[libcamera-devel] [PATCH v3 09/14] ipa: ipu3: agc: Simplify division of exposure/gain

[Kieran Bingham]

Quoting Jean-Michel Hautbois (2021-10-21 17:43:56)
> Until now, the algorithm makes complex assumptions when dividing the
> exposure and analogue gain values. Instead, use a simpler clamping of
> the shutter speed first, and then of the analogue gain, based on the
> limits configured.
> 
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/ipa/ipu3/algorithms/agc.cpp | 36 +++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> index f5bb3328..6f5b6a45 100644
> --- a/src/ipa/ipu3/algorithms/agc.cpp
> +++ b/src/ipa/ipu3/algorithms/agc.cpp
> @@ -147,7 +147,9 @@ void Agc::lockExposureGain(uint32_t &exposure, double &gain)
>                                     << " Gain " << gain;
>  
>                 currentExposure_ = currentExposureNoDg_ * newGain;
> +               utils::Duration minShutterSpeed = minExposureLines_ * lineDuration_;
>                 utils::Duration maxShutterSpeed = maxExposureLines_ * lineDuration_;
> +
>                 utils::Duration maxTotalExposure = maxShutterSpeed * kMaxGain;
>                 currentExposure_ = std::min(currentExposure_, maxTotalExposure);
>                 LOG(IPU3Agc, Debug) << "Target total exposure " << currentExposure_
> @@ -156,23 +158,23 @@ void Agc::lockExposureGain(uint32_t &exposure, double &gain)
>                 /* \todo: estimate if we need to desaturate */
>                 filterExposure();
>  
> -               utils::Duration newExposure = 0.0s;
> -               if (currentShutter < maxShutterSpeed) {
> -                       exposure = std::clamp<uint32_t>(exposure * filteredExposure_ / currentExposureNoDg_,
> -                                                       minExposureLines_,
> -                                                       maxExposureLines_);
> -                       newExposure = filteredExposure_ / exposure;
> -                       gain = std::clamp(gain * filteredExposure_ / newExposure,
> -                                         kMinGain, kMaxGain);
> -               } else {
> -                       gain = std::clamp(gain * filteredExposure_ / currentExposureNoDg_,
> -                                         kMinGain, kMaxGain);
> -                       newExposure = filteredExposure_ / gain;
> -                       exposure = std::clamp<uint32_t>(exposure * filteredExposure_ / newExposure,

Makes the previous patch a bit redundant, but at least we removed code
that was more 'correct' ;-)

> -                                                       minExposureLines_,
> -                                                       maxExposureLines_);
> -               }
> -               LOG(IPU3Agc, Debug) << "Adjust exposure " << exposure * lineDuration_ << " and gain " << gain;
> +               utils::Duration exposureValue = filteredExposure_;
> +               utils::Duration shutterTime = minShutterSpeed;
> +
> +               /*
> +                * Push the shutter time up to the maximum first, and only then
> +                * increase the gain.
> +                */
> +               shutterTime = std::clamp<utils::Duration>(exposureValue / kMinGain,
> +                                                         minShutterSpeed, maxShutterSpeed);
> +               double stepGain = std::clamp(exposureValue / shutterTime,
> +                                            kMinGain, kMaxGain);
> +               LOG(IPU3Agc, Debug) << "Divided up shutter and gain are "
> +                                   << shutterTime << " and "
> +                                   << stepGain;
> +
> +               exposure = shutterTime / lineDuration_;
> +               gain = stepGain;

This looks a lot simpler indeed. I haven't gone through and fully
validated all the equations, but as I believe I've seen this working,
locally on my device, I think I can trust that it's functional.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

>         }
>         lastFrame_ = frameCount_;
>  }
> -- 
> 2.32.0
>