[PATCH] libcamera: software_isp: Fix CCM multiplication

[Kieran Bingham]

Quoting Laurent Pinchart (2025-04-14 19:20:57)
> Hi Milan,
> 
> Thank you for the patch.
> 
> On Mon, Apr 14, 2025 at 07:32:44PM +0200, Milan Zamazal wrote:
> > A colour correction matrix (CCM) is applied like this to an RGB pixel
> > vector P:
> > 
> >   CCM * P
> > 
> > White balance must be applied before CCM.  If CCM is used, software ISP
> > makes a combined matrix by multiplying the CCM by a white balance gains
> > CCM-like matrix (WBG).  The multiplication should be as follows to do it
> > in the correct order:
> > 
> >   CCM * (WBG * P) = (CCM * WBG) * P
> > 
> > The multiplication order in Lut software ISP algorithm is reversed,
> > resulting in colour casts.  Let's fix the order.
> 
> Fixes: e2b4000dc9cc ("libcamera: software_isp: Apply CCM in debayering")
> 
> > Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

This looks great to me. Fixes the issue I reported with testing the
Saturation control which now gives clear greyscale values on 0.0
saturation.

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

> 
> > ---
> >  src/ipa/simple/algorithms/lut.cpp | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/ipa/simple/algorithms/lut.cpp b/src/ipa/simple/algorithms/lut.cpp
> > index e8638f27..d1d5f727 100644
> > --- a/src/ipa/simple/algorithms/lut.cpp
> > +++ b/src/ipa/simple/algorithms/lut.cpp
> > @@ -122,7 +122,7 @@ void Lut::prepare(IPAContext &context,
> >               Matrix<float, 3, 3> gainCcm = { { gains.r(), 0, 0,
> >                                                 0, gains.g(), 0,
> >                                                 0, 0, gains.b() } };
> > -             auto ccm = gainCcm * context.activeState.ccm.ccm;
> > +             auto ccm = context.activeState.ccm.ccm * gainCcm;
> >               auto &red = params->redCcm;
> >               auto &green = params->greenCcm;
> >               auto &blue = params->blueCcm;
> 
> -- 
> Regards,
> 
> Laurent Pinchart