[libcamera-devel] [PATCH v4 32/32] ipa: rkisp1: awb: Remove bias from gain calculation

[Kieran Bingham]

Quoting Laurent Pinchart via libcamera-devel (2022-09-08 02:42:00)
> The red and blue gains are computed by dividing the green mean by the
> red and blue means respectively. An offset of 1 is added to the dividers
> to avoid divisions by zero. This introduces a bias in the gain values.
> Fix it by clamping the divisors to a minimum of 1.0 instead of adding an
> offset.
> 

Sounds resonable. That 'hardcoded green gain of 0' is there, which
doesn't make sense to me, but I've questioned that in an earlier patch.

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

> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/ipa/rkisp1/algorithms/awb.cpp | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/src/ipa/rkisp1/algorithms/awb.cpp b/src/ipa/rkisp1/algorithms/awb.cpp
> index 404ad66e6953..fe1ba09a99a2 100644
> --- a/src/ipa/rkisp1/algorithms/awb.cpp
> +++ b/src/ipa/rkisp1/algorithms/awb.cpp
> @@ -280,10 +280,11 @@ void Awb::process(IPAContext &context,
>  
>         /*
>          * Estimate the red and blue gains to apply in a grey world. The green
> -        * gain is hardcoded to 0.
> +        * gain is hardcoded to 0. Avoid divisions by zero by clamping the
> +        * divisor to a minimum value of 1.0.
>          */
> -       double redGain = greenMean / (redMean + 1);
> -       double blueGain = greenMean / (blueMean + 1);
> +       double redGain = greenMean / std::max(redMean, 1.0);
> +       double blueGain = greenMean / std::max(blueMean, 1.0);
>  
>         /*
>          * Clamp the gain values to the hardware, which expresses gains as Q2.8
> -- 
> Regards,
> 
> Laurent Pinchart
>