# Handoff: Shader Parametrization System (February 8, 2026)

## Summary
Completed comprehensive shader parametrization system enabling dynamic per-frame parameters for visual effects via uniform buffers and .seq file syntax.

## Work Completed ✅

### Shader Parametrization System (Task #73 Phase 0)
**Goal**: Enable artists to configure visual effects with parameters (color, intensity, decay rates) via .seq files without touching C++ code.

**Use Cases**: Flash effect with custom colors, chromatic aberration strength control, blur radius adjustment, distortion parameters.

**Implementation**:

#### Phase 1: UniformHelper Template
- Created `src/gpu/uniform_helper.h` - Generic type-safe wrapper for WebGPU uniform buffers
- Template class handles buffer creation, updates, and lifetime management
- Zero-overhead abstraction over `gpu_create_buffer()` and `wgpuQueueWriteBuffer()`

```cpp
template <typename T>
class UniformBuffer {
  void init(WGPUDevice device);
  void update(WGPUQueue queue, const T& data);
  GpuBuffer& get();
};
```

#### Phase 2: FlashEffect Parameter Structs
- Added `FlashEffectParams` (constructor-time base parameters):
  - `color[3]` - RGB flash color (default: white)
  - `decay_rate` - Flash fade rate per frame (default: 0.98)
  - `trigger_threshold` - Intensity threshold to trigger flash (default: 0.7)

- Added `FlashUniforms` (GPU buffer layout with WGSL alignment):
  - `flash_intensity` (4 bytes, offset 0)
  - `intensity` (4 bytes, offset 4)
  - `_pad1[2]` (8 bytes, offset 8-15) - **Padding for vec3 alignment**
  - `color[3]` (12 bytes, offset 16-27) - **Aligned to 16 bytes**
  - `_pad2` (4 bytes, offset 28-31)
  - Total: 32 bytes (enforced with `static_assert`)

#### Phase 3: Parameterized WGSL Shader
- Updated `flash_effect.cc` shader to use `uniforms.flash_color` instead of hardcoded white
- Shader bindings: sampler (0), texture (1), uniforms (2)
- Fragment shader mixes input color with parameterized flash color based on flash intensity

```wgsl
struct Uniforms {
  flash_intensity: f32,
  intensity: f32,
  flash_color: vec3<f32>,  // Parameterized color
  _pad: f32,
};

fn fs_main(input: VertexOutput) -> @location(0) vec4<f32> {
  let color = textureSample(inputTexture, inputSampler, input.uv);
  var flashed = mix(color.rgb, uniforms.flash_color, uniforms.flash_intensity);
  return vec4<f32>(flashed, color.a);
}
```

#### Phase 4: Per-Frame Parameter Computation
- Implemented dynamic parameter animation in `FlashEffect::render()` method
- Parameters computed each frame based on time, beat, and intensity
- Example: Color modulated by sinusoidal functions of time and beat

```cpp
// Animate color based on time and beat
const float r = params_.color[0] * (0.5f + 0.5f * sinf(time * 0.5f));
const float g = params_.color[1] * (0.5f + 0.5f * cosf(time * 0.7f));
const float b = params_.color[2] * (1.0f + 0.3f * beat);

const FlashUniforms u = {
  .flash_intensity = flash_intensity_,
  .intensity = intensity,
  ._pad1 = {0.0f, 0.0f},
  .color = {r, g, b},
  .pad2 = 0.0f
};
uniforms_.update(ctx_.queue, u);
```

#### Phase 5: .seq Syntax Extension
- Extended `seq_compiler.cc` to parse `key=value` parameters after effect class name
- Added `parse_parameters()` function to extract parameter pairs
- Code generation creates `FlashEffectParams` struct initialization for parameterized effects
- Backward compatibility: Effects without parameters use default constructor

**Syntax Example**:
```
EFFECT + FlashEffect 0 1 color=1.0,0.5,0.5 decay=0.95
```

**Generated Code**:
```cpp
{
  FlashEffectParams p;
  p.color[0] = 1.0f;
  p.color[1] = 0.5f;
  p.color[2] = 0.5f;
  p.decay_rate = 0.95f;
  seq->add_effect(std::make_shared<FlashEffect>(ctx, p), 0.0f, 1.f, 0);
}
```

---

### Critical Bugfix: WGSL Alignment
**Problem**: WebGPU validation error "Buffer is bound with size 24 where the shader expects 32"

**Root Cause**: WGSL alignment rules require `vec3<f32>` to be 16-byte aligned despite having 12-byte size. Initial `FlashUniforms` struct layout was:
```cpp
struct FlashUniforms {
  float flash_intensity;  // 0-3
  float intensity;        // 4-7
  float color[3];         // 8-19 (WRONG - no padding)
  float _pad;             // 20-23
};  // Total: 24 bytes (WRONG)
```

**Solution**: Added explicit padding to align `color` array to 16-byte boundary:
```cpp
struct FlashUniforms {
  float flash_intensity;  // 0-3
  float intensity;        // 4-7
  float _pad1[2];         // 8-15 (padding)
  float color[3];         // 16-27 (aligned to 16 bytes)
  float _pad2;            // 28-31
};  // Total: 32 bytes (CORRECT)
static_assert(sizeof(FlashUniforms) == 32, "FlashUniforms must be 32 bytes");
```

**Impact**: Demo now runs without validation errors, all 32/32 tests pass.

---

## Files Modified

**New Files (3)**:
- `src/gpu/uniform_helper.h` - UniformBuffer template class
- `src/tests/test_uniform_helper.cc` - Unit test for template compilation
- `doc/SHADER_PARAMETRIZATION_PLAN.md` - Complete design document

**Modified Files (11)**:
- `src/gpu/effects/flash_effect.h` - Added FlashEffectParams, FlashUniforms structs, parameterized constructor
- `src/gpu/effects/flash_effect.cc` - Implemented per-frame computation, updated shader with parameterized color
- `src/gpu/demo_effects.h` - Removed duplicate FlashEffect definition, added include
- `tools/seq_compiler.cc` - Added parameter parsing and code generation
- `assets/demo.seq` - Added example with parameters
- `src/generated/timeline.cc` - Generated code with struct initialization
- `CMakeLists.txt` - Added test_uniform_helper target
- `TODO.md` - Added Task #73 (extend to other effects), moved completion to summary
- `PROJECT_CONTEXT.md` - Updated "Recently Completed" section
- `doc/COMPLETED.md` - Added detailed completion entry
- `doc/HANDOFF_2026-02-08.md` - This document

---

## Commits Made

**c7d1dd7** - `feat(gpu): Implement shader parametrization system`
- Phases 1-5 implementation
- 11 files changed (808+ / 40-)
- UniformHelper template + FlashEffect parametrization + .seq syntax

**775c0ea** - `fix(gpu): Correct FlashUniforms struct alignment for WGSL`
- Critical alignment bugfix (24 → 32 bytes)
- Added static_assert for future safety
- Fixed validation error

---

## Test Results

**All 32/32 tests passing (100%)**

**Test Coverage**:
- `test_uniform_helper.cc` - Verifies UniformBuffer template compiles correctly
- `test_demo_effects.cc` - All post-process and scene effects instantiate successfully
- Full shader compilation tests pass with new parameterized shaders

**Verification**:
- Demo runs without WebGPU validation errors
- Visual effects render with parameterized colors
- .seq file parsing generates correct parameter initialization code
- Per-frame animation produces time-dependent color modulation

---

## Current Status

**Completed:**
- ✅ Full shader parametrization infrastructure
- ✅ FlashEffect fully parameterized with color/decay control
- ✅ .seq syntax extension with key=value parsing
- ✅ Per-frame dynamic parameter computation
- ✅ Critical WGSL alignment bug fixed
- ✅ Documentation updated (TODO.md, PROJECT_CONTEXT.md, COMPLETED.md)

**Ready for:**
- Task #73: Extend parametrization to other effects (ChromaAberrationEffect, GaussianBlurEffect, DistortEffect, SolarizeEffect)
- Additional parameter types (vec2, vec4, enums)
- Parameter curve system for keyframe animation (future)

---

## Technical Notes

### WGSL Alignment Rules
**Critical for future implementations:**
- `vec3<f32>` has **size = 12 bytes** but **alignment = 16 bytes**
- Always add padding before vec3 fields to align to 16-byte boundary
- Use `static_assert(sizeof(YourStruct) == expected_size)` to catch misalignment
- Reference: [WebGPU WGSL Spec - Alignment and Size](https://www.w3.org/TR/WGSL/#alignment-and-size)

### UniformHelper Pattern
**Reusable template for any effect parameters:**
```cpp
// 1. Define parameter structs
struct YourEffectParams { /* constructor-time params */ };
struct YourEffectUniforms { /* GPU buffer layout with padding */ };
static_assert(sizeof(YourEffectUniforms) == expected_size);

// 2. Add UniformBuffer member
UniformBuffer<YourEffectUniforms> uniforms_;

// 3. Initialize in constructor
uniforms_.init(ctx_.device);

// 4. Update in render()
const YourEffectUniforms u = { /* compute values */ };
uniforms_.update(ctx_.queue, u);

// 5. Bind in shader
pipeline_ = create_pipeline(...);
bind_group_ = create_bind_group(..., uniforms_.get().buffer, ...);
```

### Backward Compatibility
**All existing effects continue to work:**
- Default parameter constructor delegates to parameterized constructor
- Effects without .seq parameters use default values
- No changes required to existing timeline code

---

## Design Document

**Complete design**: See `doc/SHADER_PARAMETRIZATION_PLAN.md`

**Key Sections**:
- Motivation and use cases
- 5-phase implementation plan
- WGSL alignment rules and solutions
- Per-frame vs constructor-time parameters
- .seq syntax specification
- Migration path for existing effects
- Size budget analysis (~400-500 bytes)
- Alternative approaches considered

---

## Size Impact

**Binary Size**: +400-500 bytes net
- UniformHelper template code: ~200 bytes
- FlashEffectParams/Uniforms structs: ~100 bytes
- Per-effect parameter overhead: ~50-100 bytes each
- .seq compiler parameter parsing: ~100 bytes

**Trade-off**: Acceptable size increase for significant artist workflow improvement (no C++ recompilation for parameter tweaks).

---

## Next Steps (Recommendations)

### Immediate (Task #73)
Extend parametrization to other effects using FlashEffect as template:

1. **ChromaAberrationEffect**:
   - Parameters: `offset` (vec2), `strength` (float)
   - Syntax: `EFFECT + ChromaAberrationEffect 0 5 offset=0.01,0.02 strength=1.5`

2. **GaussianBlurEffect**:
   - Parameters: `radius` (float), `sigma` (float)
   - Syntax: `EFFECT + GaussianBlurEffect 2 8 radius=5.0 sigma=2.0`

3. **DistortEffect** (if exists):
   - Parameters: `amount` (float), `speed` (float), `scale` (vec2)

4. **SolarizeEffect**:
   - Parameters: `threshold` (float), `intensity` (float)

### Future Enhancements
- Parameter curve system (keyframe animation via .seq timeline)
- vec4/mat4 parameter support (color with alpha, transformation matrices)
- Enum parameters (blend modes, filter types)
- Parameter validation and clamping (min/max ranges)

---

## Context for Next Session

### What Works
- Shader parametrization infrastructure fully functional
- FlashEffect demonstrates complete implementation pattern
- .seq syntax parsing robust with backward compatibility
- All tests passing, demo stable

### Known Limitations
- Only FlashEffect parametrized (other effects use hardcoded values)
- No parameter validation or range clamping
- No keyframe animation support (future feature)

### Migration Pattern for Other Effects
1. Copy `FlashEffectParams` / `FlashUniforms` pattern
2. Add `UniformBuffer<YourUniforms>` member
3. Update shader to use uniform values
4. Implement per-frame computation in `render()`
5. Extend `seq_compiler.cc` with effect-specific parameter parsing
6. Test with .seq file, verify alignment with static_assert

---

## User Feedback Summary

**Session Start**: User requested shader parametrization with use cases (flash colors, aberration strength)

**Key Clarifications**:
- User confirmed parameters will be "generated in the code (manually, in effect's C++ code)"
- User emphasized "precision: user change parameter values on a per-frame basis. time-dependent"
- Critical feedback: Per-frame computation required, not just static constructor params

**Critical Bug Report**: User provided log.txt showing GPU validation error (buffer size mismatch)

**Outcome**: User satisfied, requested commit + documentation update (completed)

---

## Handoff Checklist

- [x] All tests passing (32/32)
- [x] Working tree clean
- [x] Documentation updated (TODO.md, PROJECT_CONTEXT.md, COMPLETED.md)
- [x] Commits created with detailed messages (c7d1dd7, 775c0ea)
- [x] No known regressions
- [x] Design document complete (SHADER_PARAMETRIZATION_PLAN.md)
- [x] Extension task noted (Task #73)
- [x] Ready for next session

---

**handoff(Claude):** Shader parametrization system complete. FlashEffect fully parameterized with .seq syntax. Critical alignment bug fixed. 32/32 tests passing. Ready for extension to other effects (Task #73).

---

*Generated: February 8, 2026*
*Claude Sonnet 4.5*