Camera Capture Timing

How the camera server schedules captures, and what latency to expect between a control request and the start of exposure.

The Sensor Free-Runs

Once configured, the image sensor streams continuously at its configured frame rate; at 30 fps a new frame starts every 33.3 ms. Capture requests do not start an exposure directly. Each request is submitted into the camera pipeline, latches at an upcoming frame boundary, and takes effect 2 to 3 frame intervals later. At 30 fps this puts a floor of roughly 67 to 100 ms (typically ~84 ms) between submitting a request and the start of the exposure it produces.

Scheduling Fields

CameraControl.schedule has two fields:

• period: interval for periodic capture. The server submits a capture request every period nanoseconds.

• snapshot_target: absolute time for a single capture.

snapshot_target controls when the capture request is submitted to the camera pipeline, not when exposure starts. Submission accuracy is better than a millisecond; the pipeline latency described above then applies before exposure begins.

Control Message Pickup

The camera server polls for new control messages on a 100 ms cycle. Set snapshot_target at least 100 ms in the future when you send the control message; closer targets are picked up late, adding delay on top of the pipeline latency.

Jitter

Because requests latch onto the sensor’s frame boundaries, the delay between snapshot_target and the start of exposure varies by up to half a frame interval, depending on where the target lands relative to the frame grid. Repeated captures at a fixed period can appear to have very low jitter; that is the capture spacing phase-locking to the frame grid, not sub-millisecond control of exposure start.

Controlling Exposure Start Precisely

If you need exposure to start at a precise time, use external triggering: the sensor exposes on a hardware trigger pulse instead of free-running. Set external_trigger: true for the camera in camera_server_config.json (see Configuring GMSL Cameras) and follow the External Trigger tutorial. The trigger pulse can also be generated from software via GPIO, as the tutorial’s active mode demonstrates.