docs/curation_pipeline.md: add BGR-vs-RGB channel-order gotcha

docs/curation_pipeline.md

@@ -312,6 +312,41 @@ PY
 
 ---
 
+## Channel-order gotcha (read before adding any new visualiser)
+
+The three video streams use **different byte orders** inside the H5 files. Get
+this wrong and your previews come out with R/B swapped:
+
+| Stream | H5 byte order | Why |
+|---|---|---|
+| `realsense/cam{0,1,2}/color` | **BGR** | `pyrealsense2` enabled with `rs.format.bgr8` in `camera_stream/realsense_stream.py` |
+| `gelsight/{left,right}/frames` | **RGB** | Converted in `camera_stream/usb_video_stream.py` (`cv2.cvtColor(BGR2RGB)`) before writing |
+| `view` in `processed/.../*.pt` | **BGR** | Carved out of cam0 by `twm.contact_index` with no further conversion |
+| `tactile_{left,right}` in `.pt` | **RGB** | Carved out of gelsight frames, same as above |
+
+Rules of thumb:
+
+- `cv2.imshow` and `cv2.VideoWriter` expect BGR — RealSense streams + the
+  recording UI are end-to-end consistent (and incidentally hide the fact
+  that the GelSight thumbnails in `twm.visualize` are technically rendered
+  with their R/B swapped).
+- `PIL.Image.fromarray` and the GIF encoder interpret bytes as RGB. Anything
+  PIL-bound (gallery previews, dataloader sample GIFs) must convert
+  RealSense BGR → RGB once. The cheapest way is `arr[..., ::-1]`.
+- When reusing `twm.data_collection.make_preview` as the panel builder for a
+  GIF, feed the H5 streams in **as-is** so the panel matches the live viewer
+  pixel-for-pixel, then `cv2.cvtColor(panel, cv2.COLOR_BGR2RGB)` once at the
+  end before saving with PIL.
+
+If you're writing a new visualiser:
+
+```python
+import cv2, h5py, hdf5plugin  # noqa: F401
+with h5py.File(path, "r") as f:
+    cam_rgb = f["realsense/cam0/color"][i][..., ::-1].copy()  # BGR → RGB
+    gs_rgb  = f["gelsight/left/frames"][i]                    # already RGB
+```
+
 ## Failure-mode reference table
 
 | Mode                | Detector                                       | Threshold        | Where logged                              |