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

[Jacopo Mondi]

On Thu, Sep 08, 2022 at 04:42:00AM +0300, Laurent Pinchart via libcamera-devel wrote:
> 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.
>
> 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);

Dumb question, do we care about divisors in the ]0.0, 1.0[ range ?

>
>  	/*
>  	 * Clamp the gain values to the hardware, which expresses gains as Q2.8
> --
> Regards,
>
> Laurent Pinchart
>