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002_main_stream_pixel_format.md

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002 - Main Stream Pixel Format for CSI Cameras

Date

2026-05-31

Status

Decision

Category

Performance

Authors

@antoinecellerier

References

picamera2 V4L2 encoder source

Context

picamera2.create_video_configuration() defaults the main stream to XBGR8888 (32 bits/pixel). Both the V4L2 hardware encoders (MJPEGEncoder, H264Encoder) and the JpegEncoder software fallback accept YUV420 (12 bits/pixel) as a native input. Forcing YUV420 upstream of the encoders skips an implicit colour-space conversion and reduces DMA bandwidth by ~2.7×.

Options

  1. Keep the picamera2 default (XBGR8888).
  2. Request YUV420 for the main stream, falling back to the picamera2 default when the camera does not advertise it.

Decision

We request YUV420 for the main stream when libcamera reports it as supported, selected from a preference-ordered list of encoder-compatible formats with YUV420 first. If none are supported, we omit format from the config and defer to the picamera2 default.

Consequences

  • Measured CPU on a Pi 4B with Camera Module 3 at 1920×1080 @ 30 fps drops by ~28–39 % (idle and live WebRTC respectively) versus the XBGR8888 default.
  • No expected image-quality impact: JPEG and H.264 both encode in YUV internally, so this change moves the colour-space conversion earlier in the pipeline rather than introducing a new one.