diff --git a/piet-gpu/shader/coarse.comp b/piet-gpu/shader/coarse.comp
index 3ca7b5f..4e4ff19 100644
--- a/piet-gpu/shader/coarse.comp
+++ b/piet-gpu/shader/coarse.comp
@@ -42,10 +42,16 @@ shared uint sh_elements_ref;
 
 shared uint sh_bitmaps[N_SLICE][N_TILE];
 
+// scale factors useful for converting coordinates to tiles
+#define SX (1.0 / float(TILE_WIDTH_PX))
+#define SY (1.0 / float(TILE_HEIGHT_PX))
+
 void main() {
     // Could use either linear or 2d layouts for both dispatch and
     // invocations within the workgroup. We'll use variables to abstract.
     uint bin_ix = N_TILE_X * gl_WorkGroupID.y + gl_WorkGroupID.x;
+    // Top left coordinates of this bin.
+    vec2 xy0 = vec2(N_TILE_X * TILE_WIDTH_PX * gl_WorkGroupID.x, N_TILE_Y * TILE_HEIGHT_PX * gl_WorkGroupID.y);
     uint th_ix = gl_LocalInvocationID.x;
     uint wr_ix = 0;
     uint rd_ix = 0;
@@ -109,20 +115,90 @@ void main() {
             if (th_ix < chunk_n) {
                 uint el = BinInstance_read(BinInstance_index(inst_ref, th_ix)).element_ix;
                 sh_elements[(wr_ix + th_ix) % N_RINGBUF] = el;
-                probe = el;
             }
             wr_ix += chunk_n;
         }
 
         // We've done the merge and filled the buffer.
+
+        // Read one element, compute coverage.
         uint tag = Annotated_Nop;
         AnnotatedRef ref;
         if (th_ix + rd_ix < wr_ix) {
-            uint element_ix = (sh_elements[rd_ix] + th_ix) % N_RINGBUF;
+            uint element_ix = sh_elements[(rd_ix + th_ix) % N_RINGBUF];
             ref = AnnotatedRef(element_ix * Annotated_size);
             tag = Annotated_tag(ref);
-            probe = tag;
         }
+
+        int x0 = 0, y0 = 0, x1 = 0, y1 = 0;
+        switch (tag) {
+        case Annotated_Line:
+            AnnoLineSeg line = Annotated_Line_read(ref);
+            x0 = int(floor((min(line.p0.x, line.p1.x) - line.stroke.x - xy0.x) * SX));
+            y0 = int(floor((min(line.p0.y, line.p1.y) - line.stroke.y - xy0.y) * SY));
+            x1 = int(ceil((max(line.p0.x, line.p1.x) + line.stroke.x - xy0.x) * SX));
+            y1 = int(ceil((max(line.p0.y, line.p1.y) + line.stroke.y - xy0.y) * SY));
+            break;
+        case Annotated_Fill:
+        case Annotated_Stroke:
+            // Note: we take advantage of the fact that fills and strokes
+            // have compatible layout.
+            AnnoFill fill = Annotated_Fill_read(ref);
+            x0 = int(floor((fill.bbox.x - xy0.x) * SX));
+            y0 = int(floor((fill.bbox.y - xy0.y) * SY));
+            x1 = int(ceil((fill.bbox.z - xy0.x) * SX));
+            y1 = int(ceil((fill.bbox.w - xy0.y) * SY));
+            break;
+        }
+        // At this point, we run an iterator over the coverage area,
+        // trying to keep divergence low.
+        // Right now, it's just a bbox, but we'll get finer with
+        // segments.
+        x0 = clamp(x0, 0, N_TILE_X);
+        x1 = clamp(x1, x0, N_TILE_X);
+        y0 = clamp(y0, 0, N_TILE_Y);
+        y1 = clamp(y1, y0, N_TILE_Y);
+        // This loop draws a rectangle to the coverage bitmasks. For
+        // line segments, draw more precisely.
+        if (x0 == x1) y1 = y0;
+        int x = x0, y = y0;
+        uint my_slice = th_ix / 32;
+        uint my_mask = 1 << (th_ix & 31);
+        while (y < y1) {
+            atomicOr(sh_bitmaps[my_slice][y * N_TILE_X + x], my_mask);
+            x++;
+            if (x == x1) {
+                x = x0;
+                y++;
+            }
+        }
+
+        // Output elements for this tile, based on bitmaps.
+        uint slice_ix = 0;
+        uint bitmap = sh_bitmaps[0][th_ix];
+        while (true) {
+            if (bitmap == 0) {
+                slice_ix++;
+                if (slice_ix == N_SLICE) {
+                    break;
+                }
+                bitmap = sh_bitmaps[slice_ix][th_ix];
+                if (bitmap == 0) {
+                    continue;
+                }
+            }
+            uint element_ref_ix = slice_ix * 32 + findLSB(bitmap);
+            uint element_ix = sh_elements[(rd_ix + element_ref_ix) % N_RINGBUF];
+
+            // At this point, we read the element again from global memory.
+            // If that turns out to be expensive, maybe we can pack it into
+            // shared memory (or perhaps just the tag).
+            probe += 1;
+
+            // clear LSB
+            bitmap &= bitmap - 1;
+        }
+
         rd_ix += N_TILE;
     } while (wr_ix > rd_ix);
     ptcl[bin_ix * N_TILE + th_ix] = probe;
diff --git a/piet-gpu/shader/coarse.spv b/piet-gpu/shader/coarse.spv
index 6bd6f61..ded68da 100644
Binary files a/piet-gpu/shader/coarse.spv and b/piet-gpu/shader/coarse.spv differ