diff --git a/piet-gpu-types/src/scene.rs b/piet-gpu-types/src/scene.rs
index d2cc466..8e4ec3c 100644
--- a/piet-gpu-types/src/scene.rs
+++ b/piet-gpu-types/src/scene.rs
@@ -1,7 +1,7 @@
 use piet_gpu_derive::piet_gpu;
 
 pub use self::scene::{
-    Bbox, PietFill, PietItem, PietStrokeLine, PietStrokePolyLine, Point, SimpleGroup,
+    Bbox, PietCircle, PietFill, PietItem, PietStrokeLine, PietStrokePolyLine, Point, SimpleGroup,
 };
 
 piet_gpu! {
@@ -20,6 +20,11 @@ piet_gpu! {
             items: Ref<PietItem>,
             bboxes: Ref<Bbox>,
         }
+        struct PietCircle {
+            rgba_color: u32,
+            center: Point,
+            radius: f32,
+        }
         struct PietStrokeLine {
             flags: u32,
             rgba_color: u32,
@@ -40,7 +45,7 @@ piet_gpu! {
             points: Ref<Point>,
         }
         enum PietItem {
-            Circle(),
+            Circle(PietCircle),
             Line(PietStrokeLine),
             Fill(PietFill),
             Poly(PietStrokePolyLine),
diff --git a/piet-gpu/shader/build.ninja b/piet-gpu/shader/build.ninja
index d0086bc..037540b 100644
--- a/piet-gpu/shader/build.ninja
+++ b/piet-gpu/shader/build.ninja
@@ -7,4 +7,4 @@ glslang_validator = glslangValidator
 rule glsl
   command = $glslang_validator -V -o $out $in
 
-build image.spv: glsl image.comp
+build image.spv: glsl image.comp | scene.h
diff --git a/piet-gpu/shader/image.comp b/piet-gpu/shader/image.comp
index 5487366..6174724 100644
--- a/piet-gpu/shader/image.comp
+++ b/piet-gpu/shader/image.comp
@@ -26,6 +26,24 @@ void main() {
     uvec2 xy = gl_GlobalInvocationID.xy;
     vec2 uv = vec2(xy) * vec2(1.0 / IMAGE_WIDTH, 1.0 / IMAGE_HEIGHT);
     vec4 rgba = vec4(uv.xyy, 1.0);
+
+    // Render the scene. Right now, every pixel traverses the scene graph,
+    // which is horribly wasteful, but the goal is to get *some* output and
+    // then optimize.
+
+    SimpleGroup group = SimpleGroup_read(SimpleGroupRef(0));
+    for (uint i = 0; i < group.n_items; i++) {
+        // Writing this out by hand is illuminating the need for array access
+        // in piet-gpu-derive. There is some support in the `_index` functions,
+        // but those are only generated for structs, not for `Ref` or other
+        // types.
+        PietItemRef item_ref = PietItemRef(scene[(group.items.offset >> 2) + i]);
+        uint tag = PietItem_tag(item_ref);
+        if (tag == PietItem_Circle) {
+            PietCircle = PietItem_Circle_read(item_ref);
+        }
+    }
+
     uvec4 s = uvec4(round(rgba * 255.0));
     uint rgba_packed = s.x | (s.y << 8) | (s.z << 16) | (s.w << 24);
     image[xy.y * IMAGE_WIDTH + xy.x] = rgba_packed;
diff --git a/piet-gpu/shader/image.spv b/piet-gpu/shader/image.spv
index e05c8c0..5be9f6d 100644
Binary files a/piet-gpu/shader/image.spv and b/piet-gpu/shader/image.spv differ
diff --git a/piet-gpu/shader/scene.h b/piet-gpu/shader/scene.h
index eb99bd4..440f491 100644
--- a/piet-gpu/shader/scene.h
+++ b/piet-gpu/shader/scene.h
@@ -12,6 +12,10 @@ struct SimpleGroupRef {
     uint offset;
 };
 
+struct PietCircleRef {
+    uint offset;
+};
+
 struct PietStrokeLineRef {
     uint offset;
 };
@@ -60,6 +64,18 @@ SimpleGroupRef SimpleGroup_index(SimpleGroupRef ref, uint index) {
     return SimpleGroupRef(ref.offset + index * SimpleGroup_size);
 }
 
+struct PietCircle {
+    uint rgba_color;
+    Point center;
+    float radius;
+};
+
+#define PietCircle_size 16
+
+PietCircleRef PietCircle_index(PietCircleRef ref, uint index) {
+    return PietCircleRef(ref.offset + index * PietCircle_size);
+}
+
 struct PietStrokeLine {
     uint flags;
     uint rgba_color;
@@ -140,6 +156,17 @@ SimpleGroup SimpleGroup_read(SimpleGroupRef ref) {
     return s;
 }
 
+PietCircle PietCircle_read(PietCircleRef ref) {
+    uint ix = ref.offset >> 2;
+    uint raw0 = scene[ix + 0];
+    uint raw3 = scene[ix + 3];
+    PietCircle s;
+    s.rgba_color = raw0;
+    s.center = Point_read(PointRef(ref.offset + 4));
+    s.radius = uintBitsToFloat(raw3);
+    return s;
+}
+
 PietStrokeLine PietStrokeLine_read(PietStrokeLineRef ref) {
     uint ix = ref.offset >> 2;
     uint raw0 = scene[ix + 0];
@@ -186,6 +213,10 @@ uint PietItem_tag(PietItemRef ref) {
     return scene[ref.offset >> 2];
 }
 
+PietCircle PietItem_Circle_read(PietItemRef ref) {
+    return PietCircle_read(PietCircleRef(ref.offset + 4));
+}
+
 PietStrokeLine PietItem_Line_read(PietItemRef ref) {
     return PietStrokeLine_read(PietStrokeLineRef(ref.offset + 4));
 }