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LibGfx: Remove bit casting in OpenType Kern table after construction
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Do more checks at load time, including categorizing the subtables and
producing our own directory of them.

The format for Kern is a little complicated, so use a Stream instead of
manual offsets.
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@AtkinsSJ @awesomekling
AtkinsSJ authored and awesomekling committed Nov 8, 2023
1 parent 1519290 commit 3c7d654
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Showing 2 changed files with 84 additions and 57 deletions.
97 changes: 48 additions & 49 deletions Userland/Libraries/LibGfx/Font/OpenType/Tables.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -239,61 +239,67 @@ String Name::string_for_id(NameId id) const

ErrorOr<Kern> Kern::from_slice(ReadonlyBytes slice)
{
if (slice.size() < sizeof(Header))
return Error::from_string_literal("Invalid kern table header");
FixedMemoryStream stream { slice };

// We only support the old (2x u16) version of the header
auto const& header = *bit_cast<Header const*>(slice.data());
auto const& header = *TRY(stream.read_in_place<Header const>());
auto version = header.version;
auto number_of_subtables = header.n_tables;
if (version != 0)
return Error::from_string_literal("Unsupported kern table version");
if (number_of_subtables == 0)
return Error::from_string_literal("Kern table does not contain any subtables");

// Read all subtable offsets
auto subtable_offsets = TRY(FixedArray<size_t>::create(number_of_subtables));
size_t offset = sizeof(Header);
// Read subtables
Vector<Subtable> subtables;
TRY(subtables.try_ensure_capacity(number_of_subtables));
for (size_t i = 0; i < number_of_subtables; ++i) {
if (slice.size() < offset + sizeof(SubtableHeader))
return Error::from_string_literal("Invalid kern subtable header");
auto const& subtable_header = *bit_cast<SubtableHeader const*>(slice.offset_pointer(offset));
subtable_offsets[i] = offset;
offset += subtable_header.length;
auto const& subtable_header = *TRY(stream.read_in_place<SubtableHeader const>());

if (subtable_header.version != 0)
return Error::from_string_literal("Unsupported Kern subtable version");

if (stream.remaining() + sizeof(SubtableHeader) < subtable_header.length)
return Error::from_string_literal("Kern subtable is truncated");

auto subtable_format = (subtable_header.coverage & 0xFF00) >> 8;
if (subtable_format == 0) {
auto const& format0_header = *TRY(stream.read_in_place<Format0 const>());
auto pairs = TRY(stream.read_in_place<Format0Pair const>(5));

subtables.append(Subtable {
.header = subtable_header,
.table = Format0Table {
.header = format0_header,
.pairs = pairs,
},
});
} else {
dbgln("OpenType::Kern: FIXME: subtable format {} is unsupported", subtable_format);
TRY(stream.discard(subtable_header.length - sizeof(SubtableHeader)));
subtables.append(Subtable {
.header = subtable_header,
.table = UnsupportedTable {},
});
}
}

return Kern(slice, move(subtable_offsets));
return Kern(header, move(subtables));
}

i16 Kern::get_glyph_kerning(u16 left_glyph_id, u16 right_glyph_id) const
{
VERIFY(left_glyph_id > 0 && right_glyph_id > 0);

i16 glyph_kerning = 0;
for (auto subtable_offset : m_subtable_offsets) {
auto subtable_slice = m_slice.slice(subtable_offset);
auto const& subtable_header = *bit_cast<SubtableHeader const*>(subtable_slice.data());

auto version = subtable_header.version;
auto length = subtable_header.length;
auto coverage = subtable_header.coverage;

if (version != 0) {
dbgln("OpenType::Kern: unsupported subtable version {}", version);
continue;
}

if (subtable_slice.size() < length) {
dbgln("OpenType::Kern: subtable has an invalid size {}", length);
continue;
}
for (auto const& subtable : m_subtables) {
auto coverage = subtable.header.coverage;

auto is_horizontal = (coverage & (1 << 0)) > 0;
auto is_minimum = (coverage & (1 << 1)) > 0;
auto is_cross_stream = (coverage & (1 << 2)) > 0;
auto is_override = (coverage & (1 << 3)) > 0;
auto reserved_bits = (coverage & 0xF0);
auto format = (coverage & 0xFF00) >> 8;

// FIXME: implement support for these features
if (!is_horizontal || is_minimum || is_cross_stream || (reserved_bits > 0)) {
Expand All @@ -303,14 +309,12 @@ i16 Kern::get_glyph_kerning(u16 left_glyph_id, u16 right_glyph_id) const

// FIXME: implement support for subtable formats other than 0
Optional<i16> subtable_kerning;
switch (format) {
case 0:
subtable_kerning = read_glyph_kerning_format0(subtable_slice.slice(sizeof(SubtableHeader)), left_glyph_id, right_glyph_id);
break;
default:
dbgln("OpenType::Kern: FIXME: subtable format {} is unsupported", format);
continue;
}
subtable.table.visit(
[&](Format0Table const& format0) {
subtable_kerning = read_glyph_kerning_format0(format0, left_glyph_id, right_glyph_id);
},
[&](auto&) {});

if (!subtable_kerning.has_value())
continue;
auto kerning_value = subtable_kerning.release_value();
Expand All @@ -323,16 +327,12 @@ i16 Kern::get_glyph_kerning(u16 left_glyph_id, u16 right_glyph_id) const
return glyph_kerning;
}

Optional<i16> Kern::read_glyph_kerning_format0(ReadonlyBytes slice, u16 left_glyph_id, u16 right_glyph_id)
Optional<i16> Kern::read_glyph_kerning_format0(Format0Table const& format0, u16 left_glyph_id, u16 right_glyph_id)
{
if (slice.size() < sizeof(Format0))
return {};

auto const& format0 = *bit_cast<Format0 const*>(slice.data());
u16 number_of_pairs = format0.n_pairs;
u16 search_range = format0.search_range;
u16 entry_selector = format0.entry_selector;
u16 range_shift = format0.range_shift;
u16 number_of_pairs = format0.header.n_pairs;
u16 search_range = format0.header.search_range;
u16 entry_selector = format0.header.entry_selector;
u16 range_shift = format0.header.range_shift;

// Sanity checks for this table format
auto pairs_in_search_range = search_range / sizeof(Format0Pair);
Expand All @@ -346,11 +346,10 @@ Optional<i16> Kern::read_glyph_kerning_format0(ReadonlyBytes slice, u16 left_gly
return {};

// FIXME: implement a possibly slightly more efficient binary search using the parameters above
ReadonlySpan<Format0Pair> pairs { bit_cast<Format0Pair const*>(slice.slice(sizeof(Format0)).data()), number_of_pairs };

// The left and right halves of the kerning pair make an unsigned 32-bit number, which is then used to order the kerning pairs numerically.
auto needle = (static_cast<u32>(left_glyph_id) << 16u) | static_cast<u32>(right_glyph_id);
auto* pair = binary_search(pairs, nullptr, nullptr, [&](void*, Format0Pair const& pair) {
auto* pair = binary_search(format0.pairs, nullptr, nullptr, [&](void*, Format0Pair const& pair) {
auto as_u32 = (static_cast<u32>(pair.left) << 16u) | static_cast<u32>(pair.right);
return needle - as_u32;
});
Expand Down
44 changes: 36 additions & 8 deletions Userland/Libraries/LibGfx/Font/OpenType/Tables.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -424,7 +424,6 @@ class Kern {
static ErrorOr<Kern> from_slice(ReadonlyBytes);
i16 get_glyph_kerning(u16 left_glyph_id, u16 right_glyph_id) const;

private:
struct [[gnu::packed]] Header {
BigEndian<u16> version;
BigEndian<u16> n_tables;
Expand Down Expand Up @@ -454,18 +453,28 @@ class Kern {
};
static_assert(AssertSize<Format0Pair, 6>());

Header const& header() const { return *bit_cast<Header const*>(m_slice.data()); }
private:
// Non-spec structs for easier reference
struct Format0Table {
Format0 const& header;
ReadonlySpan<Format0Pair> pairs;
};
struct UnsupportedTable { };
struct Subtable {
SubtableHeader const& header;
Variant<Format0Table, UnsupportedTable> table;
};

Kern(ReadonlyBytes slice, FixedArray<size_t> subtable_offsets)
: m_slice(slice)
, m_subtable_offsets(move(subtable_offsets))
Kern(Header const& header, Vector<Subtable> subtables)
: m_header(header)
, m_subtables(move(subtables))
{
}

static Optional<i16> read_glyph_kerning_format0(ReadonlyBytes slice, u16 left_glyph_id, u16 right_glyph_id);
static Optional<i16> read_glyph_kerning_format0(Format0Table const& format0, u16 left_glyph_id, u16 right_glyph_id);

ReadonlyBytes m_slice;
FixedArray<size_t> m_subtable_offsets;
Header const& m_header;
Vector<Subtable> const m_subtables;
};

// https://learn.microsoft.com/en-us/typography/opentype/spec/eblc
Expand Down Expand Up @@ -764,3 +773,22 @@ class GPOS {
ReadonlyBytes m_slice;
};
}

namespace AK {
template<>
struct Traits<OpenType::Kern::Header const> : public GenericTraits<OpenType::Kern::Header const> {
static constexpr bool is_trivially_serializable() { return true; }
};
template<>
struct Traits<OpenType::Kern::SubtableHeader const> : public GenericTraits<OpenType::Kern::SubtableHeader const> {
static constexpr bool is_trivially_serializable() { return true; }
};
template<>
struct Traits<OpenType::Kern::Format0 const> : public GenericTraits<OpenType::Kern::Format0 const> {
static constexpr bool is_trivially_serializable() { return true; }
};
template<>
struct Traits<OpenType::Kern::Format0Pair const> : public GenericTraits<OpenType::Kern::Format0Pair const> {
static constexpr bool is_trivially_serializable() { return true; }
};
}

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