Fix performance on large canvases

src/handwriting/canvas_rasterizer.rs

@@ -9,7 +9,7 @@ use egui::{
     load::SizedTexture,
 };
 
-use crate::rasterizer::{PxBoundingBox, rasterize_onto, triangle_bounding_box};
+use crate::rasterizer::{PxBoundingBox, rasterize_triangle_onto, triangle_bounding_box};
 
 use super::StrokeBlendMode;
 
@@ -61,11 +61,6 @@ impl CanvasRasterizer {
         self.populate_tiles();
     }
 
-    pub fn clear_and_set_size(&mut self, width: usize, height: usize) {
-        self.image_size = [width, height];
-        self.clear();
-    }
-
     fn populate_tiles(&mut self) {
         let [width, height] = self.image_size;
 
@@ -96,44 +91,27 @@ impl CanvasRasterizer {
         point_to_pixel: TSTransform,
         triangles: impl Iterator<Item = [&'a Vertex; 3]> + Clone,
     ) {
-        // calculate smallest bounding box that encompasses all triangles
-        let triangles_bounding_box = triangles
-            .clone()
-            .map(|t| triangle_bounding_box(&t, point_to_pixel))
-            .reduce(|a, b| a.union(&b));
+        for triangle in triangles {
+            let triangle_bounding_box = triangle_bounding_box(&triangle, point_to_pixel);
+            for chunk in chunks_from_bounding_box(triangle_bounding_box) {
+                let Some(tile) = self.tiles.get_mut(&chunk) else {
+                    continue;
+                };
 
-        let Some(triangles_bounding_box) = triangles_bounding_box else {
-            return; // no triangles
-        };
-
-        // get all tiles that are encompassed by the bounding box
-        // TODO: smarter iteration: don't iterate over all tiles and filter
-        let mut n = 0;
-        self.tiles
-            .values_mut()
-            // skip tiles which won't be touched
-            .filter(|tile| tile.bounding_box.overlaps_with(&triangles_bounding_box))
-            // calculate TSTransform for tile
-            .map(|tile| {
                 let mut point_to_tile_pixel = point_to_pixel;
                 point_to_tile_pixel.translation -= Vec2::new(
                     tile.bounding_box.x_from as f32,
                     tile.bounding_box.y_from as f32,
                 );
-                (point_to_tile_pixel, tile)
-            })
-            // rasterize triangles onto the tile
-            .for_each(|(tile_point_to_pixel, tile)| {
-                tile.texture_is_dirty = true;
-                rasterize_onto::<StrokeBlendMode>(
-                    &mut tile.image,
-                    tile_point_to_pixel,
-                    triangles.clone(),
-                );
-                n += 1;
-            });
 
-        log::error!("rasterizing touched {n} tiles");
+                tile.texture_is_dirty = true;
+                rasterize_triangle_onto::<StrokeBlendMode>(
+                    &mut tile.image,
+                    point_to_tile_pixel,
+                    triangle,
+                );
+            }
+        }
     }
 
     /// `at` defines the location in screen-coordinates where the canvas should be drawn.
@@ -181,3 +159,18 @@ impl CanvasRasterizer {
         }
     }
 }
+
+/// Get all chunk indices that overlaps with a PxBoundingBox.
+fn chunks_from_bounding_box(
+    triangle_bounding_box: PxBoundingBox,
+) -> impl Iterator<Item = [usize; 2]> {
+    let x_from_chunk = triangle_bounding_box.x_from / CHUNK_SIZE;
+    let y_from_chunk = triangle_bounding_box.y_from / CHUNK_SIZE;
+    let x_to_chunk = triangle_bounding_box.x_to.saturating_sub(1) / CHUNK_SIZE;
+    let y_to_chunk = triangle_bounding_box.y_to.saturating_sub(1) / CHUNK_SIZE;
+
+    let xs = x_from_chunk..=x_to_chunk;
+    let ys = y_from_chunk..=y_to_chunk;
+
+    ys.flat_map(move |yi| xs.clone().map(move |xi| [xi, yi]))
+}

src/rasterizer.rs

@@ -87,6 +87,17 @@ pub fn rasterize_onto<'a, Blend: BlendFn>(
     }
 }
 
+/// Rasterize a single triangles onto an image,
+///
+/// Triangle positions must be in image-local point-coords.
+pub fn rasterize_triangle_onto<'a, Blend: BlendFn>(
+    image: &mut ColorImage,
+    point_to_pixel: TSTransform,
+    triangle: [&'a Vertex; 3],
+) {
+    rasterize_onto::<Blend>(image, point_to_pixel, [triangle].into_iter());
+}
+
 /// A bounding box, measured in pixels.
 #[derive(Debug, PartialEq, Eq)]
 pub struct PxBoundingBox {