Document grayscale fisheye WebP decode contract

README.md

@@ -133,7 +133,11 @@ This release applies a storage-only, lossless re-encoding to the published
 MOS/SOS data. Every transformed file was decoded immediately after writing and
 compared with its source array/value:
 
-* `rgb/` and `fisheye/` PNGs are stored as lossless WebP.
+* `rgb/` and `fisheye/` PNGs are stored as lossless WebP. Standard WebP
+  decoders expose the grayscale fisheye frames as RGB; use RPX's
+  `safe_load_gray` (or Pillow's `.convert("L")`) to recover the original
+  `(H, W) uint8` plane. This round-trip was verified with zero pixel
+  difference.
 * `depth/` and `sam2/masks/` remain PNG, re-encoded at compression level 9;
   their 16-bit modes and pixel values are unchanged.
 * `cam_pose/` changes from per-frame `.npz` wrappers to per-frame `.npy`