578 lines
23 KiB
Rust
578 lines
23 KiB
Rust
//! Global font and glyph specific metrics.
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//!
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//! Metrics are various measurements that define positioning and layout
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//! characteristics for a font. They come in two flavors:
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//!
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//! * Global metrics: these are applicable to all glyphs in a font and generally
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//! define values that are used for the layout of a collection of glyphs. For example,
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//! the ascent, descent and leading values determine the position of the baseline where
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//! a glyph should be rendered as well as the suggested spacing above and below it.
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//!
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//! * Glyph metrics: these apply to single glyphs. For example, the advance
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//! width value describes the distance between two consecutive glyphs on a line.
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//!
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//! ### Selecting an "instance"
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//! Both global and glyph specific metrics accept two additional pieces of information
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//! to select the desired instance of a font:
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//! * Size: represented by the [Size] type, this determines the scaling factor that is
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//! applied to all metrics.
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//! * Normalized variation coordinates: represented by the [LocationRef] type,
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//! these define the position in design space for a variable font. For a non-variable
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//! font, these coordinates are ignored and you can pass [LocationRef::default()]
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//! as an argument for this parameter.
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//!
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use read_fonts::{
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tables::{
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glyf::Glyf, gvar::Gvar, hmtx::LongMetric, hvar::Hvar, loca::Loca, os2::SelectionFlags,
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},
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types::{BigEndian, Fixed, GlyphId},
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FontRef, TableProvider,
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};
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use super::instance::{LocationRef, NormalizedCoord, Size};
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/// Type for a bounding box with single precision floating point coordinates.
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pub type BoundingBox = read_fonts::types::BoundingBox<f32>;
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/// Metrics for a text decoration.
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///
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/// This represents the suggested offset and thickness of an underline
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/// or strikeout text decoration.
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#[derive(Copy, Clone, PartialEq, Default, Debug)]
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pub struct Decoration {
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/// Offset to the top of the decoration from the baseline.
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pub offset: f32,
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/// Thickness of the decoration.
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pub thickness: f32,
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}
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/// Metrics that apply to all glyphs in a font.
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///
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/// These are retrieved for a specific position in the design space.
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///
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/// This metrics here are derived from the following tables:
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/// * [head](https://learn.microsoft.com/en-us/typography/opentype/spec/head): `units_per_em`, `bounds`
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/// * [maxp](https://learn.microsoft.com/en-us/typography/opentype/spec/maxp): `glyph_count`
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/// * [post](https://learn.microsoft.com/en-us/typography/opentype/spec/post): `is_monospace`, `italic_angle`, `underline`
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/// * [OS/2](https://learn.microsoft.com/en-us/typography/opentype/spec/os2): `average_width`, `cap_height`,
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/// `x_height`, `strikeout`, as well as the line metrics: `ascent`, `descent`, `leading` if the `USE_TYPOGRAPHIC_METRICS`
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/// flag is set or the `hhea` line metrics are zero (the Windows metrics are used as a last resort).
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/// * [hhea](https://learn.microsoft.com/en-us/typography/opentype/spec/hhea): `max_width`, as well as the line metrics:
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/// `ascent`, `descent`, `leading` if they are non-zero and the `USE_TYPOGRAPHIC_METRICS` flag is not set in the OS/2 table
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///
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/// For variable fonts, deltas are computed using the [MVAR](https://learn.microsoft.com/en-us/typography/opentype/spec/MVAR)
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/// table.
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#[derive(Copy, Clone, PartialEq, Default, Debug)]
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pub struct Metrics {
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/// Number of font design units per em unit.
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pub units_per_em: u16,
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/// Number of glyphs in the font.
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pub glyph_count: u16,
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/// True if the font is not proportionally spaced.
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pub is_monospace: bool,
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/// Italic angle in counter-clockwise degrees from the vertical. Zero for upright text,
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/// negative for text that leans to the right.
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pub italic_angle: f32,
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/// Distance from the baseline to the top of the alignment box.
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pub ascent: f32,
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/// Distance from the baseline to the bottom of the alignment box.
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pub descent: f32,
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/// Recommended additional spacing between lines.
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pub leading: f32,
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/// Distance from the baseline to the top of a typical English capital.
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pub cap_height: Option<f32>,
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/// Distance from the baseline to the top of the lowercase "x" or
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/// similar character.
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pub x_height: Option<f32>,
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/// Average width of all non-zero width characters in the font.
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pub average_width: Option<f32>,
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/// Maximum advance width of all characters in the font.
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pub max_width: Option<f32>,
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/// Metrics for an underline decoration.
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pub underline: Option<Decoration>,
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/// Metrics for a strikeout decoration.
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pub strikeout: Option<Decoration>,
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/// Union of minimum and maximum extents for all glyphs in the font.
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pub bounds: Option<BoundingBox>,
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}
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impl Metrics {
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/// Creates new metrics for the given font, size, and location in
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/// normalized variation space.
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pub fn new<'a>(font: &FontRef<'a>, size: Size, location: impl Into<LocationRef<'a>>) -> Self {
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let head = font.head();
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let mut metrics = Metrics {
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units_per_em: head.map(|head| head.units_per_em()).unwrap_or_default(),
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..Default::default()
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};
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let coords = location.into().effective_coords();
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let scale = size.linear_scale(metrics.units_per_em);
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if let Ok(head) = font.head() {
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metrics.bounds = Some(BoundingBox {
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x_min: head.x_min() as f32 * scale,
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y_min: head.y_min() as f32 * scale,
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x_max: head.x_max() as f32 * scale,
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y_max: head.y_max() as f32 * scale,
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});
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}
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if let Ok(maxp) = font.maxp() {
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metrics.glyph_count = maxp.num_glyphs();
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}
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if let Ok(post) = font.post() {
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metrics.is_monospace = post.is_fixed_pitch() != 0;
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metrics.italic_angle = post.italic_angle().to_f64() as f32;
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metrics.underline = Some(Decoration {
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offset: post.underline_position().to_i16() as f32 * scale,
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thickness: post.underline_thickness().to_i16() as f32 * scale,
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});
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}
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let hhea = font.hhea();
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if let Ok(hhea) = &hhea {
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metrics.max_width = Some(hhea.advance_width_max().to_u16() as f32 * scale);
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}
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// Choosing proper line metrics is a challenge due to the changing
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// spec, backward compatibility and broken fonts.
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//
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// We use the same strategy as FreeType:
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// 1. Use the OS/2 metrics if the table exists and the USE_TYPO_METRICS
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// flag is set.
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// 2. Otherwise, use the hhea metrics.
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// 3. If hhea metrics are zero and the OS/2 table exists:
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// 3a. Use the typo metrics if they are non-zero
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// 3b. Otherwise, use the win metrics
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//
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// See: https://github.com/freetype/freetype/blob/5c37b6406258ec0d7ab64b8619c5ea2c19e3c69a/src/sfnt/sfobjs.c#L1311
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let os2 = font.os2().ok();
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let mut used_typo_metrics = false;
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if let Some(os2) = &os2 {
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if os2
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.fs_selection()
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.contains(SelectionFlags::USE_TYPO_METRICS)
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{
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metrics.ascent = os2.s_typo_ascender() as f32 * scale;
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metrics.descent = os2.s_typo_descender() as f32 * scale;
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metrics.leading = os2.s_typo_line_gap() as f32 * scale;
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used_typo_metrics = true;
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}
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metrics.average_width = Some(os2.x_avg_char_width() as f32 * scale);
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metrics.cap_height = os2.s_cap_height().map(|v| v as f32 * scale);
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metrics.x_height = os2.sx_height().map(|v| v as f32 * scale);
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metrics.strikeout = Some(Decoration {
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offset: os2.y_strikeout_position() as f32 * scale,
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thickness: os2.y_strikeout_size() as f32 * scale,
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});
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}
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if !used_typo_metrics {
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if let Ok(hhea) = font.hhea() {
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metrics.ascent = hhea.ascender().to_i16() as f32 * scale;
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metrics.descent = hhea.descender().to_i16() as f32 * scale;
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metrics.leading = hhea.line_gap().to_i16() as f32 * scale;
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}
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if metrics.ascent == 0.0 && metrics.descent == 0.0 {
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if let Some(os2) = &os2 {
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if os2.s_typo_ascender() != 0 || os2.s_typo_descender() != 0 {
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metrics.ascent = os2.s_typo_ascender() as f32 * scale;
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metrics.descent = os2.s_typo_descender() as f32 * scale;
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metrics.leading = os2.s_typo_line_gap() as f32 * scale;
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} else {
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metrics.ascent = os2.us_win_ascent() as f32 * scale;
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// Win descent is always positive while other descent values are negative. Negate it
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// to ensure we return consistent metrics.
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metrics.descent = -(os2.us_win_descent() as f32 * scale);
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}
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}
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}
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}
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if let (Ok(mvar), true) = (font.mvar(), !coords.is_empty()) {
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use read_fonts::tables::mvar::tags::*;
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let metric_delta =
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|tag| mvar.metric_delta(tag, coords).unwrap_or_default().to_f64() as f32 * scale;
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metrics.ascent += metric_delta(HASC);
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metrics.descent += metric_delta(HDSC);
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metrics.leading += metric_delta(HLGP);
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if let Some(cap_height) = &mut metrics.cap_height {
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*cap_height += metric_delta(CPHT);
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}
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if let Some(x_height) = &mut metrics.x_height {
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*x_height += metric_delta(XHGT);
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}
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if let Some(underline) = &mut metrics.underline {
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underline.offset += metric_delta(UNDO);
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underline.thickness += metric_delta(UNDS);
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}
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if let Some(strikeout) = &mut metrics.strikeout {
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strikeout.offset += metric_delta(STRO);
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strikeout.thickness += metric_delta(STRS);
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}
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}
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metrics
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}
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}
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/// Glyph specific metrics.
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#[derive(Clone)]
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pub struct GlyphMetrics<'a> {
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glyph_count: u32,
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fixed_scale: FixedScaleFactor,
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h_metrics: &'a [LongMetric],
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default_advance_width: u16,
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lsbs: &'a [BigEndian<i16>],
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hvar: Option<Hvar<'a>>,
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gvar: Option<Gvar<'a>>,
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loca_glyf: Option<(Loca<'a>, Glyf<'a>)>,
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coords: &'a [NormalizedCoord],
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}
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impl<'a> GlyphMetrics<'a> {
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/// Creates new glyph metrics from the given font, size, and location in
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/// normalized variation space.
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pub fn new(font: &FontRef<'a>, size: Size, location: impl Into<LocationRef<'a>>) -> Self {
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let glyph_count = font
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.maxp()
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.map(|maxp| maxp.num_glyphs() as u32)
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.unwrap_or_default();
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let upem = font
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.head()
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.map(|head| head.units_per_em())
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.unwrap_or_default();
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let fixed_scale = FixedScaleFactor(size.fixed_linear_scale(upem));
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let coords = location.into().effective_coords();
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let (h_metrics, default_advance_width, lsbs) = font
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.hmtx()
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.map(|hmtx| {
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let h_metrics = hmtx.h_metrics();
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let default_advance_width = h_metrics.last().map(|m| m.advance.get()).unwrap_or(0);
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let lsbs = hmtx.left_side_bearings();
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(h_metrics, default_advance_width, lsbs)
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})
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.unwrap_or_default();
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let hvar = font.hvar().ok();
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let gvar = font.gvar().ok();
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let loca_glyf = if let (Ok(loca), Ok(glyf)) = (font.loca(None), font.glyf()) {
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Some((loca, glyf))
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} else {
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None
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};
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Self {
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glyph_count,
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fixed_scale,
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h_metrics,
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default_advance_width,
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lsbs,
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hvar,
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gvar,
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loca_glyf,
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coords,
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}
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}
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/// Returns the number of available glyphs in the font.
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pub fn glyph_count(&self) -> u32 {
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self.glyph_count
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}
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/// Returns the advance width for the specified glyph.
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///
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/// If normalized coordinates were provided when constructing glyph metrics and
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/// an `HVAR` table is present, applies the appropriate delta.
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///
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/// Returns `None` if `glyph_id >= self.glyph_count()` or the underlying font
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/// data is invalid.
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pub fn advance_width(&self, glyph_id: GlyphId) -> Option<f32> {
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if glyph_id.to_u32() >= self.glyph_count {
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return None;
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}
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let mut advance = self
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.h_metrics
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.get(glyph_id.to_u32() as usize)
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.map(|metric| metric.advance())
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.unwrap_or(self.default_advance_width) as i32;
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if let Some(hvar) = &self.hvar {
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advance += hvar
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.advance_width_delta(glyph_id, self.coords)
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// FreeType truncates metric deltas...
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// https://github.com/freetype/freetype/blob/7838c78f53f206ac5b8e9cefde548aa81cb00cf4/src/truetype/ttgxvar.c#L1027
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.map(|delta| delta.to_f64() as i32)
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.unwrap_or(0);
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} else if self.gvar.is_some() {
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advance += self.metric_deltas_from_gvar(glyph_id).unwrap_or_default()[1];
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}
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Some(self.fixed_scale.apply(advance))
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}
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/// Returns the left side bearing for the specified glyph.
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///
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/// If normalized coordinates were provided when constructing glyph metrics and
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/// an `HVAR` table is present, applies the appropriate delta.
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///
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/// Returns `None` if `glyph_id >= self.glyph_count()` or the underlying font
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/// data is invalid.
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pub fn left_side_bearing(&self, glyph_id: GlyphId) -> Option<f32> {
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if glyph_id.to_u32() >= self.glyph_count {
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return None;
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}
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let gid_index = glyph_id.to_u32() as usize;
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let mut lsb = self
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.h_metrics
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.get(gid_index)
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.map(|metric| metric.side_bearing())
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.unwrap_or_else(|| {
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self.lsbs
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.get(gid_index.saturating_sub(self.h_metrics.len()))
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.map(|lsb| lsb.get())
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.unwrap_or_default()
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}) as i32;
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if let Some(hvar) = &self.hvar {
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lsb += hvar
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.lsb_delta(glyph_id, self.coords)
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// FreeType truncates metric deltas...
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// https://github.com/freetype/freetype/blob/7838c78f53f206ac5b8e9cefde548aa81cb00cf4/src/truetype/ttgxvar.c#L1027
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.map(|delta| delta.to_f64() as i32)
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.unwrap_or(0);
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} else if self.gvar.is_some() {
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lsb += self.metric_deltas_from_gvar(glyph_id).unwrap_or_default()[0];
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}
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Some(self.fixed_scale.apply(lsb))
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}
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/// Returns the bounding box for the specified glyph.
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///
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/// Note that variations are not reflected in the bounding box returned by
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/// this method.
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///
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/// Returns `None` if `glyph_id >= self.glyph_count()`, the underlying font
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/// data is invalid, or the font does not contain TrueType outlines.
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pub fn bounds(&self, glyph_id: GlyphId) -> Option<BoundingBox> {
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let (loca, glyf) = self.loca_glyf.as_ref()?;
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Some(match loca.get_glyf(glyph_id, glyf).ok()? {
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Some(glyph) => BoundingBox {
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x_min: self.fixed_scale.apply(glyph.x_min() as i32),
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y_min: self.fixed_scale.apply(glyph.y_min() as i32),
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x_max: self.fixed_scale.apply(glyph.x_max() as i32),
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y_max: self.fixed_scale.apply(glyph.y_max() as i32),
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},
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// Empty glyphs have an empty bounding box
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None => BoundingBox::default(),
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})
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}
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}
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impl GlyphMetrics<'_> {
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fn metric_deltas_from_gvar(&self, glyph_id: GlyphId) -> Option<[i32; 2]> {
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let (loca, glyf) = self.loca_glyf.as_ref()?;
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let mut deltas = self
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.gvar
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.as_ref()?
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.phantom_point_deltas(glyf, loca, self.coords, glyph_id)
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.ok()
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.flatten()?;
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deltas[1] -= deltas[0];
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Some([deltas[0], deltas[1]].map(|delta| delta.x.to_i32()))
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}
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}
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#[derive(Copy, Clone)]
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struct FixedScaleFactor(Fixed);
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impl FixedScaleFactor {
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#[inline(always)]
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fn apply(self, value: i32) -> f32 {
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// Match FreeType metric scaling
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// <https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/base/ftadvanc.c#L50>
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self.0
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.mul_div(Fixed::from_bits(value), Fixed::from_bits(64))
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.to_f32()
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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use crate::MetadataProvider as _;
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use font_test_data::{SIMPLE_GLYF, VAZIRMATN_VAR};
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use read_fonts::FontRef;
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#[test]
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fn metrics() {
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let font = FontRef::new(SIMPLE_GLYF).unwrap();
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let metrics = font.metrics(Size::unscaled(), LocationRef::default());
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let expected = Metrics {
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units_per_em: 1024,
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glyph_count: 3,
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bounds: Some(BoundingBox {
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x_min: 51.0,
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y_min: -250.0,
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x_max: 998.0,
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y_max: 950.0,
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}),
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average_width: Some(1275.0),
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max_width: None,
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x_height: Some(512.0),
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cap_height: Some(717.0),
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is_monospace: false,
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italic_angle: 0.0,
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ascent: 950.0,
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descent: -250.0,
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leading: 0.0,
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underline: None,
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strikeout: Some(Decoration {
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offset: 307.0,
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thickness: 51.0,
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}),
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};
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assert_eq!(metrics, expected);
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}
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#[test]
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fn metrics_missing_os2() {
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let font = FontRef::new(VAZIRMATN_VAR).unwrap();
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let metrics = font.metrics(Size::unscaled(), LocationRef::default());
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let expected = Metrics {
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units_per_em: 2048,
|
|
glyph_count: 4,
|
|
bounds: Some(BoundingBox {
|
|
x_min: 29.0,
|
|
y_min: 0.0,
|
|
x_max: 1310.0,
|
|
y_max: 1847.0,
|
|
}),
|
|
average_width: None,
|
|
max_width: Some(1336.0),
|
|
x_height: None,
|
|
cap_height: None,
|
|
is_monospace: false,
|
|
italic_angle: 0.0,
|
|
ascent: 2100.0,
|
|
descent: -1100.0,
|
|
leading: 0.0,
|
|
underline: None,
|
|
strikeout: None,
|
|
};
|
|
assert_eq!(metrics, expected);
|
|
}
|
|
|
|
#[test]
|
|
fn glyph_metrics() {
|
|
let font = FontRef::new(VAZIRMATN_VAR).unwrap();
|
|
let glyph_metrics = font.glyph_metrics(Size::unscaled(), LocationRef::default());
|
|
// (advance_width, lsb) in glyph order
|
|
let expected = &[
|
|
(908.0, 100.0),
|
|
(1336.0, 29.0),
|
|
(1336.0, 29.0),
|
|
(633.0, 57.0),
|
|
];
|
|
let result = (0..4)
|
|
.map(|i| {
|
|
let gid = GlyphId::new(i as u32);
|
|
let advance_width = glyph_metrics.advance_width(gid).unwrap();
|
|
let lsb = glyph_metrics.left_side_bearing(gid).unwrap();
|
|
(advance_width, lsb)
|
|
})
|
|
.collect::<Vec<_>>();
|
|
assert_eq!(expected, &result[..]);
|
|
}
|
|
|
|
/// Asserts that the results generated with Size::unscaled() and
|
|
/// Size::new(upem) are equal.
|
|
///
|
|
/// See <https://github.com/googlefonts/fontations/issues/590#issuecomment-1711595882>
|
|
#[test]
|
|
fn glyph_metrics_unscaled_matches_upem_scale() {
|
|
let font = FontRef::new(VAZIRMATN_VAR).unwrap();
|
|
let upem = font.head().unwrap().units_per_em() as f32;
|
|
let unscaled_metrics = font.glyph_metrics(Size::unscaled(), LocationRef::default());
|
|
let upem_metrics = font.glyph_metrics(Size::new(upem), LocationRef::default());
|
|
for i in 0..unscaled_metrics.glyph_count() {
|
|
let gid = GlyphId::new(i);
|
|
assert_eq!(
|
|
unscaled_metrics.advance_width(gid),
|
|
upem_metrics.advance_width(gid)
|
|
);
|
|
assert_eq!(
|
|
unscaled_metrics.left_side_bearing(gid),
|
|
upem_metrics.left_side_bearing(gid)
|
|
);
|
|
}
|
|
}
|
|
|
|
#[test]
|
|
fn glyph_metrics_var() {
|
|
let font = FontRef::new(VAZIRMATN_VAR).unwrap();
|
|
let coords = &[NormalizedCoord::from_f32(-0.8)];
|
|
let glyph_metrics = font.glyph_metrics(Size::unscaled(), LocationRef::new(coords));
|
|
// (advance_width, lsb) in glyph order
|
|
let expected = &[
|
|
(908.0, 100.0),
|
|
(1246.0, 29.0),
|
|
(1246.0, 29.0),
|
|
(556.0, 57.0),
|
|
];
|
|
let result = (0..4)
|
|
.map(|i| {
|
|
let gid = GlyphId::new(i as u32);
|
|
let advance_width = glyph_metrics.advance_width(gid).unwrap();
|
|
let lsb = glyph_metrics.left_side_bearing(gid).unwrap();
|
|
(advance_width, lsb)
|
|
})
|
|
.collect::<Vec<_>>();
|
|
assert_eq!(expected, &result[..]);
|
|
}
|
|
|
|
#[test]
|
|
fn glyph_metrics_missing_hvar() {
|
|
let font = FontRef::new(VAZIRMATN_VAR).unwrap();
|
|
let glyph_count = font.maxp().unwrap().num_glyphs();
|
|
// Test a few different locations in variation space
|
|
for coord in [-1.0, -0.8, 0.0, 0.75, 1.0] {
|
|
let coords = &[NormalizedCoord::from_f32(coord)];
|
|
let location = LocationRef::new(coords);
|
|
let glyph_metrics = font.glyph_metrics(Size::unscaled(), location);
|
|
let mut glyph_metrics_no_hvar = glyph_metrics.clone();
|
|
// Setting hvar to None forces use of gvar for metric deltas
|
|
glyph_metrics_no_hvar.hvar = None;
|
|
for gid in 0..glyph_count {
|
|
let gid = GlyphId::from(gid);
|
|
assert_eq!(
|
|
glyph_metrics.advance_width(gid),
|
|
glyph_metrics_no_hvar.advance_width(gid)
|
|
);
|
|
assert_eq!(
|
|
glyph_metrics.left_side_bearing(gid),
|
|
glyph_metrics_no_hvar.left_side_bearing(gid)
|
|
);
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Ensure our fixed point scaling code matches FreeType for advances.
|
|
///
|
|
/// <https://github.com/googlefonts/fontations/issues/590>
|
|
#[test]
|
|
fn match_freetype_glyph_metric_scaling() {
|
|
// fontations:
|
|
// gid: 36 advance: 15.33600044250488281250 gid: 68 advance: 13.46399974822998046875 gid: 47 advance: 12.57600021362304687500 gid: 79 advance: 6.19199991226196289062
|
|
// ft:
|
|
// gid: 36 advance: 15.33595275878906250000 gid: 68 advance: 13.46395874023437500000 gid: 47 advance: 12.57595825195312500000 gid: 79 advance: 6.19198608398437500000
|
|
// with font.setSize(24);
|
|
//
|
|
// Raw advances for gids 36, 68, 47, and 79 in NotoSans-Regular
|
|
let font_unit_advances = [639, 561, 524, 258];
|
|
#[allow(clippy::excessive_precision)]
|
|
let scaled_advances = [
|
|
15.33595275878906250000,
|
|
13.46395874023437500000,
|
|
12.57595825195312500000,
|
|
6.19198608398437500000,
|
|
];
|
|
let fixed_scale = FixedScaleFactor(Size::new(24.0).fixed_linear_scale(1000));
|
|
for (font_unit_advance, expected_scaled_advance) in
|
|
font_unit_advances.iter().zip(scaled_advances)
|
|
{
|
|
let scaled_advance = fixed_scale.apply(*font_unit_advance);
|
|
assert_eq!(scaled_advance, expected_scaled_advance);
|
|
}
|
|
}
|
|
}
|