: 0xF758323A, len: 0x00000380 },
},
// DFMing-Md-WIN-HK-BF, issue #1087
FontId {
cvt: TableId { checksum: 0x122FEB0B, len: 0x00000350 },
fpgm: TableId { checksum: 0x7F10919A, len: 0x000070A9 },
prep: TableId { checksum: 0x7CD7E7B7, len: 0x0000025C },
},
];

#[cfg(test)]
mod tests {
use super::*;
Expand Down Expand Up @@ -127,14 +361,14 @@ mod tests {
#[test]
fn all_hint_reliant_names() {
for name in HINT_RELIANT_NAMES {
assert!(matches_hint_reliant_list(name));
assert!(matches_hint_reliant_name_list(name));
}
}

#[test]
fn non_hint_reliant_names() {
for not_tricky in ["Roboto", "Arial", "Helvetica", "Blah", ""] {
assert!(!matches_hint_reliant_list(not_tricky));
assert!(!matches_hint_reliant_name_list(not_tricky));
}
}
}
18 changes: 2 additions & 16 deletions write-fonts/src/font_builder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -136,23 +136,9 @@ fn round4(sz: usize) -> usize {
}

fn checksum_and_padding(table: &[u8]) -> (u32, u32) {
let checksum = read_fonts::tables::compute_checksum(table);
let padding = round4(table.len()) - table.len();
let mut sum = 0u32;
let mut iter = table.chunks_exact(4);
for quad in &mut iter {
// this can't fail, and we trust the compiler to avoid a branch
let array: [u8; 4] = quad.try_into().unwrap_or_default();
sum = sum.wrapping_add(u32::from_be_bytes(array));
}

let rem = match *iter.remainder() {
[a] => u32::from_be_bytes([a, 0, 0, 0]),
[a, b] => u32::from_be_bytes([a, b, 0, 0]),
[a, b, c] => u32::from_be_bytes([a, b, c, 0]),
_ => 0,
};

(sum.wrapping_add(rem), padding as u32)
(checksum, padding as u32)
}

impl TTCHeader {
Expand Down

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