# Fix for 404 Error - OpenCV Model Download

## Problem

The OpenCV fallback detector was trying to download face detection models from URLs that returned 404 errors:

```
urllib.error.HTTPError: HTTP Error 404: Not Found
```

## Root Cause

The original implementation attempted to download OpenCV DNN models from GitHub URLs that are no longer available or have moved.

## Solution

✅ **Replaced with OpenCV's Built-in Haar Cascade Classifier**

- **No downloads required** - Uses OpenCV's built-in face detection
- **Always available** - Included with OpenCV installation
- **More reliable** - No dependency on external URLs
- **Faster initialization** - No network requests

## What Changed

### Before:
- Tried to download models from GitHub (404 errors)
- Required external dependencies
- Failed if URLs were unavailable

### After:
- Uses `cv2.data.haarcascades` (built-in)
- No network requests
- Works offline
- More reliable

## Technical Details

The fix changes the face detector from:
```python
# OLD: Download DNN models
urllib.request.urlretrieve(FACE_DETECTION_MODEL_URL, face_model)
self.face_net = cv2.dnn.readNetFromTensorflow(...)
```

To:
```python
# NEW: Use built-in Haar Cascade
cascade_path = cv2.data.haarcascades + 'haarcascade_frontalface_default.xml'
self.face_cascade = cv2.CascadeClassifier(cascade_path)
```

## Verification

The fix ensures:
1. ✅ No HTTP requests for model downloads
2. ✅ Uses OpenCV's built-in resources
3. ✅ Works offline
4. ✅ Compatible with all OpenCV installations
5. ✅ Maintains MediaPipe-compatible interface

## Performance

- **Haar Cascade**: Slightly faster than DNN for face detection
- **Accuracy**: Good for face detection (suitable for POC)
- **Memory**: Lower memory usage than DNN models

## Testing

To verify the fix works:

```bash
# Activate virtual environment
source venv/bin/activate

# Test the import
python3 -c "from src.face_pose_detector import get_face_detector; print('OK')"

# Run the application
streamlit run src/poc_demo.py
```

## Expected Behavior

When MediaPipe is not available:
1. Code detects MediaPipe import failure
2. Automatically uses OpenCV Haar Cascade
3. Logs: `"✓ OpenCV Face Detector loaded (Haar Cascade)"`
4. Application runs normally

## Notes

- Haar Cascade is less accurate than MediaPipe for detailed face landmarks
- However, it's sufficient for:
  - Face presence detection
  - Basic face tracking
  - Driver absent detection
  - Simplified PERCLOS calculation

For production, MediaPipe is still recommended for better accuracy.

## Summary

✅ **Fixed**: 404 error eliminated
✅ **Improved**: No external dependencies
✅ **Reliable**: Works with standard OpenCV installation
✅ **Compatible**: Maintains same interface as MediaPipe

The application should now work without any download errors!