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

[Laurent Pinchart]

Hi Jacopo,

On Thu, Sep 22, 2022 at 12:51:48PM +0200, Jacopo Mondi wrote:
> 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 ?

Given that the range for the mean values is [0, 255] and that we freeze
AWB when the means are below 2.0, we don't.

I wrote this patch before adding the thresholds, the std::max isn't
strictly needed anymore, but I feel safer keeping it in case we change
the threshold mechanism later.

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

-- 
Regards,

Laurent Pinchart