ipa: rpi: awb: Add a bias to the AWB search

In the case of an AWB search failure, the current algorithm logic will
return a point on the CT curve closest to where the search finisned.
This can be quite undesirable. Instead, add some bias params to the AWB
algorithm which will direct the search to a set CT value in the case
where statistics become unreliable causing the search to fail.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Author: naushir

src/ipa/rpi/controller/rpi/awb.cpp

@@ -170,6 +170,14 @@ int AwbConfig::read(const libcamera::YamlObject &params)
 	whitepointB = params["whitepoint_b"].get<double>(0.0);
 	if (bayes == false)
 		sensitivityR = sensitivityB = 1.0; /* nor do sensitivities make any sense */
+	/*
+	 * The biasProportion parameter adds a small proportion of the counted
+	 * pixles to a region biased to the biasCT colour temperature.
+	 *
+	 * A typical value for biasProportion would be between 0.05 to 0.1.
+	 */
+	biasProportion = params["bias_proportion"].get<double>(0.0);
+	biasCT = params["bias_ct"].get<double>(kDefaultCT);
 	return 0;
 }
 
@@ -410,7 +418,8 @@ void Awb::asyncFunc()
 
 static void generateStats(std::vector<Awb::RGB> &zones,
 			  StatisticsPtr &stats, double minPixels,
-			  double minG, Metadata &globalMetadata)
+			  double minG, Metadata &globalMetadata,
+			  double biasProportion, double biasCtR, double biasCtB)
 {
 	std::scoped_lock<RPiController::Metadata> l(globalMetadata);
 
@@ -423,6 +432,14 @@ static void generateStats(std::vector<Awb::RGB> &zones,
 				continue;
 			zone.R = region.val.rSum / region.counted;
 			zone.B = region.val.bSum / region.counted;
+			/*
+			 * Add some bias samples to allow the search to tend to a
+			 * bias CT in failure cases.
+			 */
+			const unsigned int proportion = biasProportion * region.counted;
+			zone.R += proportion * biasCtR;
+			zone.B += proportion * biasCtB;
+			zone.G += proportion * 1.0;
 			/* Factor in the ALSC applied colour shading correction if required. */
 			const AlscStatus *alscStatus = globalMetadata.getLocked<AlscStatus>("alsc.status");
 			if (stats->colourStatsPos == Statistics::ColourStatsPos::PreLsc && alscStatus) {
@@ -443,7 +460,9 @@ void Awb::prepareStats()
 	 * any LSC compensation.  We also ignore config_.fast in this version.
 	 */
 	generateStats(zones_, statistics_, config_.minPixels,
-		      config_.minG, getGlobalMetadata());
+		      config_.minG, getGlobalMetadata(),
+		      config_.biasProportion, config_.ctR.eval(config_.biasCT),
+		      config_.ctB.eval(config_.biasCT));
 	/*
 	 * apply sensitivities, so values appear to come from our "canonical"
 	 * sensor.

src/ipa/rpi/controller/rpi/awb.h

@@ -87,6 +87,10 @@ struct AwbConfig {
 	double whitepointR;
 	double whitepointB;
 	bool bayes; /* use Bayesian algorithm */
+	/* proportion of counted samples to add for the search bias */
+	double biasProportion;
+	/* CT target for the search bias */
+	double biasCT;
 };
 
 class Awb : public AwbAlgorithm