# CNN v3 How-To

Practical playbook for the CNN v3 pipeline: G-buffer effect, training data,
training the U-Net+FiLM network, and wiring everything into the demo.

See `CNN_V3.md` for the full architecture design.

---

## 1. Using GBufferEffect in the Demo

`GBufferEffect` is a full-class effect (Path B in `doc/EFFECT_WORKFLOW.md`).
It rasterizes proxy geometry to MRT G-buffer textures and packs them into two
`rgba32uint` feature textures (`feat_tex0`, `feat_tex1`) consumed by the CNN.

### Registration (already done)

- Shaders in `assets.txt`: `SHADER_GBUF_RASTER`, `SHADER_GBUF_PACK`
- Source in `cmake/DemoSourceLists.cmake`: `cnn_v3/src/gbuffer_effect.cc`
- Header included in `src/gpu/demo_effects.h`
- Test in `src/tests/gpu/test_demo_effects.cc`

### Adding to a Sequence

`GBufferEffect` does not exist in `seq_compiler.py` as a named effect yet
(no `.seq` syntax integration for Phase 1). Wire it directly in C++ alongside
your scene code, or add it to the timeline when the full CNNv3Effect is ready.

**C++ wiring example** (e.g. inside a Sequence or main.cc):

```cpp
#include "../../cnn_v3/src/gbuffer_effect.h"

// Allocate once alongside your scene
auto gbuf = std::make_shared<GBufferEffect>(
    ctx, /*inputs=*/{"prev_cnn"},  // or any dummy node
    /*outputs=*/{"gbuf_feat0", "gbuf_feat1"},
    /*start=*/0.0f, /*end=*/60.0f);

gbuf->set_scene(&my_scene, &my_camera);

// In render loop, call before CNN pass:
gbuf->render(encoder, params, nodes);
```

### Internal passes

Each frame, `GBufferEffect::render()` executes:

1. **Pass 1 — MRT rasterization** (`gbuf_raster.wgsl`)
   - Proxy box (36 verts) × N objects, instanced
   - MRT outputs: `gbuf_albedo` (rgba16float), `gbuf_normal_mat` (rgba16float)
   - Depth test + write into `gbuf_depth` (depth32float)

2. **Pass 2/3 — SDF + Lighting** — TODO (placeholder: shadow=1, transp=0)

3. **Pass 4 — Pack compute** (`gbuf_pack.wgsl`)
   - Reads all G-buffer textures + `prev_cnn` input
   - Writes `feat_tex0` + `feat_tex1` (rgba32uint, 20 channels, 32 bytes/pixel)

### Output node names

By default the outputs are named from the `outputs` vector passed to the
constructor. Use these names when binding the CNN effect input:

```
outputs[0]  → feat_tex0   (rgba32uint: albedo.rgb, normal.xy, depth, depth_grad.xy)
outputs[1]  → feat_tex1   (rgba32uint: mat_id, prev.rgb, mip1.rgb, mip2.rgb, shadow, transp)
```

### Scene data

Call `set_scene(scene, camera)` before the first render. The effect uploads
`GlobalUniforms` (view-proj, camera pos, resolution) and `ObjectData` (model
matrix, color) to GPU storage buffers each frame.

---

## 2. Preparing Training Data

CNN v3 supports two data sources: Blender renders and real photos.

### 2a. From Blender Renders

```bash
# 1. In Blender: run the export script (requires Blender 3.x+)
blender --background scene.blend --python cnn_v3/training/blender_export.py \
    -- --output /tmp/renders/ --frames 200

# 2. Pack into sample directory
python3 cnn_v3/training/pack_blender_sample.py \
    --render-dir /tmp/renders/frame_0001/ \
    --output dataset/blender/sample_0001/
```

Each sample directory contains:
```
sample_XXXX/
  albedo.png    — RGB uint8 (material color, pre-lighting)
  normal.png    — RG uint8 (oct-encoded XY, remap [0,1])
  depth.png     — R uint16 (1/z normalized, 16-bit)
  matid.png     — R uint8 (object index / 255)
  shadow.png    — R uint8 (0=dark, 255=lit)
  transp.png    — R uint8 (0=opaque, 255=transparent)
  target.png    — RGB/RGBA (stylized ground truth)
```

### 2b. From Real Photos

Geometric channels are zeroed; the network degrades gracefully due to
channel-dropout training.

```bash
python3 cnn_v3/training/pack_photo_sample.py \
    --photo cnn_v3/training/input/photo1.jpg \
    --output dataset/photos/sample_001/
```

The output `target.png` defaults to the input photo (no style). Copy in
your stylized version as `target.png` before training.

### Dataset layout

```
dataset/
  blender/
    sample_0001/  sample_0002/  ...
  photos/
    sample_001/   sample_002/   ...
```

Mix freely; the dataloader treats all sample directories uniformly.

---

## 3. Training

*(Script not yet written — see TODO.md. Architecture spec in `CNN_V3.md` §Training.)*

**Planned command:**
```bash
python3 cnn_v3/training/train_cnn_v3.py \
    --dataset dataset/ \
    --epochs 500 \
    --output cnn_v3/weights/cnn_v3_weights.bin
```

**FiLM conditioning** during training:
- Beat/audio inputs randomized per sample
- MLP: `Linear(5→16) → ReLU → Linear(16→40)` trained jointly with U-Net
- Output: γ/β for enc0(4ch) + enc1(8ch) + dec1(4ch) + dec0(4ch) = 40 floats

---

## 4. Running the CNN v3 Effect

`CNNv3Effect` is implemented. Wire into a sequence:

```seq
# BPM 120
SEQUENCE 0 0 "Scene with CNN v3"
  EFFECT + GBufferEffect prev_cnn -> gbuf_feat0 gbuf_feat1  0 60
  EFFECT + CNNv3Effect   gbuf_feat0 gbuf_feat1 -> sink       0 60
```

FiLM parameters uploaded each frame:
```cpp
cnn_v3_effect->set_film_params(
    params.beat_phase, params.beat_time / 8.0f, params.audio_intensity,
    style_p0, style_p1);
```

FiLM γ/β default to identity (γ=1, β=0) until `train_cnn_v3.py` produces a trained MLP.

---

## 5. Per-Pixel Validation

C++ parity test passes: `src/tests/gpu/test_cnn_v3_parity.cc` (2 tests).

```bash
cmake -B build -DDEMO_BUILD_TESTS=ON && cmake --build build -j4
cd build && ./test_cnn_v3_parity
```

Results (8×8 test tensors, random weights):
- enc0 max_err = 1.95e-3 ✓
- dec1 max_err = 1.95e-3 ✓
- final max_err = 4.88e-4 ✓  (all ≤ 1/255 = 3.92e-3)

Test vectors generated by `cnn_v3/training/gen_test_vectors.py` (PyTorch reference).

---

## 6. Phase Status

| Phase | Status | Notes |
|-------|--------|-------|
| 1 — G-buffer (raster + pack) | ✅ Done | Integrated, 36/36 tests pass |
| 1 — G-buffer (SDF + shadow passes) | TODO | Placeholder: shadow=1, transp=0 |
| 2 — Training infrastructure | ✅ Done | blender_export.py, pack_*_sample.py |
| 3 — WGSL U-Net shaders | ✅ Done | 5 compute shaders + cnn_v3/common snippet |
| 4 — C++ CNNv3Effect | ✅ Done | FiLM uniform upload, 36/36 tests pass |
| 5 — Parity validation | ✅ Done | test_cnn_v3_parity.cc, max_err=4.88e-4 |
| 6 — FiLM MLP training | TODO | train_cnn_v3.py not yet written |

---

## 7. CNN v3 Inference Shaders (Phase 3)

Five compute passes, each a standalone WGSL shader using `#include "cnn_v3/common"`.
The common snippet provides `get_w()` and `unpack_8ch()`.

| Pass | Shader | Input(s) | Output | Dims |
|------|--------|----------|--------|------|
| enc0 | `cnn_v3_enc0.wgsl` | feat_tex0+feat_tex1 (20ch) | enc0_tex rgba16float (4ch) | full |
| enc1 | `cnn_v3_enc1.wgsl` | enc0_tex (AvgPool2×2 inline) | enc1_tex rgba32uint (8ch) | ½ |
| bottleneck | `cnn_v3_bottleneck.wgsl` | enc1_tex (AvgPool2×2 inline) | bottleneck_tex rgba32uint (8ch) | ¼ |
| dec1 | `cnn_v3_dec1.wgsl` | bottleneck_tex + enc1_tex (skip) | dec1_tex rgba16float (4ch) | ½ |
| dec0 | `cnn_v3_dec0.wgsl` | dec1_tex + enc0_tex (skip) | output_tex rgba16float (4ch) | full |

**Parity rules baked into the shaders:**
- Zero-padding (not clamp) at conv borders
- AvgPool 2×2 for downsampling (exact, deterministic)
- Nearest-neighbor for upsampling (integer `coord / 2`)
- Skip connections: channel concatenation (not add)
- FiLM applied after conv+bias, before ReLU: `max(0, γ·x + β)`
- No batch norm at inference
- Weight layout: OIHW (out × in × kH × kW), biases after conv weights

**Params uniform per shader** (`group 0, binding 3`):
```
struct Params {
    weight_offset: u32,  // f16 index into shared weights buffer
    _pad: vec3u,
    gamma: vec4f,        // FiLM γ  (enc1: gamma_lo+gamma_hi for 8ch)
    beta:  vec4f,        // FiLM β  (enc1: beta_lo+beta_hi for 8ch)
}
```
FiLM γ/β are computed CPU-side by the FiLM MLP (Phase 4) and uploaded each frame.

**Weight offsets** (f16 units, including bias):
| Layer | Weights | Bias | Total f16 |
|-------|---------|------|-----------|
| enc0  | 20×4×9=720 | +4 | 724 |
| enc1  | 4×8×9=288  | +8 | 296 |
| bottleneck | 8×8×1=64 | +8 | 72 |
| dec1  | 16×4×9=576 | +4 | 580 |
| dec0  | 8×4×9=288  | +4 | 292 |
| **Total** | | | **2064 f16 = ~4 KB** |

**Asset IDs** (registered in `workspaces/main/assets.txt` + `src/effects/shaders.cc`):
`SHADER_CNN_V3_COMMON`, `SHADER_CNN_V3_ENC0`, `SHADER_CNN_V3_ENC1`,
`SHADER_CNN_V3_BOTTLENECK`, `SHADER_CNN_V3_DEC1`, `SHADER_CNN_V3_DEC0`

**C++ usage (Phase 4):**
```cpp
auto src = ShaderComposer::Get().Compose({"cnn_v3/common"}, raw_wgsl);
```

---

## 8. Quick Troubleshooting

**GBufferEffect renders nothing / albedo is black**
- Check `set_scene()` was called before `render()`
- Verify scene has at least one object
- Check camera matrix is not degenerate (near/far, aspect)

**Pack shader fails to compile**
- `gbuf_pack.wgsl` uses no `#include`s; ShaderComposer compose is a no-op
- Check `ASSET_SHADER_GBUF_PACK` resolves in assets.txt

**Raster shader fails with `#include "common_uniforms"` error**
- `ShaderComposer::Get().Compose({"common_uniforms"}, src)` must be called
  before passing to `wgpuDeviceCreateShaderModule` — already done in effect.cc

**G-buffer outputs wrong resolution**
- `resize()` is not yet implemented in GBufferEffect; textures are fixed
  at construction size. Will be added when resize support is needed.

---

## 9. See Also

- `cnn_v3/docs/CNN_V3.md` — Full architecture design (U-Net, FiLM, feature layout)
- `doc/EFFECT_WORKFLOW.md` — General effect integration guide
- `cnn_v2/docs/CNN_V2.md` — Reference implementation (simpler, operational)
- `src/tests/gpu/test_demo_effects.cc` — GBufferEffect construction test