# STORY-05: Classification & Confidence Aggregation

## Epic
**E1: Core Pipeline** — I can classify face crops and aggregate video confidence.

## Related Requirements
| ID | Requirement |
|----|------|
| FR-03 | Binary demographic classification of detected face crops: Outputs probability p ∈ [0,1] for target class |
| FR-04 | Video-level confidence aggregation & threshold routing: Aggregates frame-level scores → video confidence C; Routes to MATCH, REVIEW, or SKIP |
| NFR-01 | Throughput: ≥ 30 videos/hour/GPU |
| NFR-03 | GPU Memory Safety: ≤ 18GB per GPU sustained |
| NFR-04 | Determinism & Reproducibility: Config-seeded randomness, versioned models |
| TC-01 | Hardware: 2× Tesla P40 24GB (compute capability 5.2, PCIe 3.0, no Tensor Cores) |
| TC-02 | CUDA/Torch Compatibility: CUDA ≤ 11.8, PyTorch ≤ 2.1.0, FP32 inference only |

## Description
Implement demographic classification of detected face crops using a lightweight classifier (MobileNetV3 or equivalent). Apply temperature scaling for calibrated confidence scores. Aggregate frame-level confidence to video-level confidence using configurable aggregation logic. Route videos to MATCH, REVIEW, or SKIP based on threshold comparison.

## Scope

### In Scope
- MobileNetV3 (or equivalent) classification model via TensorRT FP32
- Temperature scaling for confidence calibration
- Frame-level confidence computation (softmax with temperature)
- Video-level confidence aggregation (max or weighted mean, configurable)
- Threshold-based routing (T_high, T_low)
- Routing decision assignment (MATCH, REVIEW, SKIP)
- Confidence score persistence to DB and output files

### Out of Scope
- Frame sampling (covered in STORY-03)
- Face detection (covered in STORY-04)
- Results export format (covered in STORY-06)
- Review UI (covered in STORY-07)
- Active learning / model retraining (covered in STORY-08)

## Deliverables

### 5.1 Classification Model
**File:** `models/classifier/`

Components:
- **Source Model:** MobileNetV3-small (or equivalent lightweight classifier)
  - Trained on demographic dataset (fairness considerations documented)
  - Binary classification: Black male subject (class 1) vs. not (class 0)
- **ONNX Export:** `classifier.onnx`
  - Input: `[1, 3, 224, 224]` (RGB, resized face crop)
  - Output: `[1, 2]` (logits for class 0 and class 1)
- **TensorRT Engine:** `classifier.trt` (FP32)
  - Builder config: max_batch_size=32, max_workspace_size=2GB
- **Calibration Data:** Temperature parameter T stored with model
- **Model Metadata:** `model.json` with architecture, training dataset, validation metrics

### 5.2 Classification Runner
**File:** `src/classifier.py`

Core components:
- **Model Loader:** Load TensorRT engine at startup
  ```python
  class FaceClassifier:
      def __init__(self, engine_path, temperature=1.0, device='cuda'):
          self.engine = load_trt_engine(engine_path)
          self.context = self.engine.create_execution_context()
          self.temperature = temperature
          self.input_shape = (1, 3, 224, 224)
  ```
- **Preprocessing:** Convert face crop to model input
  - Resize to 224×224
  - Normalize (ImageNet statistics)
  - Convert to tensor (NCHW format)
- **Inference:** Run classification on face crops
  - Dynamic batching (shared with face detection batcher)
  - Execute TensorRT engine
  - Get raw logits output
- **Temperature Scaling:** Apply temperature to logits before softmax
  ```python
  def calibrated_softmax(logits, temperature):
      scaled_logits = logits / temperature
      return softmax(scaled_logits, axis=-1)
  ```
- **Frame-level Confidence:** Extract probability for target class (class 1)
  ```python
  p_i = calibrated_probs[:, 1]  # probability of target class
  ```

### 5.3 Confidence Aggregation
**File:** `src/aggregator.py`

Features:
- **Aggregation Strategies (configurable):**
  1. **Max:** `C = max(p_i)` — use highest confidence frame
  2. **Weighted Mean:** `C = softmax(α·mean(p_i) + β·var(p_i))`
  3. **Top-K Mean:** `C = mean(top_k(p_i))` — average of top K confidences
- **Default Strategy:** Max (most conservative, aligns with FR-04)
- **Parameter Configuration:** α, β, K configurable in config.yaml
- **Variance Calculation:** Compute variance of frame-level confidences (for uncertainty estimation)

### 5.4 Threshold Routing
**File:** `src/router.py`

Features:
- **Threshold Comparison:**
  ```python
  if C >= T_high:  # default 0.75
      routing = 'MATCH'
  elif C >= T_low:  # default 0.45
      routing = 'REVIEW'
  else:
      routing = 'SKIP'
  ```
- **Configurable Thresholds:** T_high and T_low in config.yaml
- **Routing Decision Logging:** Log routing decision with confidence score
- **Edge Case Handling:**
  - No faces detected → routing = SKIP (with confidence = 0.0)
  - All frames have same confidence → routing based on threshold comparison
  - Confidence exactly at threshold → use >= comparison (inclusive)

### 5.5 Configuration Updates
**File:** `config.yaml` (updates)

New fields:
```yaml
classifier:
  model: mobilenetv3-small
  model_path: /models/classifier/classifier.trt
  input_size: 224
  temperature: 1.0  # calibration temperature
  default_strategy: max  # max, weighted_mean, top_k_mean

aggregation:
  strategy: max
  alpha: 1.0   # for weighted_mean
  beta: 0.1    # for weighted_mean
  top_k: 3     # for top_k_mean

routing:
  T_high: 0.75
  T_low: 0.45
  no_faces_decision: SKIP
```

## Acceptance Criteria

### Functional
- [ ] MobileNetV3 model loads and runs inference via TensorRT FP32
- [ ] Classification outputs calibrated probability p ∈ [0,1] for target class
- [ ] Temperature scaling is applied correctly (verified on test set)
- [ ] Frame-level confidence scores are deterministic (same input → same output)
- [ ] Max aggregation produces C = max(p_i) correctly
- [ ] Weighted mean aggregation produces correct result with configurable α, β
- [ ] Threshold routing assigns exactly one of: MATCH, REVIEW, SKIP
- [ ] Videos with no detected faces are routed to SKIP with confidence 0.0
- [ ] Confidence exactly at threshold uses >= comparison (inclusive)

### Non-Functional
- [ ] Classification for 16 face crops (batched) completes in < 1 second on Tesla P40
- [ ] GPU memory usage stays ≤ 18GB per GPU during classification
- [ ] Aggregation computation is negligible (< 10ms per video)
- [ ] Temperature parameter is stored with model checkpoint
- [ ] All routing decisions are logged with confidence score and threshold values

### Technical Constraints
- [ ] TensorRT engine is FP32 only
- [ ] No Tensor Cores used (CC 5.2 constraint)
- [ ] CUDA 11.8 compatible
- [ ] ONNX model exports without errors
- [ ] Confidence scores are reproducible (deterministic inference)
- [ ] Aggregation strategy is configurable without code changes

## Dependencies
- **Prerequisites:** STORY-01 (Foundation), STORY-03 (Frame Sampling), STORY-04 (Face Detection — provides crops)
- **Depends on:** None (runs after face detection)
- **Enables:** STORY-06 (Results Persistence), STORY-07 (Review UI — provides routing decisions)

## Risks & Mitigations
| Risk | Mitigation |
|------|--|
| Temperature scaling parameters need tuning | Evaluate on held-out calibration set during training cycle |
| Max aggregation may be overly conservative | Offer weighted mean as alternative; allow per-job strategy selection |
| Model bias concerns with demographic classification | Document training dataset; audit fairness metrics; legal review required |
| TensorRT engine build time | Pre-build and cache engines; version control model files |

## Estimated Effort
- **Sprint:** 5-6 (second half)
- **Story Points:** 21
- **Dependencies:** STORY-01, STORY-03, STORY-04