zed/crates/gpui/src/text_system.rs

mod font_features;
mod line;
mod line_layout;
mod line_wrapper;

pub use font_features::*;
pub use line::*;
pub use line_layout::*;
pub use line_wrapper::*;

use crate::{
    px, Bounds, DevicePixels, Hsla, Pixels, PlatformTextSystem, Point, Result, SharedString, Size,
    StrikethroughStyle, UnderlineStyle,
};
use anyhow::anyhow;
use collections::{BTreeSet, FxHashMap};
use core::fmt;
use derive_more::Deref;
use itertools::Itertools;
use parking_lot::{Mutex, RwLock, RwLockUpgradableReadGuard};
use smallvec::{smallvec, SmallVec};
use std::{
    borrow::Cow,
    cmp,
    fmt::{Debug, Display, Formatter},
    hash::{Hash, Hasher},
    ops::{Deref, DerefMut, Range},
    sync::Arc,
};

/// An opaque identifier for a specific font.
#[derive(Hash, PartialEq, Eq, Clone, Copy, Debug)]
#[repr(C)]
pub struct FontId(pub usize);

/// An opaque identifier for a specific font family.
#[derive(Hash, PartialEq, Eq, Clone, Copy, Debug)]
pub struct FontFamilyId(pub usize);

pub(crate) const SUBPIXEL_VARIANTS: u8 = 4;

/// The GPUI text rendering sub system.
pub struct TextSystem {
    platform_text_system: Arc<dyn PlatformTextSystem>,
    font_ids_by_font: RwLock<FxHashMap<Font, Result<FontId>>>,
    font_metrics: RwLock<FxHashMap<FontId, FontMetrics>>,
    raster_bounds: RwLock<FxHashMap<RenderGlyphParams, Bounds<DevicePixels>>>,
    wrapper_pool: Mutex<FxHashMap<FontIdWithSize, Vec<LineWrapper>>>,
    font_runs_pool: Mutex<Vec<Vec<FontRun>>>,
    fallback_font_stack: SmallVec<[Font; 2]>,
}

impl TextSystem {
    pub(crate) fn new(platform_text_system: Arc<dyn PlatformTextSystem>) -> Self {
        TextSystem {
            platform_text_system,
            font_metrics: RwLock::default(),
            raster_bounds: RwLock::default(),
            font_ids_by_font: RwLock::default(),
            wrapper_pool: Mutex::default(),
            font_runs_pool: Mutex::default(),
            fallback_font_stack: smallvec![
                // TODO: This is currently Zed-specific.
                // We should allow GPUI users to provide their own fallback font stack.
                font("Zed Mono"),
                font("Helvetica"),
                font("Cantarell"), // Gnome
                font("Ubuntu"),    // Gnome (Ubuntu)
                font("Noto Sans"), // KDE
            ],
        }
    }

    /// Get a list of all available font names from the operating system.
    pub fn all_font_names(&self) -> Vec<String> {
        let mut names: BTreeSet<_> = self
            .platform_text_system
            .all_font_names()
            .into_iter()
            .collect();
        names.extend(self.platform_text_system.all_font_families());
        names.extend(
            self.fallback_font_stack
                .iter()
                .map(|font| font.family.to_string()),
        );
        names.into_iter().collect()
    }

    /// Add a font's data to the text system.
    pub fn add_fonts(&self, fonts: Vec<Cow<'static, [u8]>>) -> Result<()> {
        self.platform_text_system.add_fonts(fonts)
    }

    /// Get the FontId for the configure font family and style.
    pub fn font_id(&self, font: &Font) -> Result<FontId> {
        fn clone_font_id_result(font_id: &Result<FontId>) -> Result<FontId> {
            match font_id {
                Ok(font_id) => Ok(*font_id),
                Err(err) => Err(anyhow!("{}", err)),
            }
        }

        let font_id = self
            .font_ids_by_font
            .read()
            .get(font)
            .map(clone_font_id_result);
        if let Some(font_id) = font_id {
            font_id
        } else {
            let font_id = self.platform_text_system.font_id(font);
            self.font_ids_by_font
                .write()
                .insert(font.clone(), clone_font_id_result(&font_id));
            font_id
        }
    }

    /// Resolves the specified font, falling back to the default font stack if
    /// the font fails to load.
    ///
    /// # Panics
    ///
    /// Panics if the font and none of the fallbacks can be resolved.
    pub fn resolve_font(&self, font: &Font) -> FontId {
        if let Ok(font_id) = self.font_id(font) {
            return font_id;
        }
        for fallback in &self.fallback_font_stack {
            if let Ok(font_id) = self.font_id(fallback) {
                return font_id;
            }
        }

        panic!(
            "failed to resolve font '{}' or any of the fallbacks: {}",
            font.family,
            self.fallback_font_stack
                .iter()
                .map(|fallback| &fallback.family)
                .join(", ")
        );
    }

    /// Get the bounding box for the given font and font size.
    /// A font's bounding box is the smallest rectangle that could enclose all glyphs
    /// in the font. superimposed over one another.
    pub fn bounding_box(&self, font_id: FontId, font_size: Pixels) -> Bounds<Pixels> {
        self.read_metrics(font_id, |metrics| metrics.bounding_box(font_size))
    }

    /// Get the typographic bounds for the given character, in the given font and size.
    pub fn typographic_bounds(
        &self,
        font_id: FontId,
        font_size: Pixels,
        character: char,
    ) -> Result<Bounds<Pixels>> {
        let glyph_id = self
            .platform_text_system
            .glyph_for_char(font_id, character)
            .ok_or_else(|| anyhow!("glyph not found for character '{}'", character))?;
        let bounds = self
            .platform_text_system
            .typographic_bounds(font_id, glyph_id)?;
        Ok(self.read_metrics(font_id, |metrics| {
            (bounds / metrics.units_per_em as f32 * font_size.0).map(px)
        }))
    }

    /// Get the advance width for the given character, in the given font and size.
    pub fn advance(&self, font_id: FontId, font_size: Pixels, ch: char) -> Result<Size<Pixels>> {
        let glyph_id = self
            .platform_text_system
            .glyph_for_char(font_id, ch)
            .ok_or_else(|| anyhow!("glyph not found for character '{}'", ch))?;
        let result = self.platform_text_system.advance(font_id, glyph_id)?
            / self.units_per_em(font_id) as f32;

        Ok(result * font_size)
    }

    /// Get the number of font size units per 'em square',
    /// Per MDN: "an abstract square whose height is the intended distance between
    /// lines of type in the same type size"
    pub fn units_per_em(&self, font_id: FontId) -> u32 {
        self.read_metrics(font_id, |metrics| metrics.units_per_em)
    }

    /// Get the height of a capital letter in the given font and size.
    pub fn cap_height(&self, font_id: FontId, font_size: Pixels) -> Pixels {
        self.read_metrics(font_id, |metrics| metrics.cap_height(font_size))
    }

    /// Get the height of the x character in the given font and size.
    pub fn x_height(&self, font_id: FontId, font_size: Pixels) -> Pixels {
        self.read_metrics(font_id, |metrics| metrics.x_height(font_size))
    }

    /// Get the recommended distance from the baseline for the given font
    pub fn ascent(&self, font_id: FontId, font_size: Pixels) -> Pixels {
        self.read_metrics(font_id, |metrics| metrics.ascent(font_size))
    }

    /// Get the recommended distance below the baseline for the given font,
    /// in single spaced text.
    pub fn descent(&self, font_id: FontId, font_size: Pixels) -> Pixels {
        self.read_metrics(font_id, |metrics| metrics.descent(font_size))
    }

    /// Get the recommended baseline offset for the given font and line height.
    pub fn baseline_offset(
        &self,
        font_id: FontId,
        font_size: Pixels,
        line_height: Pixels,
    ) -> Pixels {
        let ascent = self.ascent(font_id, font_size);
        let descent = self.descent(font_id, font_size);
        let padding_top = (line_height - ascent - descent) / 2.;
        padding_top + ascent
    }

    fn read_metrics<T>(&self, font_id: FontId, read: impl FnOnce(&FontMetrics) -> T) -> T {
        let lock = self.font_metrics.upgradable_read();

        if let Some(metrics) = lock.get(&font_id) {
            read(metrics)
        } else {
            let mut lock = RwLockUpgradableReadGuard::upgrade(lock);
            let metrics = lock
                .entry(font_id)
                .or_insert_with(|| self.platform_text_system.font_metrics(font_id));
            read(metrics)
        }
    }

    /// Returns a handle to a line wrapper, for the given font and font size.
    pub fn line_wrapper(self: &Arc<Self>, font: Font, font_size: Pixels) -> LineWrapperHandle {
        let lock = &mut self.wrapper_pool.lock();
        let font_id = self.resolve_font(&font);
        let wrappers = lock
            .entry(FontIdWithSize { font_id, font_size })
            .or_default();
        let wrapper = wrappers.pop().unwrap_or_else(|| {
            LineWrapper::new(font_id, font_size, self.platform_text_system.clone())
        });

        LineWrapperHandle {
            wrapper: Some(wrapper),
            text_system: self.clone(),
        }
    }

    /// Get the rasterized size and location of a specific, rendered glyph.
    pub(crate) fn raster_bounds(&self, params: &RenderGlyphParams) -> Result<Bounds<DevicePixels>> {
        let raster_bounds = self.raster_bounds.upgradable_read();
        if let Some(bounds) = raster_bounds.get(params) {
            Ok(*bounds)
        } else {
            let mut raster_bounds = RwLockUpgradableReadGuard::upgrade(raster_bounds);
            let bounds = self.platform_text_system.glyph_raster_bounds(params)?;
            raster_bounds.insert(params.clone(), bounds);
            Ok(bounds)
        }
    }

    pub(crate) fn rasterize_glyph(
        &self,
        params: &RenderGlyphParams,
    ) -> Result<(Size<DevicePixels>, Vec<u8>)> {
        let raster_bounds = self.raster_bounds(params)?;
        self.platform_text_system
            .rasterize_glyph(params, raster_bounds)
    }
}

/// The GPUI text layout subsystem.
#[derive(Deref)]
pub struct WindowTextSystem {
    line_layout_cache: LineLayoutCache,
    #[deref]
    text_system: Arc<TextSystem>,
}

impl WindowTextSystem {
    pub(crate) fn new(text_system: Arc<TextSystem>) -> Self {
        Self {
            line_layout_cache: LineLayoutCache::new(text_system.platform_text_system.clone()),
            text_system,
        }
    }

    pub(crate) fn layout_index(&self) -> LineLayoutIndex {
        self.line_layout_cache.layout_index()
    }

    pub(crate) fn reuse_layouts(&self, index: Range<LineLayoutIndex>) {
        self.line_layout_cache.reuse_layouts(index)
    }

    /// Shape the given line, at the given font_size, for painting to the screen.
    /// Subsets of the line can be styled independently with the `runs` parameter.
    ///
    /// Note that this method can only shape a single line of text. It will panic
    /// if the text contains newlines. If you need to shape multiple lines of text,
    /// use `TextLayout::shape_text` instead.
    pub fn shape_line(
        &self,
        text: SharedString,
        font_size: Pixels,
        runs: &[TextRun],
    ) -> Result<ShapedLine> {
        debug_assert!(
            text.find('\n').is_none(),
            "text argument should not contain newlines"
        );

        let mut decoration_runs = SmallVec::<[DecorationRun; 32]>::new();
        for run in runs {
            if let Some(last_run) = decoration_runs.last_mut() {
                if last_run.color == run.color
                    && last_run.underline == run.underline
                    && last_run.strikethrough == run.strikethrough
                    && last_run.background_color == run.background_color
                {
                    last_run.len += run.len as u32;
                    continue;
                }
            }
            decoration_runs.push(DecorationRun {
                len: run.len as u32,
                color: run.color,
                background_color: run.background_color,
                underline: run.underline,
                strikethrough: run.strikethrough,
            });
        }

        let layout = self.layout_line(text.as_ref(), font_size, runs)?;

        Ok(ShapedLine {
            layout,
            text,
            decoration_runs,
        })
    }

    /// Shape a multi line string of text, at the given font_size, for painting to the screen.
    /// Subsets of the text can be styled independently with the `runs` parameter.
    /// If `wrap_width` is provided, the line breaks will be adjusted to fit within the given width.
    pub fn shape_text(
        &self,
        text: SharedString,
        font_size: Pixels,
        runs: &[TextRun],
        wrap_width: Option<Pixels>,
    ) -> Result<SmallVec<[WrappedLine; 1]>> {
        let mut runs = runs.iter().cloned().peekable();
        let mut font_runs = self.font_runs_pool.lock().pop().unwrap_or_default();

        let mut lines = SmallVec::new();
        let mut line_start = 0;

        let mut process_line = |line_text: SharedString| {
            let line_end = line_start + line_text.len();

            let mut last_font: Option<Font> = None;
            let mut decoration_runs = SmallVec::<[DecorationRun; 32]>::new();
            let mut run_start = line_start;
            while run_start < line_end {
                let Some(run) = runs.peek_mut() else {
                    break;
                };

                let run_len_within_line = cmp::min(line_end, run_start + run.len) - run_start;

                if last_font == Some(run.font.clone()) {
                    font_runs.last_mut().unwrap().len += run_len_within_line;
                } else {
                    last_font = Some(run.font.clone());
                    font_runs.push(FontRun {
                        len: run_len_within_line,
                        font_id: self.resolve_font(&run.font),
                    });
                }

                if decoration_runs.last().map_or(false, |last_run| {
                    last_run.color == run.color
                        && last_run.underline == run.underline
                        && last_run.strikethrough == run.strikethrough
                        && last_run.background_color == run.background_color
                }) {
                    decoration_runs.last_mut().unwrap().len += run_len_within_line as u32;
                } else {
                    decoration_runs.push(DecorationRun {
                        len: run_len_within_line as u32,
                        color: run.color,
                        background_color: run.background_color,
                        underline: run.underline,
                        strikethrough: run.strikethrough,
                    });
                }

                if run_len_within_line == run.len {
                    runs.next();
                } else {
                    // Preserve the remainder of the run for the next line
                    run.len -= run_len_within_line;
                }
                run_start += run_len_within_line;
            }

            let layout = self
                .line_layout_cache
                .layout_wrapped_line(&line_text, font_size, &font_runs, wrap_width);

            lines.push(WrappedLine {
                layout,
                decoration_runs,
                text: line_text,
            });

            // Skip `\n` character.
            line_start = line_end + 1;
            if let Some(run) = runs.peek_mut() {
                run.len = run.len.saturating_sub(1);
                if run.len == 0 {
                    runs.next();
                }
            }

            font_runs.clear();
        };

        let mut split_lines = text.split('\n');
        let mut processed = false;

        if let Some(first_line) = split_lines.next() {
            if let Some(second_line) = split_lines.next() {
                processed = true;
                process_line(first_line.to_string().into());
                process_line(second_line.to_string().into());
                for line_text in split_lines {
                    process_line(line_text.to_string().into());
                }
            }
        }

        if !processed {
            process_line(text);
        }

        self.font_runs_pool.lock().push(font_runs);

        Ok(lines)
    }

    pub(crate) fn finish_frame(&self) {
        self.line_layout_cache.finish_frame()
    }

    /// Layout the given line of text, at the given font_size.
    /// Subsets of the line can be styled independently with the `runs` parameter.
    /// Generally, you should prefer to use `TextLayout::shape_line` instead, which
    /// can be painted directly.
    pub fn layout_line(
        &self,
        text: &str,
        font_size: Pixels,
        runs: &[TextRun],
    ) -> Result<Arc<LineLayout>> {
        let mut font_runs = self.font_runs_pool.lock().pop().unwrap_or_default();
        for run in runs.iter() {
            let font_id = self.resolve_font(&run.font);
            if let Some(last_run) = font_runs.last_mut() {
                if last_run.font_id == font_id {
                    last_run.len += run.len;
                    continue;
                }
            }
            font_runs.push(FontRun {
                len: run.len,
                font_id,
            });
        }

        let layout = self
            .line_layout_cache
            .layout_line(text, font_size, &font_runs);

        font_runs.clear();
        self.font_runs_pool.lock().push(font_runs);

        Ok(layout)
    }
}

#[derive(Hash, Eq, PartialEq)]
struct FontIdWithSize {
    font_id: FontId,
    font_size: Pixels,
}

/// A handle into the text system, which can be used to compute the wrapped layout of text
pub struct LineWrapperHandle {
    wrapper: Option<LineWrapper>,
    text_system: Arc<TextSystem>,
}

impl Drop for LineWrapperHandle {
    fn drop(&mut self) {
        let mut state = self.text_system.wrapper_pool.lock();
        let wrapper = self.wrapper.take().unwrap();
        state
            .get_mut(&FontIdWithSize {
                font_id: wrapper.font_id,
                font_size: wrapper.font_size,
            })
            .unwrap()
            .push(wrapper);
    }
}

impl Deref for LineWrapperHandle {
    type Target = LineWrapper;

    fn deref(&self) -> &Self::Target {
        self.wrapper.as_ref().unwrap()
    }
}

impl DerefMut for LineWrapperHandle {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.wrapper.as_mut().unwrap()
    }
}

/// The degree of blackness or stroke thickness of a font. This value ranges from 100.0 to 900.0,
/// with 400.0 as normal.
#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
pub struct FontWeight(pub f32);

impl Default for FontWeight {
    #[inline]
    fn default() -> FontWeight {
        FontWeight::NORMAL
    }
}

impl Hash for FontWeight {
    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write_u32(u32::from_be_bytes(self.0.to_be_bytes()));
    }
}

impl Eq for FontWeight {}

impl FontWeight {
    /// Thin weight (100), the thinnest value.
    pub const THIN: FontWeight = FontWeight(100.0);
    /// Extra light weight (200).
    pub const EXTRA_LIGHT: FontWeight = FontWeight(200.0);
    /// Light weight (300).
    pub const LIGHT: FontWeight = FontWeight(300.0);
    /// Normal (400).
    pub const NORMAL: FontWeight = FontWeight(400.0);
    /// Medium weight (500, higher than normal).
    pub const MEDIUM: FontWeight = FontWeight(500.0);
    /// Semibold weight (600).
    pub const SEMIBOLD: FontWeight = FontWeight(600.0);
    /// Bold weight (700).
    pub const BOLD: FontWeight = FontWeight(700.0);
    /// Extra-bold weight (800).
    pub const EXTRA_BOLD: FontWeight = FontWeight(800.0);
    /// Black weight (900), the thickest value.
    pub const BLACK: FontWeight = FontWeight(900.0);
}

/// Allows italic or oblique faces to be selected.
#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash, Default)]
pub enum FontStyle {
    /// A face that is neither italic not obliqued.
    #[default]
    Normal,
    /// A form that is generally cursive in nature.
    Italic,
    /// A typically-sloped version of the regular face.
    Oblique,
}

impl Display for FontStyle {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        Debug::fmt(self, f)
    }
}

/// A styled run of text, for use in [`TextLayout`].
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct TextRun {
    /// A number of utf8 bytes
    pub len: usize,
    /// The font to use for this run.
    pub font: Font,
    /// The color
    pub color: Hsla,
    /// The background color (if any)
    pub background_color: Option<Hsla>,
    /// The underline style (if any)
    pub underline: Option<UnderlineStyle>,
    /// The strikethrough style (if any)
    pub strikethrough: Option<StrikethroughStyle>,
}

/// An identifier for a specific glyph, as returned by [`TextSystem::layout_line`].
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
#[repr(C)]
pub struct GlyphId(pub(crate) u32);

#[derive(Clone, Debug, PartialEq)]
pub(crate) struct RenderGlyphParams {
    pub(crate) font_id: FontId,
    pub(crate) glyph_id: GlyphId,
    pub(crate) font_size: Pixels,
    pub(crate) subpixel_variant: Point<u8>,
    pub(crate) scale_factor: f32,
    pub(crate) is_emoji: bool,
}

impl Eq for RenderGlyphParams {}

impl Hash for RenderGlyphParams {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.font_id.0.hash(state);
        self.glyph_id.0.hash(state);
        self.font_size.0.to_bits().hash(state);
        self.subpixel_variant.hash(state);
        self.scale_factor.to_bits().hash(state);
    }
}

/// The parameters for rendering an emoji glyph.
#[derive(Clone, Debug, PartialEq)]
pub struct RenderEmojiParams {
    pub(crate) font_id: FontId,
    pub(crate) glyph_id: GlyphId,
    pub(crate) font_size: Pixels,
    pub(crate) scale_factor: f32,
}

impl Eq for RenderEmojiParams {}

impl Hash for RenderEmojiParams {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.font_id.0.hash(state);
        self.glyph_id.0.hash(state);
        self.font_size.0.to_bits().hash(state);
        self.scale_factor.to_bits().hash(state);
    }
}

/// The configuration details for identifying a specific font.
#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub struct Font {
    /// The font family name.
    pub family: SharedString,

    /// The font features to use.
    pub features: FontFeatures,

    /// The font weight.
    pub weight: FontWeight,

    /// The font style.
    pub style: FontStyle,
}

/// Get a [`Font`] for a given name.
pub fn font(family: impl Into<SharedString>) -> Font {
    Font {
        family: family.into(),
        features: FontFeatures::default(),
        weight: FontWeight::default(),
        style: FontStyle::default(),
    }
}

impl Font {
    /// Set this Font to be bold
    pub fn bold(mut self) -> Self {
        self.weight = FontWeight::BOLD;
        self
    }

    /// Set this Font to be italic
    pub fn italic(mut self) -> Self {
        self.style = FontStyle::Italic;
        self
    }
}

/// A struct for storing font metrics.
/// It is used to define the measurements of a typeface.
#[derive(Clone, Copy, Debug)]
pub struct FontMetrics {
    /// The number of font units that make up the "em square",
    /// a scalable grid for determining the size of a typeface.
    pub(crate) units_per_em: u32,

    /// The vertical distance from the baseline of the font to the top of the glyph covers.
    pub(crate) ascent: f32,

    /// The vertical distance from the baseline of the font to the bottom of the glyph covers.
    pub(crate) descent: f32,

    /// The recommended additional space to add between lines of type.
    pub(crate) line_gap: f32,

    /// The suggested position of the underline.
    pub(crate) underline_position: f32,

    /// The suggested thickness of the underline.
    pub(crate) underline_thickness: f32,

    /// The height of a capital letter measured from the baseline of the font.
    pub(crate) cap_height: f32,

    /// The height of a lowercase x.
    pub(crate) x_height: f32,

    /// The outer limits of the area that the font covers.
    /// Corresponds to the xMin / xMax / yMin / yMax values in the OpenType `head` table
    pub(crate) bounding_box: Bounds<f32>,
}

impl FontMetrics {
    /// Returns the vertical distance from the baseline of the font to the top of the glyph covers in pixels.
    pub fn ascent(&self, font_size: Pixels) -> Pixels {
        Pixels((self.ascent / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the vertical distance from the baseline of the font to the bottom of the glyph covers in pixels.
    pub fn descent(&self, font_size: Pixels) -> Pixels {
        Pixels((self.descent / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the recommended additional space to add between lines of type in pixels.
    pub fn line_gap(&self, font_size: Pixels) -> Pixels {
        Pixels((self.line_gap / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the suggested position of the underline in pixels.
    pub fn underline_position(&self, font_size: Pixels) -> Pixels {
        Pixels((self.underline_position / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the suggested thickness of the underline in pixels.
    pub fn underline_thickness(&self, font_size: Pixels) -> Pixels {
        Pixels((self.underline_thickness / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the height of a capital letter measured from the baseline of the font in pixels.
    pub fn cap_height(&self, font_size: Pixels) -> Pixels {
        Pixels((self.cap_height / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the height of a lowercase x in pixels.
    pub fn x_height(&self, font_size: Pixels) -> Pixels {
        Pixels((self.x_height / self.units_per_em as f32) * font_size.0)
    }

    /// Returns the outer limits of the area that the font covers in pixels.
    pub fn bounding_box(&self, font_size: Pixels) -> Bounds<Pixels> {
        (self.bounding_box / self.units_per_em as f32 * font_size.0).map(px)
    }
}