ipa: rkisp1: awb: Freeze AWB when means are too small

When the RGB means are too small, gains and color temperature can't be
meaningfully calculated. Freeze the AWB in that case, using the
previously calculated values.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/ipa/rkisp1/algorithms/awb.cpp

@@ -31,6 +31,9 @@ namespace ipa::rkisp1::algorithms {
 
 LOG_DEFINE_CATEGORY(RkISP1Awb)
 
+/* Minimum mean value below which AWB can't operate. */
+constexpr double kMeanMinThreshold = 2.0;
+
 Awb::Awb()
 	: rgbMode_(false)
 {
@@ -263,7 +266,17 @@ void Awb::process(IPAContext &context,
 	greenMean /= frameContext.awb.gains.green;
 	blueMean /= frameContext.awb.gains.blue;
 
-	frameContext.awb.temperatureK = estimateCCT(redMean, greenMean, blueMean);
+	/*
+	 * If the means are too small we don't have enough information to
+	 * meaningfully calculate gains. Freeze the algorithm in that case.
+	 */
+	if (redMean < kMeanMinThreshold && greenMean < kMeanMinThreshold &&
+	    blueMean < kMeanMinThreshold) {
+		frameContext.awb.temperatureK = activeState.awb.temperatureK;
+		return;
+	}
+
+	activeState.awb.temperatureK = estimateCCT(redMean, greenMean, blueMean);
 
 	/*
 	 * Estimate the red and blue gains to apply in a grey world. The green
@@ -290,6 +303,8 @@ void Awb::process(IPAContext &context,
 	activeState.awb.gains.automatic.blue = blueGain;
 	activeState.awb.gains.automatic.green = 1.0;
 
+	frameContext.awb.temperatureK = activeState.awb.temperatureK;
+
 	LOG(RkISP1Awb, Debug) << std::showpoint
 		<< "Means [" << redMean << ", " << greenMean << ", " << blueMean
 		<< "], gains [" << activeState.awb.gains.automatic.red << ", "

src/ipa/rkisp1/ipa_context.cpp

@@ -156,6 +156,9 @@ namespace libcamera::ipa::rkisp1 {
  * \var IPAActiveState::awb.gains.automatic.blue
  * \brief Automatic white balance gain for B channel
  *
+ * \var IPAActiveState::awb.temperatureK
+ * \brief Estimated color temperature
+ *
  * \var IPAActiveState::awb.autoEnabled
  * \brief Whether the Auto White Balance algorithm is enabled
  */

src/ipa/rkisp1/ipa_context.h

@@ -68,6 +68,7 @@ struct IPAActiveState {
 			} automatic;
 		} gains;
 
+		unsigned int temperatureK;
 		bool autoEnabled;
 	} awb;