[PATCH] libcamera: software_isp: Fix CCM multiplication

[Milan Zamazal]

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.

Signed-off-by: Milan Zamazal <mzamazal at redhat.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;
-- 
2.49.0