fix(cnn_v3): blender_export --view-layer flag + fallback to layer[0]

Fixes KeyError when blend file uses a non-default view layer name.
Adds --view-layer NAME arg; pass '?' to list available layers.
Defaults to index 0 with a clear error if the name is not found.

handoff(Gemini): blender_export.py view layer selection now robust

2 files changed, 26 insertions, 2 deletions

diff --git a/cnn_v3/docs/HOW_TO_CNN.md b/cnn_v3/docs/HOW_TO_CNN.md
index 6214b20..c9c16a7 100644
--- a/cnn_v3/docs/HOW_TO_CNN.md
+++ b/cnn_v3/docs/HOW_TO_CNN.md
@@ -163,6 +163,7 @@ not to Blender. Each `#` in `--output` is replaced by a zero-padded frame digit.
 | `--height N` | 360 | Render resolution |
 | `--start-frame N` | scene start | First frame |
 | `--end-frame N` | scene end | Last frame |
+| `--view-layer NAME` | first layer | View layer name; pass `?` to list available layers |
 
 **Render pass → CNN channel mapping:**
 
diff --git a/cnn_v3/training/blender_export.py b/cnn_v3/training/blender_export.py
index 63dd0e3..5d07c26 100644
--- a/cnn_v3/training/blender_export.py
+++ b/cnn_v3/training/blender_export.py
@@ -6,6 +6,12 @@ then renders the current scene to a multi-layer EXR.
 Usage (headless):
     blender -b scene.blend -P blender_export.py -- --output renders/frame_###
 
+    # List available view layers in the blend file:
+    blender -b scene.blend -P blender_export.py -- --view-layer ?
+
+    # Use a specific view layer:
+    blender -b scene.blend -P blender_export.py -- --output renders/frame_### --view-layer "MyLayer"
+
 Each '#' in the output path is replaced by Blender with the frame number (zero-padded).
 The script writes one multi-layer EXR per frame containing all required passes.
 
@@ -57,6 +63,11 @@ def parse_args():
         "--end-frame", type=int, default=None,
         help="Last frame to render (default: scene end frame)"
     )
+    parser.add_argument(
+        "--view-layer", default=None, metavar="NAME",
+        help="View layer name to use (default: first available). "
+             "Use --view-layer without a value (or pass '?') to list available layers."
+    )
     return parser.parse_args(argv)
 
 
@@ -77,8 +88,20 @@ def configure_scene(args):
     # Use Cycles for best multi-pass support
     scene.render.engine = "CYCLES"
 
-    # Enable required render passes on the active view layer
-    vl = scene.view_layers["ViewLayer"]
+    # Enable required render passes on the selected view layer
+    available = list(scene.view_layers.keys())
+    if getattr(args, "view_layer", None) in (None, "?"):
+        if args.view_layer == "?":
+            print("Available view layers:", available)
+            sys.exit(0)
+        vl = scene.view_layers[0]
+    else:
+        try:
+            vl = scene.view_layers[args.view_layer]
+        except KeyError:
+            print(f"ERROR: view layer '{args.view_layer}' not found.")
+            print(f"Available: {available}")
+            sys.exit(1)
     vl.use_pass_combined           = True   # beauty target
     vl.use_pass_diffuse_color      = True   # albedo
     vl.use_pass_normal             = True   # world normals