[libcamera-devel] [PATCH 09/10] libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images

[Naushir Patuck]

Hi David,


On Mon, 16 Nov 2020 at 16:49, David Plowman <david.plowman at raspberrypi.com>
wrote:

> When parts of an image saturate then the image brightness no longer
> increases linearly with increased exposure/gain. Having calculated a
> linear gain value it's better then to try it, allowing for saturating
> regions, and if necessary increase the gain some more. We repeat this
> several times.
>
> Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
>

Reviewed-by: Naushir Patuck <naush at raspberrypi.com>


> ---
>  src/ipa/raspberrypi/controller/rpi/agc.cpp | 34 +++++++++++++++-------
>  1 file changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/src/ipa/raspberrypi/controller/rpi/agc.cpp
> b/src/ipa/raspberrypi/controller/rpi/agc.cpp
> index 1037f968..93b46a28 100644
> --- a/src/ipa/raspberrypi/controller/rpi/agc.cpp
> +++ b/src/ipa/raspberrypi/controller/rpi/agc.cpp
> @@ -422,17 +422,21 @@ void Agc::fetchAwbStatus(Metadata *image_metadata)
>  }
>
>  static double compute_initial_Y(bcm2835_isp_stats *stats, AwbStatus const
> &awb,
> -                               double weights[])
> +                               double weights[], double gain)
>  {
>         bcm2835_isp_stats_region *regions = stats->agc_stats;
>         // Note how the calculation below means that equal weights give you
>         // "average" metering (i.e. all pixels equally important).
>         double R_sum = 0, G_sum = 0, B_sum = 0, pixel_sum = 0;
>         for (int i = 0; i < AGC_STATS_SIZE; i++) {
> -               R_sum += regions[i].r_sum * weights[i];
> -               G_sum += regions[i].g_sum * weights[i];
> -               B_sum += regions[i].b_sum * weights[i];
> -               pixel_sum += regions[i].counted * weights[i];
> +               double counted = regions[i].counted;
> +               double r_sum = std::min(regions[i].r_sum * gain, ((1 <<
> PIPELINE_BITS) - 1) * counted);
> +               double g_sum = std::min(regions[i].g_sum * gain, ((1 <<
> PIPELINE_BITS) - 1) * counted);
> +               double b_sum = std::min(regions[i].b_sum * gain, ((1 <<
> PIPELINE_BITS) - 1) * counted);
> +               R_sum += r_sum * weights[i];
> +               G_sum += g_sum * weights[i];
> +               B_sum += b_sum * weights[i];
> +               pixel_sum += counted * weights[i];
>         }
>         if (pixel_sum == 0.0) {
>                 LOG(RPiAgc, Warning) << "compute_initial_Y: pixel_sum is
> zero";
> @@ -476,11 +480,21 @@ void Agc::computeGain(bcm2835_isp_stats *statistics,
> Metadata *image_metadata,
>         target_Y =
>
> config_.Y_target.Eval(config_.Y_target.Domain().Clip(lux.lux));
>         target_Y = std::min(EV_GAIN_Y_TARGET_LIMIT, target_Y * ev_gain);
> -       double initial_Y = compute_initial_Y(statistics, awb_,
> -                                            metering_mode_->weights);
> -       gain = std::min(10.0, target_Y / (initial_Y + .001));
> -       LOG(RPiAgc, Debug) << "Initially Y " << initial_Y << " target " <<
> target_Y
> -                          << " gives gain " << gain;
> +
> +       // Do this calculation a few times as brightness increase can be
> +       // non-linear when there are saturated regions.
> +       gain = 1.0;
> +       for (int i = 0; i < 8; i++) {
> +               double initial_Y = compute_initial_Y(statistics, awb_,
> +
> metering_mode_->weights, gain);
> +               double extra_gain = std::min(10.0, target_Y / (initial_Y +
> .001));
> +               gain *= extra_gain;
> +               LOG(RPiAgc, Debug) << "Initial Y " << initial_Y << "
> target " << target_Y
> +                                  << " gives gain " << gain;
> +               if (extra_gain < 1.01) // close enough
> +                       break;
> +       }
> +
>         for (auto &c : *constraint_mode_) {
>                 double new_target_Y;
>                 double new_gain =
> --
> 2.20.1
>
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