Lux metadata always 400 with --immediate and manual AWB gains (single-frame capture)

[ByteHD]

Hi, I found a problem where the Lux metadata is always stuck at 400.0 when using --immediate with --awbgains set to non-zero values. I've been running with rpicam-still --immediate --awbgains 0,0 --shutter ... --gain ... and Lux was always correct. When I switched to --awbgains 1.3,1.6, Lux became permanently stuck at 400.0.

Setup:

• Pi 5, HQ camera (IMX477 NoIR)
• rpicam-still --immediate --nopreview --raw --shutter 2155 --gain 1 --awbgains 1.3,1.6 --denoise off --sharpness 0
• libcamera 0.7.1+rpt20260429-1
• Running LIBCAMERA_LOG_LEVELS=RPiLux:DEBUG produces zero RPiLux output, confirming Lux::process() never executes

I don't know cpp well so I asked Claude to check source code and it found what appears to be the following problem. I'd appreciate confirmation if this analysis is correct.

Root cause:

When --awbgains is set to non-zero values, Awb::setManualGains() disables auto AWB. This causes Awb::getConvergenceFrames() to return 0 (awb.cpp:192-201). Combined with manual exposure and gain, AgcChannel::getConvergenceFrames() also returns 0.

In ipa_base.cpp, startupFrameCount is calculated as:

result->startupFrameCount = std::max({ agcConvergenceFrames, awbConvergenceFrames });

With both at 0, startupFrameCount = 0. This means no frames are tagged as FrameStartup in the pipeline, and the very first frame is returned as FrameSuccess.

rpicam-apps drops FrameStartup frames (rpicam_app.cpp:1134-1136), so with --immediate this first FrameSuccess frame is captured immediately.

However, mistrustCount_ is still 1 (from CamHelper::mistrustFramesStartup()). In ipa_base.cpp:527:

if (processPending_ && frameCount_ >= mistrustCount_)

On frame 0, 0 >= 1 is false, so controller_.process() is skipped entirely. Lux::process() never runs. The reported Lux value is the constructor default of 400 (lux.cpp:29).

Why --awbgains 0,0 worked: Awb::setManualGains() checks for zero values and switches back to auto mode (awb.cpp:209). Auto AWB returns convergenceFrames = 3, so startupFrameCount becomes 4 (3 + mistrustCount). Those 4 frames get dropped as FrameStartup, and by the time the first FrameSuccess arrives, frameCount_ is well past mistrustCount_ and Lux::process() has run multiple times.

Impact: This affects any single-frame --immediate capture with fully manual controls (shutter + gain + awbgains). This is common in astrophotography and scientific imaging where exposures can be minutes long, making it impractical to throw away extra frames just to let lux process.

Possible fix: Lux is a passive read-only estimate that doesn't feed back into any control loop. There's no reason to gate it behind the mistrustCount_ guard — the ISP Y statistics are valid from frame 0. Exempting Lux::process() from the mistrust check would fix this without affecting any other algorithm.

Thanks for looking into it and for your help!