ttf_parser/tables/
hmtx.rs

1//! A [Horizontal/Vertical Metrics Table](
2//! https://docs.microsoft.com/en-us/typography/opentype/spec/hmtx) implementation.
3
4use core::num::NonZeroU16;
5
6use crate::parser::{FromData, LazyArray16, Stream};
7use crate::GlyphId;
8
9/// Horizontal/Vertical Metrics.
10#[derive(Clone, Copy, Debug)]
11pub struct Metrics {
12    /// Width/Height advance for `hmtx`/`vmtx`.
13    pub advance: u16,
14    /// Left/Top side bearing for `hmtx`/`vmtx`.
15    pub side_bearing: i16,
16}
17
18impl FromData for Metrics {
19    const SIZE: usize = 4;
20
21    #[inline]
22    fn parse(data: &[u8]) -> Option<Self> {
23        let mut s = Stream::new(data);
24        Some(Metrics {
25            advance: s.read::<u16>()?,
26            side_bearing: s.read::<i16>()?,
27        })
28    }
29}
30
31/// A [Horizontal/Vertical Metrics Table](
32/// https://docs.microsoft.com/en-us/typography/opentype/spec/hmtx).
33///
34/// `hmtx` and `vmtx` tables has the same structure, so we're reusing the same struct for both.
35#[derive(Clone, Copy, Debug)]
36pub struct Table<'a> {
37    /// A list of metrics indexed by glyph ID.
38    pub metrics: LazyArray16<'a, Metrics>,
39    /// Side bearings for glyph IDs greater than or equal to the number of `metrics` values.
40    pub bearings: LazyArray16<'a, i16>,
41    /// Sum of long metrics + bearings.
42    pub number_of_metrics: u16,
43}
44
45impl<'a> Table<'a> {
46    /// Parses a table from raw data.
47    ///
48    /// - `number_of_metrics` is from the `hhea`/`vhea` table.
49    /// - `number_of_glyphs` is from the `maxp` table.
50    pub fn parse(
51        mut number_of_metrics: u16,
52        number_of_glyphs: NonZeroU16,
53        data: &'a [u8],
54    ) -> Option<Self> {
55        if number_of_metrics == 0 {
56            return None;
57        }
58
59        let mut s = Stream::new(data);
60        let metrics = s.read_array16::<Metrics>(number_of_metrics)?;
61
62        // 'If the number_of_metrics is less than the total number of glyphs,
63        // then that array is followed by an array for the left side bearing values
64        // of the remaining glyphs.'
65        let bearings_count = number_of_glyphs.get().checked_sub(number_of_metrics);
66        let bearings = if let Some(count) = bearings_count {
67            number_of_metrics += count;
68            // Some malformed fonts can skip "left side bearing values"
69            // even when they are expected.
70            // Therefore if we weren't able to parser them, simply fallback to an empty array.
71            // No need to mark the whole table as malformed.
72            s.read_array16::<i16>(count).unwrap_or_default()
73        } else {
74            LazyArray16::default()
75        };
76
77        Some(Table {
78            metrics,
79            bearings,
80            number_of_metrics,
81        })
82    }
83
84    /// Returns advance for a glyph.
85    #[inline]
86    pub fn advance(&self, glyph_id: GlyphId) -> Option<u16> {
87        if glyph_id.0 >= self.number_of_metrics {
88            return None;
89        }
90
91        if let Some(metrics) = self.metrics.get(glyph_id.0) {
92            Some(metrics.advance)
93        } else {
94            // 'As an optimization, the number of records can be less than the number of glyphs,
95            // in which case the advance value of the last record applies
96            // to all remaining glyph IDs.'
97            self.metrics.last().map(|m| m.advance)
98        }
99    }
100
101    /// Returns side bearing for a glyph.
102    #[inline]
103    pub fn side_bearing(&self, glyph_id: GlyphId) -> Option<i16> {
104        if let Some(metrics) = self.metrics.get(glyph_id.0) {
105            Some(metrics.side_bearing)
106        } else {
107            // 'If the number_of_metrics is less than the total number of glyphs,
108            // then that array is followed by an array for the side bearing values
109            // of the remaining glyphs.'
110            self.bearings
111                .get(glyph_id.0.checked_sub(self.metrics.len())?)
112        }
113    }
114}
ttf_parser/tables/hmtx.rs

ttf_parser/tables/
hmtx.rs
