Fix boundary condition for active edges precisely on tile boundaries

renderer/src/gpu_data.rs

@@ -115,6 +115,7 @@ impl BuiltObject {
     // TODO(pcwalton): SIMD-ify `tile_x` and `tile_y`.
     fn add_fill(&mut self, segment: &LineSegmentF32, tile_x: i16, tile_y: i16) {
         //println!("add_fill({:?} ({}, {}))", segment, tile_x, tile_y);
+
         let mut segment = (segment.0 * F32x4::splat(256.0)).to_i32x4();
 
         let tile_origin_x = (TILE_WIDTH as i32) * 256 * (tile_x as i32);
@@ -143,11 +144,14 @@ impl BuiltObject {
         // Cull degenerate fills.
         if (px.0 & 0xf) as u8 == ((px.0 >> 8) & 0xf) as u8 &&
                 (subpx.0 & 0xff) as u8 == ((subpx.0 >> 16) & 0xff) as u8 {
+            //println!("... ... culling!");
             return;
         }
 
         let tile_index = self.tile_coords_to_index(tile_x, tile_y);
 
+        //println!("... ... OK, pushing");
+
         self.fills.push(FillObjectPrimitive {
             px,
             subpx,
@@ -175,13 +179,12 @@ impl BuiltObject {
             LineSegmentF32::new(&right, &left)
         };
 
-        /*
-        println!("... emitting active fill {} -> {} winding {} @ tile {}",
+        /*println!("... emitting active fill {} -> {} winding {} @ tile {},{}",
                  left.x(),
                  right.x(),
                  winding,
-                 tile_x);
-        */
+                 tile_x,
+                 tile_y);*/
 
         while winding != 0 {
             self.add_fill(&segment, tile_x, tile_y);

renderer/src/tiles.rs

@@ -18,7 +18,6 @@ use pathfinder_geometry::outline::{Contour, Outline, PointIndex};
 use pathfinder_geometry::point::Point2DF32;
 use pathfinder_geometry::segment::Segment;
 use pathfinder_geometry::util;
-use smallvec::SmallVec;
 use std::cmp::Ordering;
 use std::mem;
 
@@ -92,9 +91,14 @@ impl<'o, 'z> Tiler<'o, 'z> {
         // Add new active edges.
         let strip_max_y = ((i32::from(strip_origin_y) + 1) * TILE_HEIGHT as i32) as f32;
         while let Some(queued_endpoint) = self.point_queue.peek() {
-            if queued_endpoint.y >= strip_max_y {
+            // We're done when we see an endpoint that belongs to the next tile.
+            //
+            // Note that this test must be `>`, not `>=`, in order to make sure we don't miss
+            // active edges that lie precisely on the tile boundary.
+            if queued_endpoint.y > strip_max_y {
                 break;
             }
+
             self.add_new_active_edge(strip_origin_y);
         }
     }
@@ -121,8 +125,8 @@ impl<'o, 'z> Tiler<'o, 'z> {
         let mut last_segment_x = -9999.0;
 
         let tile_top = (i32::from(tile_y) * TILE_HEIGHT as i32) as f32;
-        /*println!("---------- tile y {}({}) ----------", tile_y, tile_top);
-        println!("old active edges: {:#?}", self.old_active_edges);*/
+        //println!("---------- tile y {}({}) ----------", tile_y, tile_top);
+        //println!("old active edges: {:#?}", self.old_active_edges);
 
         for mut active_edge in self.old_active_edges.drain(..) {
             // Determine x-intercept and winding.