gen_identity_weights: Add --mix option for static feature blending

Adds --mix flag to blend input channels with static features:
- p0+p4 → p0 (RGBA + UV.x)
- p1+p5 → p1 (RGBA + UV.y)
- p2+p6 → p2 (RGBA + sin encoding)
- p3+p7 → p3 (RGBA + bias)

Useful for debugging static feature contribution in CNN v2.

Updated doc/CNN_V2_DEBUG_TOOLS.md with --mix usage examples.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

2 files changed, 25 insertions, 6 deletions

diff --git a/doc/CNN_V2_DEBUG_TOOLS.md b/doc/CNN_V2_DEBUG_TOOLS.md
index b6dc65f..25e5205 100644
--- a/doc/CNN_V2_DEBUG_TOOLS.md
+++ b/doc/CNN_V2_DEBUG_TOOLS.md
@@ -18,14 +18,17 @@ Tools for investigating CNN v2 mismatch between HTML tool and cnn_test.
 # 3×3 identity
 ./training/gen_identity_weights.py workspaces/main/weights/cnn_v2_identity_3x3.bin --kernel-size 3
 
+# Mix mode: p0+p4, p1+p5, p2+p6, p3+p7
+./training/gen_identity_weights.py output.bin --mix
+
 # Custom mip level
 ./training/gen_identity_weights.py output.bin --kernel-size 1 --mip-level 2
 ```
 
 **Output:**
 - Single layer, 12D→4D (4 input channels + 8 static features)
-- Identity matrix: Output Ch{0,1,2,3} = Input Ch{0,1,2,3}
-- Static features (Ch 4-11) are zeroed
+- Identity mode: Output Ch{0,1,2,3} = Input Ch{0,1,2,3}
+- Mix mode (--mix): Output Ch{i} = Input Ch{i} + Input Ch{i+4} (blends input with static features)
 - Minimal file size (~136 bytes for 1×1, ~904 bytes for 3×3)
 
 **Validation:**
diff --git a/training/gen_identity_weights.py b/training/gen_identity_weights.py
index a84ea87..c996758 100755
--- a/training/gen_identity_weights.py
+++ b/training/gen_identity_weights.py
@@ -4,8 +4,12 @@
 Creates trivial .bin with 1 layer, 1×1 kernel, identity passthrough.
 Output Ch{0,1,2,3} = Input Ch{0,1,2,3} (ignores static features).
 
+With --mix: Output Ch{i} = Input Ch{i} + Input Ch{i+4}
+  (p0+p4, p1+p5, p2+p6, p3+p7)
+
 Usage:
   ./training/gen_identity_weights.py [output.bin]
+  ./training/gen_identity_weights.py --mix [output.bin]
 """
 
 import argparse
@@ -14,9 +18,11 @@ import struct
 from pathlib import Path
 
 
-def generate_identity_weights(output_path, kernel_size=1, mip_level=0):
+def generate_identity_weights(output_path, kernel_size=1, mip_level=0, mix=False):
     """Generate identity weights: output = input (ignores static features).
 
+    If mix=True, adds p4→p0, p5→p1, p6→p2, p7→p3 (blends input with static).
+
     Binary format:
       Header (20 bytes):
         uint32 magic ('CNN2')
@@ -34,7 +40,8 @@ def generate_identity_weights(output_path, kernel_size=1, mip_level=0):
 
       Weights (u32 packed f16):
         Identity matrix for first 4 input channels
-        Zeros for static features (channels 4-11)
+        Zeros for static features (channels 4-11) OR
+        Mix matrix (p0+p4, p1+p5, p2+p6, p3+p7) if mix=True
     """
     # Identity: 4 output channels, 12 input channels
     # Weight shape: [out_ch, in_ch, kernel_h, kernel_w]
@@ -51,15 +58,22 @@ def generate_identity_weights(output_path, kernel_size=1, mip_level=0):
     for i in range(out_channels):
         weights[i, i, center, center] = 1.0
 
+    # If mix, add p4→p0, p5→p1, p6→p2, p7→p3
+    if mix:
+        for i in range(out_channels):
+            weights[i, i + 4, center, center] = 1.0
+
     # Flatten
     weights_flat = weights.flatten()
     weight_count = len(weights_flat)
 
-    print(f"Generating identity weights:")
+    print(f"Generating {'mix' if mix else 'identity'} weights:")
     print(f"  Kernel size: {kernel_size}×{kernel_size}")
     print(f"  Channels: 12D→4D")
     print(f"  Weights: {weight_count}")
     print(f"  Mip level: {mip_level}")
+    if mix:
+        print(f"  Mode: p0+p4, p1+p5, p2+p6, p3+p7")
 
     # Convert to f16
     weights_f16 = np.array(weights_flat, dtype=np.float16)
@@ -122,11 +136,13 @@ def main():
                        help='Kernel size (default: 1×1)')
     parser.add_argument('--mip-level', type=int, default=0,
                        help='Mip level for p0-p3 features (default: 0)')
+    parser.add_argument('--mix', action='store_true',
+                       help='Mix mode: p0+p4, p1+p5, p2+p6, p3+p7')
 
     args = parser.parse_args()
 
     print("=== Identity Weight Generator ===\n")
-    generate_identity_weights(args.output, args.kernel_size, args.mip_level)
+    generate_identity_weights(args.output, args.kernel_size, args.mip_level, args.mix)
     print("\nDone!")