[libcamera-devel] [PATCH 04/10] libcamera: ipa: raspberrypi: agc: Improve centre-weighted luminance calucation

[Naushir Patuck]

Hi David,


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

> Previously the calculation computed Y for each region before returning
> the weighted average, which "baked in" the over-importance of small
> statistics regions. The revised calculation will treat all pixels
> equally when the region weights are the same, making it easier to
> use. With the previous scheme, proper "average" metering was difficult
> to implement.
>
> 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 | 25 +++++++++++++---------
>  1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/src/ipa/raspberrypi/controller/rpi/agc.cpp
> b/src/ipa/raspberrypi/controller/rpi/agc.cpp
> index d29b1156..ead28398 100644
> --- a/src/ipa/raspberrypi/controller/rpi/agc.cpp
> +++ b/src/ipa/raspberrypi/controller/rpi/agc.cpp
> @@ -386,18 +386,23 @@ static double compute_initial_Y(bcm2835_isp_stats
> *stats, Metadata *image_metada
>         awb.gain_r = awb.gain_g = awb.gain_b = 1.0; // in case no metadata
>         if (image_metadata->Get("awb.status", awb) != 0)
>                 LOG(RPiAgc, Warning) << "Agc: no AWB status found";
> -       double Y_sum = 0, weight_sum = 0;
> +       // 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++) {
> -               if (regions[i].counted == 0)
> -                       continue;
> -               weight_sum += weights[i];
> -               double Y = regions[i].r_sum * awb.gain_r * .299 +
> -                          regions[i].g_sum * awb.gain_g * .587 +
> -                          regions[i].b_sum * awb.gain_b * .114;
> -               Y /= regions[i].counted;
> -               Y_sum += Y * weights[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];
>         }
> -       return Y_sum / weight_sum / (1 << PIPELINE_BITS);
> +       if (pixel_sum == 0.0) {
> +               LOG(RPiAgc, Warning) << "compute_initial_Y: pixel_sum is
> zero";
> +               return 0;
> +       }
> +       double Y_sum = R_sum * awb.gain_r * .299 +
> +                      G_sum * awb.gain_g * .587 +
> +                      B_sum * awb.gain_b * .114;
> +       return Y_sum / pixel_sum / (1 << PIPELINE_BITS);
>  }
>
>  // We handle extra gain through EV by adjusting our Y targets. However,
> you
> --
> 2.20.1
>
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