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diff --git a/doc/HOWTO.md b/doc/HOWTO.md
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--- a/doc/HOWTO.md
+++ b/doc/HOWTO.md
@@ -162,6 +162,45 @@ See `doc/ASSET_SYSTEM.md` and `doc/WORKSPACE_SYSTEM.md`.
 
 ---
 
+## CNN Testing
+
+### Offline Shader Validation
+```bash
+# Test trained CNN on PNG input
+./build/cnn_test input.png output.png
+
+# Adjust blend amount (0.0 = original, 1.0 = full CNN)
+./build/cnn_test input.png output.png --blend 0.5
+
+# PPM output format
+./build/cnn_test input.png output.ppm --format ppm
+```
+
+### Ground Truth Comparison
+```bash
+# Generate Python ground truth
+./training/train_cnn.py --infer input.png \
+  --export-only checkpoints/checkpoint_epoch_1000.pth \
+  --output ground_truth.png
+
+# Run tool
+./build/cnn_test input.png tool_output.png
+
+# Compare (Python required)
+python3 -c "
+import numpy as np
+from PIL import Image
+gt = np.array(Image.open('ground_truth.png').convert('RGB'))
+out = np.array(Image.open('tool_output.png').convert('RGB'))
+mse = np.mean((gt.astype(float) - out.astype(float)) ** 2)
+print(f'MSE: {mse:.4f} (target: < 10.0)')
+"
+```
+
+See `doc/CNN_TEST_TOOL.md` for full documentation.
+
+---
+
 ## Additional Documentation
 
 - **Build System:** `doc/BUILD.md` - Multi-platform, size optimization