Add a canvas based font fallback

Cargo.lock

@@ -1622,6 +1622,8 @@ dependencies = [
  "similar-asserts",
  "skrifa",
  "vello_cpu",
+ "wasm-bindgen",
+ "web-sys",
 ]
 
 [[package]]

crates/epaint/Cargo.toml

@@ -83,6 +83,17 @@ serde = { workspace = true, optional = true, features = ["derive", "rc"] }
 
 epaint_default_fonts = { workspace = true, optional = true }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+wasm-bindgen.workspace = true
+web-sys = { workspace = true, features = [
+    "CanvasRenderingContext2d",
+    "HtmlCanvasElement",
+    "ImageData",
+    "TextMetrics",
+    "Document",
+    "Window",
+] }
+
 [dev-dependencies]
 criterion.workspace = true
 mimalloc.workspace = true

crates/epaint/src/text/canvas_renderer.rs

@@ -0,0 +1,225 @@
+use std::cell::RefCell;
+
+use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsCast;
+use web_sys::{CanvasRenderingContext2d, HtmlCanvasElement};
+
+use crate::text::font::ScaledMetrics;
+
+/// Data for a glyph rendered via HTML5 canvas
+#[derive(Debug, Clone)]
+pub struct CanvasGlyphData {
+    /// RGBA pixel data from canvas ImageData
+    pub image_data: Vec<u8>,
+    /// Width of the glyph in pixels
+    pub width: u32,
+    /// Height of the glyph in pixels
+    pub height: u32,
+    /// Advance width for horizontal text layout
+    pub advance_width: f32,
+    /// Horizontal offset from origin
+    pub offset_x: f32,
+    /// Vertical offset from origin (baseline)
+    pub offset_y: f32,
+}
+
+/// Renders glyphs using HTML5 canvas (WASM only)
+///
+/// This renderer uses the browser's native text rendering capabilities
+/// to rasterize glyphs that are not available in the bundled fonts.
+pub struct CanvasGlyphRenderer {
+    canvas: HtmlCanvasElement,
+    context: CanvasRenderingContext2d,
+}
+
+impl CanvasGlyphRenderer {
+    /// Create a new canvas glyph renderer
+    ///
+    /// Returns an error if canvas creation or context acquisition fails
+    pub fn new() -> Result<Self, JsValue> {
+        let window = web_sys::window().ok_or("No window object")?;
+        let document = window.document().ok_or("No document object")?;
+
+        let canvas = document
+            .create_element("canvas")?
+            .dyn_into::<HtmlCanvasElement>()?;
+
+        // Start with a small canvas, will resize as needed
+        canvas.set_width(128);
+        canvas.set_height(128);
+
+        let context = canvas
+            .get_context("2d")?
+            .ok_or("Failed to get 2d context")?
+            .dyn_into::<CanvasRenderingContext2d>()?;
+
+        Ok(Self { canvas, context })
+    }
+
+    /// Render a glyph using canvas
+    ///
+    /// Tries each font family in order until one renders the character successfully.
+    /// Returns None if the character cannot be rendered or has zero width.
+    pub fn render_glyph(
+        &mut self,
+        chr: char,
+        metrics: &ScaledMetrics,
+        font_families: &[String],
+        bin: crate::text::font::SubpixelBin,
+    ) -> Option<CanvasGlyphData> {
+        // metrics.scale is the absolute font size in pixels (includes DPI and zoom)
+        let font_size_px = metrics.scale;
+        let subpixel_offset = bin.as_float();
+
+        // Try each font family in the fallback chain
+        for family in font_families {
+            if let Some(data) = self.try_render_with_font(chr, font_size_px, subpixel_offset, family) {
+                return Some(data);
+            }
+        }
+
+        // Try with generic sans-serif as last resort
+        self.try_render_with_font(chr, font_size_px, subpixel_offset, "sans-serif")
+    }
+
+    /// Try to render a glyph with a specific font family
+    fn try_render_with_font(
+        &mut self,
+        chr: char,
+        font_size_px: f32,
+        subpixel_offset: f32,
+        font_family: &str,
+    ) -> Option<CanvasGlyphData> {
+        let font_string = format!("{}px {}", font_size_px, font_family);
+        self.context.set_font(&font_string);
+
+        let text = chr.to_string();
+
+        // Measure the text to get metrics
+        let text_metrics = self.context.measure_text(&text).ok()?;
+        let advance_width = text_metrics.width() as f32;
+
+        // Skip if character is not supported (zero width)
+        if advance_width < 0.1 {
+            return None;
+        }
+
+        // Get bounding box metrics
+        let ascent = text_metrics.actual_bounding_box_ascent();
+        let descent = text_metrics.actual_bounding_box_descent();
+        let left = text_metrics.actual_bounding_box_left();
+        let right = text_metrics.actual_bounding_box_right();
+
+        // Canvas measureText returns values at the specified font size
+        // We render at font_size_px which is already in the right scale
+        let width = (right + left).ceil() as u32;
+        let height = (ascent + descent).ceil() as u32;
+
+        // Skip zero-size glyphs
+        if width == 0 || height == 0 {
+            return None;
+        }
+
+        // Limit maximum glyph size to prevent excessive memory usage
+        const MAX_GLYPH_SIZE: u32 = 256;
+        if width > MAX_GLYPH_SIZE || height > MAX_GLYPH_SIZE {
+            log::warn!(
+                "Glyph '{}' too large ({}x{}), max size is {}x{}",
+                chr,
+                width,
+                height,
+                MAX_GLYPH_SIZE,
+                MAX_GLYPH_SIZE
+            );
+            return None;
+        }
+
+        // Resize canvas if needed
+        if width > self.canvas.width() || height > self.canvas.height() {
+            self.canvas.set_width(width.max(128));
+            self.canvas.set_height(height.max(128));
+        }
+
+        // Clear the canvas
+        self.context.clear_rect(
+            0.0,
+            0.0,
+            self.canvas.width() as f64,
+            self.canvas.height() as f64,
+        );
+
+        // Set up rendering
+        self.context.set_fill_style_str("white");
+        self.context.set_text_baseline("alphabetic");
+
+        // Render the text at the correct position
+        // The baseline is at y = ascent, and we offset x by left bearing + subpixel offset
+        if let Err(e) = self.context.fill_text(&text, left + subpixel_offset as f64, ascent) {
+            log::debug!("Failed to render '{}': {:?}", chr, e);
+            return None;
+        }
+
+        // Extract image data (now at device pixel resolution)
+        let image_data = self
+            .context
+            .get_image_data(0.0, 0.0, width as f64, height as f64)
+            .ok()?;
+
+        let rgba_data = image_data.data().0;
+
+        log::debug!(
+            "Canvas glyph '{}': advance={}, size={}x{}, offset=({}, {}), font_size={}px",
+            chr, advance_width, width, height, -left as f32, -ascent as f32, font_size_px
+        );
+
+        Some(CanvasGlyphData {
+            image_data: rgba_data,
+            width,
+            height,
+            advance_width,
+            offset_x: -left as f32,
+            offset_y: -ascent as f32,
+        })
+    }
+}
+
+thread_local! {
+    static CANVAS_RENDERER: RefCell<Option<CanvasGlyphRenderer>> = RefCell::new(None);
+}
+
+/// Initialize the canvas renderer if not already initialized
+fn ensure_canvas_renderer() -> Result<(), JsValue> {
+    CANVAS_RENDERER.with(|renderer_cell| {
+        if renderer_cell.borrow().is_none() {
+            match CanvasGlyphRenderer::new() {
+                Ok(renderer) => {
+                    *renderer_cell.borrow_mut() = Some(renderer);
+                    Ok(())
+                }
+                Err(e) => {
+                    log::warn!("Failed to create canvas renderer: {:?}", e);
+                    Err(e)
+                }
+            }
+        } else {
+            Ok(())
+        }
+    })
+}
+
+/// Render a glyph using the thread-local canvas renderer
+pub fn render_glyph_with_canvas(
+    chr: char,
+    metrics: &ScaledMetrics,
+    font_families: &[String],
+    bin: crate::text::font::SubpixelBin,
+) -> Option<CanvasGlyphData> {
+    ensure_canvas_renderer().ok()?;
+
+    CANVAS_RENDERER.with(|renderer_cell| {
+        renderer_cell
+            .borrow_mut()
+            .as_mut()?
+            .render_glyph(chr, metrics, font_families, bin)
+    })
+}

crates/epaint/src/text/font.rs

@@ -204,10 +204,10 @@ impl FontCell {
     ) -> Option<GlyphAllocation> {
         let glyph_id = glyph_info.id?;
 
-        debug_assert!(
-            glyph_id != skrifa::GlyphId::NOTDEF,
-            "Can't allocate glyph for id 0"
-        );
+        // Return None for NOTDEF - this will trigger canvas fallback on WASM
+        if glyph_id == skrifa::GlyphId::NOTDEF {
+            return None;
+        }
 
         let mut path = kurbo::BezPath::new();
         let mut pen = VelloPen {
@@ -341,6 +341,11 @@ pub struct FontFace {
     location: skrifa::instance::Location,
     glyph_info_cache: ahash::HashMap<char, GlyphInfo>,
     glyph_alloc_cache: ahash::HashMap<GlyphCacheKey, GlyphAllocation>,
+
+    /// Cache for canvas-rendered glyphs (WASM only)
+    /// Key: (character, pixels_per_point bits, px_scale_factor bits, subpixel bin)
+    #[cfg(target_arch = "wasm32")]
+    canvas_glyph_cache: ahash::HashMap<(char, u32, u32, SubpixelBin), GlyphAllocation>,
 }
 
 impl FontFace {
@@ -436,6 +441,8 @@ impl FontFace {
             location,
             glyph_info_cache: Default::default(),
             glyph_alloc_cache: Default::default(),
+            #[cfg(target_arch = "wasm32")]
+            canvas_glyph_cache: Default::default(),
         })
     }
 
@@ -614,26 +621,65 @@ impl FontFace {
             SubpixelBin::new(h_pos)
         };
 
-        let entry = match self
-            .glyph_alloc_cache
-            .entry(GlyphCacheKey::new(glyph_id, metrics, bin))
-        {
-            std::collections::hash_map::Entry::Occupied(glyph_alloc) => {
-                let mut glyph_alloc = *glyph_alloc.get();
-                glyph_alloc.advance_width_px = advance_width_px; // Hack to get `\t` and thin space to work, since they use the same glyph id as ` ` (space).
-                return (glyph_alloc, h_pos_round);
-            }
-            std::collections::hash_map::Entry::Vacant(entry) => entry,
-        };
+        // For canvas glyphs (NOTDEF), we need to use the character for caching
+        // because all canvas glyphs have the same glyph_id
+        #[cfg(target_arch = "wasm32")]
+        if glyph_id == skrifa::GlyphId::NOTDEF {
+            // Use character-based cache key for canvas glyphs
+            // Include bin for subpixel positioning
+            let canvas_cache_key = (chr, metrics.pixels_per_point.to_bits(), metrics.px_scale_factor.to_bits(), bin);
 
+            // Check canvas glyph cache
+            if let Some(&cached_alloc) = self.canvas_glyph_cache.get(&canvas_cache_key) {
+                return (cached_alloc, h_pos_round);
+            }
+
+            // Render with canvas at subpixel offset
+            let allocation = Self::try_allocate_canvas_glyph_static(atlas, metrics, chr, bin).unwrap_or_default();
+
+            // Cache it
+            self.canvas_glyph_cache.insert(canvas_cache_key, allocation);
+            return (allocation, h_pos_round);
+        }
+
+        // Check cache first (for non-canvas glyphs)
+        let cache_key = GlyphCacheKey::new(glyph_id, metrics, bin);
+        if let Some(&cached_alloc) = self.glyph_alloc_cache.get(&cache_key) {
+            let mut glyph_alloc = cached_alloc;
+            glyph_alloc.advance_width_px = advance_width_px; // Hack to get `\t` and thin space to work, since they use the same glyph id as ` ` (space).
+            return (glyph_alloc, h_pos_round);
+        }
+
+        // Allocate the glyph
         let allocation = self
             .font
             .allocate_glyph_uncached(atlas, metrics, &glyph_info, bin, &self.location)
             .unwrap_or_default();
 
-        entry.insert(allocation);
+        // Insert into cache
+        self.glyph_alloc_cache.insert(cache_key, allocation);
         (allocation, h_pos_round)
     }
+
+    #[cfg(target_arch = "wasm32")]
+    fn try_allocate_canvas_glyph_static(
+        atlas: &mut TextureAtlas,
+        metrics: &ScaledMetrics,
+        chr: char,
+        bin: SubpixelBin,
+    ) -> Option<GlyphAllocation> {
+        use crate::text::canvas_renderer;
+
+        // Build font family list - just use a generic font for now
+        // TODO: We could pass the full font family list from CachedFamily
+        let font_families: Vec<String> = vec!["sans-serif".to_string()];
+
+        // Try to render the glyph with subpixel offset
+        let canvas_data = canvas_renderer::render_glyph_with_canvas(chr, metrics, &font_families, bin)?;
+
+        // Allocate the canvas-rendered glyph
+        Some(allocate_canvas_glyph(atlas, metrics, canvas_data))
+    }
 }
 
 // TODO(emilk): rename?
@@ -787,6 +833,64 @@ fn invisible_char(c: char) -> bool {
     )
 }
 
+// ----------------------------------------------------------------------------
+
+#[cfg(target_arch = "wasm32")]
+pub(super) fn allocate_canvas_glyph(
+    atlas: &mut TextureAtlas,
+    metrics: &ScaledMetrics,
+    canvas_data: crate::text::canvas_renderer::CanvasGlyphData,
+) -> GlyphAllocation {
+    use crate::text::canvas_renderer::CanvasGlyphData;
+
+    let CanvasGlyphData {
+        image_data,
+        width,
+        height,
+        advance_width,
+        offset_x,
+        offset_y,
+    } = canvas_data;
+
+    // Get alpha_from_coverage before allocating
+    let alpha_from_coverage = atlas.options().alpha_from_coverage;
+
+    // Allocate space in the texture atlas
+    let (glyph_pos, image) = atlas.allocate((width as usize, height as usize));
+
+    // Convert RGBA to alpha channel
+    // Canvas ImageData is RGBA, we need alpha channel only
+    for y in 0..height as usize {
+        for x in 0..width as usize {
+            let idx = (y * width as usize + x) * 4;
+            // Use alpha channel from ImageData
+            let alpha = image_data[idx + 3] as f32 / 255.0;
+            image[(x + glyph_pos.0, y + glyph_pos.1)] =
+                alpha_from_coverage.color_from_coverage(alpha);
+        }
+    }
+
+    // Calculate offset in points
+    let offset_in_points =
+        Vec2::new(offset_x, offset_y) / metrics.pixels_per_point + metrics.y_offset_in_points * Vec2::Y;
+
+    GlyphAllocation {
+        id: skrifa::GlyphId::NOTDEF, // Mark as canvas-rendered
+        advance_width_px: advance_width,
+        uv_rect: UvRect {
+            offset: offset_in_points,
+            size: Vec2::new(width as f32, height as f32) / metrics.pixels_per_point,
+            min: [glyph_pos.0 as u16, glyph_pos.1 as u16],
+            max: [
+                (glyph_pos.0 + width as usize) as u16,
+                (glyph_pos.1 + height as usize) as u16,
+            ],
+        },
+    }
+}
+
+// ----------------------------------------------------------------------------
+
 #[inline]
 pub(super) fn is_cjk_ideograph(c: char) -> bool {
     ('\u{4E00}' <= c && c <= '\u{9FFF}')

crates/epaint/src/text/fonts.rs

@@ -525,6 +525,23 @@ impl CachedFamily {
                 return Some((*font_key, glyph_info));
             }
         }
+
+        // Try canvas fallback for WASM
+        #[cfg(target_arch = "wasm32")]
+        {
+            // Use a special GlyphInfo to indicate canvas rendering is needed
+            // We use NOTDEF as the glyph ID to mark this as a canvas glyph
+            // The advance width will be filled in when we actually render it
+            let canvas_glyph_info = GlyphInfo {
+                id: Some(skrifa::GlyphId::NOTDEF),
+                advance_width_unscaled: OrderedFloat(0.0), // Will be filled in during rendering
+            };
+            // Use first font key as placeholder (the actual font family list will be used during rendering)
+            if let Some(&first_font_key) = self.fonts.first() {
+                return Some((first_font_key, canvas_glyph_info));
+            }
+        }
+
         None
     }
 }

crates/epaint/src/text/mod.rs

@@ -6,6 +6,9 @@ mod fonts;
 mod text_layout;
 mod text_layout_types;
 
+#[cfg(target_arch = "wasm32")]
+mod canvas_renderer;
+
 /// One `\t` character is this many spaces wide.
 pub const TAB_SIZE: usize = 4;