fix: Correct UV coordinate computation to match PyTorch linspace

Critical mismatch: shader used pixel-center coordinates while PyTorch
uses pixel-corner coordinates, causing 0.5-pixel offset.

PyTorch: linspace(0, 1, H) → [0, 1/(H-1), ..., 1]
Shader: (p.xy - 0.5) / (resolution - 1.0) to match

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

1 files changed, 2 insertions, 1 deletions

diff --git a/training/train_cnn.py b/training/train_cnn.py
index 7a2e85a..2b60d15 100755
--- a/training/train_cnn.py
+++ b/training/train_cnn.py
@@ -169,7 +169,8 @@ def generate_layer_shader(output_path, num_layers, kernel_sizes):
         f.write("    return vec4<f32>(pos[i], 0.0, 1.0);\n")
         f.write("}\n\n")
         f.write("@fragment fn fs_main(@builtin(position) p: vec4<f32>) -> @location(0) vec4<f32> {\n")
-        f.write("    let uv = p.xy / uniforms.resolution;\n")
+        f.write("    // Match PyTorch linspace: pixel_idx / (size - 1), not pixel_center / size\n")
+        f.write("    let uv = (p.xy - 0.5) / (uniforms.resolution - 1.0);\n")
         f.write("    let original_raw = textureSample(original_input, smplr, uv);\n")
         f.write("    let original = (original_raw - 0.5) * 2.0;  // Normalize to [-1,1]\n")
         f.write("    let gray = dot(original.rgb, vec3<f32>(0.2126, 0.7152, 0.0722));\n")