[libcamera-devel] [PATCH v4 29/32] ipa: rkisp1: awb: Prevent RGB means from being negative

[Laurent Pinchart]

Hi Jacopo,

On Thu, Sep 22, 2022 at 12:42:48PM +0200, Jacopo Mondi wrote:
> Hi Laurent
> 
> On Thu, Sep 08, 2022 at 04:41:57AM +0300, Laurent Pinchart via libcamera-devel wrote:
> > Due to hardware rounding errors in the YCbCr means, the calculated RGB
> 
> Is it due to rounding or simply because
> the values of yMean, cbMean and crMean
> 
> 		double yMean = awb->awb_mean[0].mean_y_or_g;
> 		double cbMean = awb->awb_mean[0].mean_cb_or_b;
> 		double crMean = awb->awb_mean[0].mean_cr_or_r;
> 
> 		/*
> 		 * Convert from YCbCr to RGB.
> 		 * The hardware uses the following formulas:
> 		 * Y = 16 + 0.2500 R + 0.5000 G + 0.1094 B
> 		 * Cb = 128 - 0.1406 R - 0.2969 G + 0.4375 B
> 		 * Cr = 128 + 0.4375 R - 0.3750 G - 0.0625 B
> 		 *
> 		 * The inverse matrix is thus:
> 		 * [[1,1636, -0,0623,  1,6008]
> 		 *  [1,1636, -0,4045, -0,7949]
> 		 *  [1,1636,  1,9912, -0,0250]]
> 		 */
> 		yMean -= 16;
> 		cbMean -= 128;
> 		crMean -= 128;
> 
> are smaller than the above offsets ? I guess this can happen when the
> image is particularly dark, right ?

When the image is fully black yMean should be equal to 16 and cbMean and crMean
to 128. I haven't seen yMean smaller than 16, but cbMean and crMean were
often 127. This leads to RGB means smaller than 0. The only plausible
cause I found was hardware rounding errors.

> Anyway, this certainly fixes an issue
> 
> Reviewed-by: Jacopo Mondi <jacopo at jmondi.org>
> 
> > means may be negative. This would lead to negative gains, messing up
> > calculation. Prevent this by clamping the means to positive values.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/ipa/rkisp1/algorithms/awb.cpp | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/src/ipa/rkisp1/algorithms/awb.cpp b/src/ipa/rkisp1/algorithms/awb.cpp
> > index ed91e9277a16..de54c4d24650 100644
> > --- a/src/ipa/rkisp1/algorithms/awb.cpp
> > +++ b/src/ipa/rkisp1/algorithms/awb.cpp
> > @@ -242,6 +242,16 @@ void Awb::process(IPAContext &context,
> >  		redMean = 1.1636 * yMean - 0.0623 * cbMean + 1.6008 * crMean;
> >  		greenMean = 1.1636 * yMean - 0.4045 * cbMean - 0.7949 * crMean;
> >  		blueMean = 1.1636 * yMean + 1.9912 * cbMean - 0.0250 * crMean;
> > +
> > +		/*
> > +		 * Due to hardware rounding errors in the YCbCr means, the
> > +		 * calculated RGB means may be negative. This would lead to
> > +		 * negative gains, messing up calculation. Prevent this by
> > +		 * clamping the means to positive values.
> > +		 */
> > +		redMean = std::max(redMean, 0.0);
> > +		greenMean = std::max(greenMean, 0.0);
> > +		blueMean = std::max(blueMean, 0.0);
> >  	}
> >
> >  	/*

-- 
Regards,

Laurent Pinchart