# Diagnosis: 255 → 253 Loss (-2 LSBs)

## Findings

### F16 Precision
✅ **No loss:** 1.0 → f16(0x3c00) → 1.0 (exact round-trip)

### Visualization Scale
⚠️ **Inconsistent:**
- Layer 1 uses `vizScale = 0.5` (line 1530)
- Should render as 128, not 253
- **User seeing 253 suggests viewing Static Features (scale=1.0), not CNN output**

### Suspected Issue: Input Alpha Channel

**Code:** `tools/cnn_v2_test/index.html` line 1233
```javascript
depthData[i] = pixels[i * 4 + 3] / 255.0;  // Alpha from canvas
```

**Hypothesis:** Input PNG alpha channel = 253 (not 255)
- Browsers may set alpha < 255 for certain images
- Pre-multiplied alpha corrections
- PNG encoder compression artifacts

### Test

**Check input alpha:**
```javascript
// In HTML tool console:
const canvas = document.createElement('canvas');
canvas.width = tester.image.width;
canvas.height = tester.image.height;
const ctx = canvas.getContext('2d');
ctx.drawImage(tester.image, 0, 0);
const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
const alpha = imgData.data[3];  // First pixel alpha
console.log('First pixel alpha:', alpha);
```

### Alternative: C++ Reference

Check if `cnn_test` tool produces same -2 loss:
```bash
# Generate solid white 8×8 test image with alpha=255
python3 -c "
from PIL import Image
import numpy as np
img = np.ones((8, 8, 4), dtype=np.uint8) * 255
Image.fromarray(img, 'RGBA').save('test_white_255.png')
print('Created test_white_255.png: all pixels RGBA=(255,255,255,255)')
"

# Test with HTML tool → check if p3 = 1.0 or 0.9921875
# Test with cnn_test → compare output
./build/cnn_test test_white_255.png output.png --cnn-version 2 --debug-hex
```

### Next Steps

1. **Verify input:** Check alpha channel of user's input image
2. **Add debug:** Log first pixel RGBA values in HTML tool
3. **Compare:** Run same image through C++ cnn_test
4. **Isolate:** Test with synthetic 255 alpha image

## Conclusion

**Most likely:** Input image alpha ≠ 255, already 253 before CNN processing.
**Verify:** User should check input PNG metadata and alpha channel values.