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+# GPU Effects Test Coverage Analysis
+
+## Current State
+
+### Coverage Status
+**GPU Directory Coverage: Very Low** (~20% estimated)
+
+### File Breakdown (32 files total)
+
+#### Core Infrastructure (5 files)
+- `gpu.{h,cc}` - WebGPU initialization and management
+- `effect.{h,cc}` - Base Effect/Sequence/MainSequence classes
+- `texture_manager.{h,cc}` - Texture loading and procedural generation
+
+#### Effects System (27 files)
+**Base Effects:**
+- `demo_effects.{h,cc}` - Effect registration and factory
+
+**Individual Effects (19 implementations):**
+1. `chroma_aberration_effect.cc` - Chromatic aberration post-process
+2. `distort_effect.cc` - Distortion effect
+3. `fade_effect.{h,cc}` - Fade in/out
+4. `flash_cube_effect.{h,cc}` - Flashing cube
+5. `flash_effect.{h,cc}` - Screen flash on beat
+6. `gaussian_blur_effect.cc` - Blur post-process
+7. `heptagon_effect.cc` - Heptagon shape
+8. `hybrid_3d_effect.{h,cc}` - 3D + SDF hybrid rendering
+9. `moving_ellipse_effect.cc` - Animated ellipse
+10. `particle_spray_effect.cc` - Particle spray
+11. `particles_effect.cc` - Particle system
+12. `passthrough_effect.cc` - Identity post-process
+13. `solarize_effect.cc` - Solarize color effect
+14. `theme_modulation_effect.{h,cc}` - Theme color modulation
+
+**Utilities:**
+- `post_process_helper.{h,cc}` - Post-process pipeline utilities
+- `shader_composer.{h,cc}` - Shader composition system
+- `shaders.{h,cc}` - Shader registration
+
+### Existing Tests (4 files)
+1. ✅ `test_shader_assets.cc` - Basic shader keyword validation
+2. ✅ `test_shader_compilation.cc` - Real WebGPU shader compilation
+3. ✅ `test_shader_composer.cc` - Shader composition logic
+4. ⚠️ `test_texture_manager.cc` - Minimal (just compilation check)
+
+### Coverage Gaps
+**Completely Untested:**
+- ❌ `effect.cc` - Sequence/Effect lifecycle (0% coverage)
+- ❌ All 19 effect implementations (0% coverage)
+- ❌ `demo_effects.cc` - Effect factory (0% coverage)
+- ❌ `post_process_helper.cc` - Pipeline utilities (0% coverage)
+- ❌ `gpu.cc` - Initialization logic (partially tested via integration tests)
+
+---
+
+## Testing Challenges
+
+### Challenge 1: WebGPU Initialization Overhead
+**Problem**: Every test needs a full WebGPU device/queue/surface setup.
+
+**Current Approach** (test_shader_compilation.cc):
+```cpp
+// 100+ lines of initialization boilerplate
+WGPUInstance instance = wgpuCreateInstance(nullptr);
+WGPUAdapter adapter = ...; // Complex callback-based async request
+WGPUDevice device = ...;   // Complex callback-based async request
+```
+
+**Issues**:
+- Heavy: ~150ms per test for GPU initialization
+- Unreliable: May fail in headless CI environments
+- Verbose: 100+ lines of boilerplate per test file
+
+### Challenge 2: Rendering Requires a Surface
+**Problem**: Most tests need a real window for `render_frame()`.
+
+**Current Approach** (test_3d_render.cc, test_mesh.cc):
+```cpp
+PlatformState state = platform_init(false, 640, 480); // Creates GLFW window
+WGPUSurface surface = platform_create_wgpu_surface(instance, &state);
+main_seq.render_frame(time, beat, peak, aspect, surface);
+```
+
+**Issues**:
+- GUI dependency: Can't run in headless CI
+- Manual testing: Requires human to verify visual output
+- No assertions: Can't validate correctness automatically
+
+### Challenge 3: No Frame Validation
+**Problem**: How do you assert that a visual effect is correct?
+
+**Current Gap**:
+- No pixel readback infrastructure
+- No reference image comparison
+- No "golden master" test images
+
+---
+
+## Proposed Solution: Headless Rendering with Frame Sink
+
+### Architecture Overview
+
+```
+┌─────────────────────────────────────────────────────────┐
+│                  Test Framework                          │
+├─────────────────────────────────────────────────────────┤
+│                                                           │
+│  ┌──────────────┐      ┌──────────────────────────┐    │
+│  │ MockSurface  │─────▶│ OffscreenFramebuffer     │    │
+│  └──────────────┘      │ (WGPUTexture)            │    │
+│        │               └──────────────────────────┘    │
+│        │                         │                       │
+│        │                         ▼                       │
+│        │               ┌──────────────────────────┐    │
+│        └──────────────▶│ PixelReadback            │    │
+│                        │ (wgpuBufferMapAsync)     │    │
+│                        └──────────────────────────┘    │
+│                                  │                       │
+│                                  ▼                       │
+│                        ┌──────────────────────────┐    │
+│                        │ Assertions                │    │
+│                        │ (pixel checks, hashes)   │    │
+│                        └──────────────────────────┘    │
+└─────────────────────────────────────────────────────────┘
+```
+
+### Key Components
+
+#### 1. WebGPU Test Fixture (Reusable)
+```cpp
+// src/tests/webgpu_test_fixture.h
+class WebGPUTestFixture {
+ public:
+  bool init();  // One-time setup
+  void shutdown();
+
+  WGPUDevice device() const { return device_; }
+  WGPUQueue queue() const { return queue_; }
+  WGPUTextureFormat format() const { return WGPUTextureFormat_BGRA8Unorm; }
+
+ private:
+  WGPUInstance instance_ = nullptr;
+  WGPUAdapter adapter_ = nullptr;
+  WGPUDevice device_ = nullptr;
+  WGPUQueue queue_ = nullptr;
+};
+```
+
+**Benefits**:
+- Shared initialization across all GPU tests
+- ~100 lines of boilerplate → 3 lines per test
+- Can gracefully skip tests if GPU unavailable
+
+#### 2. Offscreen Render Target
+```cpp
+// src/tests/offscreen_render_target.h
+class OffscreenRenderTarget {
+ public:
+  OffscreenRenderTarget(WGPUDevice device, int width, int height);
+  ~OffscreenRenderTarget();
+
+  WGPUTexture texture() const { return texture_; }
+  WGPUTextureView view() const { return view_; }
+
+  // Simulate surface behavior without actual window
+  WGPUSurfaceTexture get_current_texture();
+
+  // Readback pixels for validation
+  std::vector<uint8_t> read_pixels();
+
+ private:
+  WGPUDevice device_;
+  WGPUTexture texture_;
+  WGPUTextureView view_;
+  int width_, height_;
+};
+```
+
+**Key Feature**: No window needed! Pure GPU texture operations.
+
+#### 3. Effect Test Helpers
+```cpp
+// src/tests/effect_test_helpers.h
+
+// Helper: Test effect lifecycle
+void test_effect_lifecycle(Effect* effect, MainSequence* demo);
+
+// Helper: Test effect render (smoke test - no crash)
+void test_effect_render(Effect* effect, WGPURenderPassEncoder pass);
+
+// Helper: Test post-process bind group update
+void test_post_process_bind_group(PostProcessEffect* effect,
+                                    WGPUTextureView input);
+
+// Helper: Validate rendered output (pixel checks)
+bool validate_pixels(const std::vector<uint8_t>& pixels,
+                     int width, int height,
+                     std::function<bool(uint8_t r, uint8_t g, uint8_t b, uint8_t a)> predicate);
+
+// Helper: Hash-based validation (deterministic output check)
+uint64_t hash_pixels(const std::vector<uint8_t>& pixels);
+```
+
+---
+
+## Proposed Test Structure
+
+### Test 1: Effect Base Classes (`test_effect_base.cc`)
+
+**Goal**: Test Effect/Sequence/MainSequence lifecycle
+
+```cpp
+void test_effect_construction() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return; // Skip if no GPU
+
+  FlashEffect effect(fixture.device(), fixture.queue(), fixture.format());
+  assert(!effect.is_initialized);
+
+  MainSequence main_seq;
+  main_seq.init_test(fixture.device(), fixture.queue(), fixture.format());
+
+  effect.init(&main_seq);
+  assert(effect.is_initialized);
+}
+
+void test_sequence_add_effect() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return;
+
+  auto effect = std::make_shared<FlashEffect>(/*...*/);
+
+  Sequence seq;
+  seq.add_effect(effect, 0.0f, 10.0f, 0);
+
+  // Should not activate before start time
+  seq.update_active_list(-1.0f);
+  // Verify not active
+
+  // Should activate at start time
+  seq.update_active_list(0.0f);
+  // Verify active
+
+  // Should deactivate after end time
+  seq.update_active_list(11.0f);
+  // Verify not active
+}
+
+void test_sequence_priority_sorting() {
+  // Add effects with different priorities
+  // Verify render order via collect_active_effects()
+}
+```
+
+**Coverage Impact**: effect.cc 0% → ~60%
+
+---
+
+### Test 2: Post-Process Effects (`test_post_process_effects.cc`)
+
+**Goal**: Test all post-process effects (flash, blur, distort, etc.)
+
+```cpp
+void test_flash_effect_basic() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return;
+
+  FlashEffect effect(fixture.device(), fixture.queue(), fixture.format());
+
+  MainSequence main_seq;
+  main_seq.init_test(fixture.device(), fixture.queue(), fixture.format());
+  effect.init(&main_seq);
+
+  OffscreenRenderTarget target(fixture.device(), 256, 256);
+  effect.update_bind_group(target.view());
+
+  // Create render pass
+  WGPUCommandEncoder encoder = wgpuDeviceCreateCommandEncoder(fixture.device(), nullptr);
+  WGPURenderPassDescriptor pass_desc = create_simple_pass(target.view());
+  WGPURenderPassEncoder pass = wgpuCommandEncoderBeginRenderPass(encoder, &pass_desc);
+
+  // Render with low intensity (no flash)
+  effect.render(pass, 0.0f, 0.0f, 0.1f, 1.0f);
+  wgpuRenderPassEncoderEnd(pass);
+
+  // Submit and readback
+  WGPUCommandBuffer cmd = wgpuCommandEncoderFinish(encoder, nullptr);
+  wgpuQueueSubmit(fixture.queue(), 1, &cmd);
+
+  // Validate: should not be pure white (no flash triggered)
+  auto pixels = target.read_pixels();
+  bool has_non_white = validate_pixels(pixels, 256, 256,
+    [](uint8_t r, uint8_t g, uint8_t b, uint8_t a) {
+      return r < 255 || g < 255 || b < 255; // At least one pixel not white
+    });
+  assert(has_non_white);
+}
+
+void test_flash_effect_trigger() {
+  // Test with high intensity (>0.7) - should trigger flash
+  // Validate some pixels are white/bright
+}
+
+// Similar tests for:
+// - test_gaussian_blur_effect()
+// - test_chroma_aberration_effect()
+// - test_distort_effect()
+// - test_solarize_effect()
+// - test_passthrough_effect()
+```
+
+**Coverage Impact**: 6 post-process effects 0% → ~50%
+
+---
+
+### Test 3: Scene Effects (`test_scene_effects.cc`)
+
+**Goal**: Test scene-rendering effects (geometry, 3D, particles)
+
+```cpp
+void test_heptagon_effect() {
+  // Test basic render without crash
+  // Optionally: readback and check for non-black pixels (shape rendered)
+}
+
+void test_moving_ellipse_effect() {
+  // Test animation over time (different time values)
+}
+
+void test_particle_effects() {
+  // Test ParticleSprayEffect
+  // Test ParticlesEffect
+}
+
+void test_hybrid_3d_effect() {
+  // Test 3D rendering with SDF raymarching
+  // This is complex - may just be smoke test
+}
+```
+
+**Coverage Impact**: 8 scene effects 0% → ~40%
+
+---
+
+### Test 4: Effect Factory (`test_demo_effects.cc`)
+
+**Goal**: Test effect registration and instantiation
+
+```cpp
+void test_effect_factory_registration() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return;
+
+  // All effects should be auto-registered
+  const auto& effects = GetEffectRegistry();
+  assert(effects.size() >= 15); // We have ~19 effects
+
+  // Check specific effects exist
+  assert(effects.find("FlashEffect") != effects.end());
+  assert(effects.find("FlashCubeEffect") != effects.end());
+  // etc.
+}
+
+void test_effect_factory_instantiation() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return;
+
+  MainSequence main_seq;
+  main_seq.init_test(fixture.device(), fixture.queue(), fixture.format());
+
+  // Create effect via factory
+  auto effect = CreateEffect("FlashEffect", fixture.device(),
+                             fixture.queue(), fixture.format(), nullptr);
+  assert(effect != nullptr);
+  assert(dynamic_cast<FlashEffect*>(effect.get()) != nullptr);
+
+  // Init should not crash
+  effect->init(&main_seq);
+}
+```
+
+**Coverage Impact**: demo_effects.cc 0% → ~70%
+
+---
+
+### Test 5: Integration Test (`test_mainsequence_render.cc`)
+
+**Goal**: Test full render pipeline with multiple effects
+
+```cpp
+void test_mainsequence_full_render() {
+  WebGPUTestFixture fixture;
+  if (!fixture.init()) return;
+
+  MainSequence main_seq;
+  main_seq.init_test(fixture.device(), fixture.queue(), fixture.format());
+
+  // Add a sequence with multiple effects
+  auto seq = std::make_shared<Sequence>();
+  seq->add_effect(std::make_shared<FlashEffect>(/*...*/), 0.0f, 10.0f, 0);
+  seq->add_effect(std::make_shared<PassthroughEffect>(/*...*/), 0.0f, 10.0f, 1);
+
+  main_seq.add_sequence(seq, 0.0f, 0);
+  seq->init(&main_seq);
+
+  // Create offscreen target
+  OffscreenRenderTarget target(fixture.device(), 256, 256);
+
+  // Render a frame
+  // Note: Need to adapt render_frame() to accept texture instead of surface
+  // OR create a mock surface that wraps the offscreen texture
+
+  main_seq.render_frame_offscreen(0.5f, 0.0f, 0.5f, 1.0f, target.texture());
+
+  // Validate: frame rendered without crash, some pixels non-black
+  auto pixels = target.read_pixels();
+  bool rendered = validate_pixels(pixels, 256, 256,
+    [](uint8_t r, uint8_t g, uint8_t b, uint8_t a) {
+      return r > 0 || g > 0 || b > 0; // At least one pixel not black
+    });
+  assert(rendered);
+}
+```
+
+**Coverage Impact**: effect.cc MainSequence 0% → ~50%
+
+---
+
+## Implementation Plan
+
+### Phase 1: Foundation (Week 1) - 8 hours
+1. **Create WebGPUTestFixture** (2h)
+   - Extract init code from test_shader_compilation.cc
+   - Add graceful skip if GPU unavailable
+   - Test on macOS, Linux
+
+2. **Create OffscreenRenderTarget** (3h)
+   - WGPUTexture creation without surface
+   - Pixel readback via wgpuBufferMapAsync
+   - Validation helpers (hash, predicate checks)
+
+3. **Create Effect Test Helpers** (2h)
+   - Lifecycle test helper
+   - Render smoke test helper
+   - Pixel validation utilities
+
+4. **Add render_frame_offscreen() to MainSequence** (1h)
+   - Variant of render_frame() that targets offscreen texture
+   - Or: Mock surface that wraps offscreen texture
+
+---
+
+### Phase 2: Base Tests (Week 2) - 10 hours
+1. **test_effect_base.cc** (4h)
+   - Effect construction/init
+   - Sequence lifecycle (add, activate, deactivate)
+   - Priority sorting
+   - Collect active effects
+
+2. **test_demo_effects.cc** (2h)
+   - Effect registry validation
+   - Effect factory instantiation
+
+3. **test_post_process_helper.cc** (2h)
+   - Pipeline creation
+   - Bind group management
+
+4. **CI Integration** (2h)
+   - Update CMakeLists.txt
+   - Test graceful skip in headless CI
+   - Document GPU test requirements
+
+---
+
+### Phase 3: Individual Effects (Week 3-4) - 20 hours
+1. **test_post_process_effects.cc** (8h)
+   - FlashEffect (with trigger logic)
+   - GaussianBlurEffect
+   - ChromaAberrationEffect
+   - DistortEffect
+   - SolarizeEffect
+   - PassthroughEffect
+
+2. **test_scene_effects.cc** (8h)
+   - HeptagonEffect
+   - MovingEllipseEffect
+   - FlashCubeEffect
+   - ParticleSprayEffect
+   - ParticlesEffect
+   - FadeEffect
+   - ThemeModulationEffect
+
+3. **test_hybrid_3d.cc** (4h)
+   - Hybrid3DEffect (complex, may be smoke test only)
+
+---
+
+### Phase 4: Integration (Week 5) - 6 hours
+1. **test_mainsequence_render.cc** (4h)
+   - Full render pipeline
+   - Multiple sequences
+   - Effect priority ordering
+   - Frame-by-frame simulation
+
+2. **Coverage Report** (2h)
+   - Generate coverage with new tests
+   - Update documentation
+   - Identify remaining gaps
+
+---
+
+## Expected Coverage Impact
+
+### Before (Current State)
+```
+gpu/
+├── gpu.cc                   ~20% (partially via integration tests)
+├── effect.cc                  0% ❌
+├── texture_manager.cc        10% ⚠️
+├── demo_effects.cc             0% ❌
+├── effects/
+│   ├── (19 effect files)       0% ❌
+│   ├── post_process_helper.cc  0% ❌
+│   ├── shader_composer.cc     80% ✅ (already tested)
+│   └── shaders.cc              5% ⚠️
+
+Overall GPU Coverage: ~20%
+```
+
+### After (With Proposed Tests)
+```
+gpu/
+├── gpu.cc                   ~30% (some init paths tested)
+├── effect.cc                 ~60% ✅ (lifecycle, sequence logic)
+├── texture_manager.cc        ~40% (basic ops tested)
+├── demo_effects.cc           ~70% ✅ (factory tested)
+├── effects/
+│   ├── (19 effect files)     ~45% ✅ (basic render tested)
+│   ├── post_process_helper.cc ~65% ✅
+│   ├── shader_composer.cc     80% (unchanged)
+│   └── shaders.cc            ~30% (registration tested)
+
+Overall GPU Coverage: ~50% ✅ (20% → 50% = +150% increase)
+```
+
+---
+
+## Alternative: Minimal Approach (If Time-Constrained)
+
+### Quick Wins (2-3 hours total)
+
+1. **test_effect_lifecycle.cc** (1.5h)
+   - Just test Effect/Sequence construction, add_effect, activation
+   - No rendering, just state checks
+   - Gets effect.cc from 0% → 40%
+
+2. **test_effect_factory.cc** (1h)
+   - Test effect registry and instantiation
+   - Gets demo_effects.cc from 0% → 60%
+
+3. **Smoke Test for Each Effect** (30min)
+   - Loop through all effects, construct + init
+   - Verify no crashes
+   - Gets effects/*.cc from 0% → 20%
+
+**Result**: GPU coverage 20% → 35% with minimal effort
+
+---
+
+## Recommendations
+
+### Priority 1: Foundation (Do First)
+- ✅ Create WebGPUTestFixture (shared across all GPU tests)
+- ✅ Create OffscreenRenderTarget (enables headless testing)
+- ✅ test_effect_base.cc (high impact, tests core logic)
+
+### Priority 2: Smoke Tests (Quick Wins)
+- ✅ test_demo_effects.cc (factory validation)
+- ✅ Loop through all effects, test construction
+
+### Priority 3: Detailed Effect Tests (Long-Term)
+- ⚠️ Individual effect render validation (time-consuming)
+- ⚠️ Pixel-level assertions (requires golden masters)
+
+### Priority 4: Integration Tests (Optional)
+- ⚠️ Full MainSequence render pipeline (complex setup)
+
+---
+
+## Open Questions
+
+1. **Pixel Readback Performance**: Is `wgpuBufferMapAsync` fast enough for CI?
+   - Typical latency: ~5-10ms per frame
+   - For 50 tests × 1 frame each = ~500ms (acceptable)
+
+2. **Headless CI Support**: Will WebGPU work without display server?
+   - macOS: ✅ Works (Metal backend)
+   - Linux: ⚠️ Requires Vulkan/X11/Wayland
+   - Win32: ✅ Works (D3D12 backend)
+   - Solution: Gracefully skip if init fails
+
+3. **Golden Master Images**: Do we need reference images?
+   - For now: ❌ No (too fragile, GPU differences)
+   - Instead: Use coarse checks (non-black, brightness, hash)
+
+4. **Mock vs Real GPU**: Should we mock WebGPU API?
+   - Verdict: ❌ No mocking - use real GPU but offscreen
+   - Mocking WebGPU is too complex and low-value
+
+---
+
+## Files to Create
+
+### Test Infrastructure (3 files)
+```
+src/tests/webgpu_test_fixture.{h,cc}       # Shared WebGPU init
+src/tests/offscreen_render_target.{h,cc}   # Headless rendering
+src/tests/effect_test_helpers.h            # Common test utilities
+```
+
+### Test Suites (6 files)
+```
+src/tests/test_effect_base.cc              # Effect/Sequence lifecycle
+src/tests/test_demo_effects.cc             # Effect factory
+src/tests/test_post_process_helper.cc      # Pipeline utilities
+src/tests/test_post_process_effects.cc     # All post-process effects
+src/tests/test_scene_effects.cc            # All scene effects
+src/tests/test_mainsequence_render.cc      # Integration test
+```
+
+**Total**: 9 new files (~2000 lines)
+
+---
+
+## Conclusion
+
+**Recommended Approach**: Start with foundation + base tests (Phase 1-2)
+- Achieves GPU coverage: 20% → 40% with moderate effort
+- Provides infrastructure for future detailed tests
+- Low risk: graceful skip if GPU unavailable
+- No GUI dependency: fully headless
+
+**Stretch Goal**: Add individual effect tests (Phase 3)
+- Achieves GPU coverage: 40% → 50%
+- Higher effort, diminishing returns
+- Nice-to-have, not critical
+
+**Key Innovation**: Offscreen rendering eliminates need for windows/displays while still testing real GPU code paths.