USB camera report: manual exposure and gain sweep

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/impl/reports/USB-CAMERA.md

@@ -91,6 +91,34 @@ frames found*). UVC MJPEG commonly omits the Huffman tables, which `jpegdec`
 fills in and NVIDIA's decoder does not; not investigated further. The CPU
 figures include software JPEG decoding.
 
+### Manual exposure and gain (1280×720, 60 fps requested, same scene)
+
+Both controls are honored and read back as set. Brightness is the frame mean (0–255);
+noise is the standard deviation over a flat patch of ceiling.
+
+| Exposure | Gain | Delivered | Mean | Flat patch: mean / std |
+|---|---|---|---|---|
+| auto | auto | 18.7 fps | 137 | 160 / 3.9 |
+| 0.1 ms | 0 | 50 fps | 3 | 2 / 0.1 |
+| 0.1 ms | 240 | 50 fps | 135 | 151 / **31.3** |
+| 1 ms | 240 | 50 fps | 219 | 250 (clipped) |
+| 10 ms | 0 | 50 fps | 9 | 7 / 0.9 |
+| 16.6 ms | 0 | 50 fps | 16 | 12 / 1.0 |
+| 33.3 ms | 0 | **23 fps** | 31 | 26 / 1.9 |
+| 100 ms requested | 0 | **16.5 fps** | 44 | 41 / 2.5 |
+
+- **Frame rate holds to ~16.6 ms of exposure**, then the frame time follows the
+  exposure. Auto exposure takes that trade freely, and **nothing tells it not to**:
+  there is no `exposure_dynamic_framerate` control. A pipeline that needs a fixed
+  rate has to run exposure manually and do its own metering.
+- **Exposure tops out near 50–60 ms**, whatever the control's 1 s range says:
+  asking for 100 ms gave 1.45× the brightness of 33 ms and a 60 ms frame time.
+- **Gain is very large:** 0.1 ms at full gain matches auto exposure's brightness,
+  roughly 1000–2000× over gain 0, and is very noisy (std 31 against 4).
+- **Sensor identity stays unproven.** The gain range is consistent with a
+  STARVIS-class sensor; a night test against a reference camera would settle
+  low-light performance, and the bridge cannot.
+
 ## What it means
 
 | Use | Fits? |
@@ -106,7 +134,7 @@ The USB 2.0 hub it shares with the modem has headroom: 7.3 MB/s of MJPEG is
 ## Open
 
 - Daylight and night frame rates and frame sizes in the car, on a real road
-  scene.
+  scene; noise at the gain night driving needs.
 - Mounting and orientation (the image can be flipped in the pipeline; the
   camera has no flip control).
 - Whether the bridge's MJPEG can be made acceptable to `nvjpegdec` (inserting