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+# 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.