[PATCH 2/5] libcamera: software_isp: Honor black level in AWB

[Milan Zamazal]

The white balance computation didn't consider black level.  This is
wrong because then the computed ratios are off when they are computed
from the whole brightness range rather than the sensor range.

This patch adjusts white balance computation for the black level.  There
is no need to change white balance application in debayering as this is
already applied after black level correction.

Exposure computation already subtracts black level, no changes are
needed there.

Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
---
 src/ipa/simple/soft_simple.cpp | 43 ++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/src/ipa/simple/soft_simple.cpp b/src/ipa/simple/soft_simple.cpp
index b9fb58b5..30956a19 100644
--- a/src/ipa/simple/soft_simple.cpp
+++ b/src/ipa/simple/soft_simple.cpp
@@ -5,6 +5,8 @@
  * soft_simple.cpp - Simple Software Image Processing Algorithm module
  */
 
+#include <numeric>
+#include <stdint.h>
 #include <sys/mman.h>
 
 #include <linux/v4l2-controls.h>
@@ -240,28 +242,45 @@ void IPASoftSimple::stop()
 
 void IPASoftSimple::processStats(const ControlList &sensorControls)
 {
+	SwIspStats::Histogram histogram = stats_->yHistogram;
+	if (ignoreUpdates_ > 0)
+		blackLevel_.update(histogram);
+	const uint8_t blackLevel = blackLevel_.get();
+	params_->blackLevel = blackLevel;
+
 	/*
 	 * Calculate red and blue gains for AWB.
 	 * Clamp max gain at 4.0, this also avoids 0 division.
 	 */
-	if (stats_->sumR_ <= stats_->sumG_ / 4)
-		params_->gainR = 1024;
-	else
-		params_->gainR = 256 * stats_->sumG_ / stats_->sumR_;
+	{
+		const uint64_t nPixels = std::accumulate(
+			histogram.begin(), histogram.end(), 0);
+		auto subtractBlackLevel = [blackLevel, nPixels](
+						  uint64_t sum, uint64_t div)
+			-> uint64_t {
+			uint64_t reducedSum = sum - blackLevel * nPixels / div;
+			uint64_t spreadSum = reducedSum * 256 / (256 - blackLevel);
+			return spreadSum;
+		};
+		const uint64_t sumR = subtractBlackLevel(stats_->sumR_, 4);
+		const uint64_t sumG = subtractBlackLevel(stats_->sumG_, 2);
+		const uint64_t sumB = subtractBlackLevel(stats_->sumB_, 4);
+
+		if (sumR <= sumG / 4)
+			params_->gainR = 1024;
+		else
+			params_->gainR = 256 * sumG / sumR;
 
-	if (stats_->sumB_ <= stats_->sumG_ / 4)
-		params_->gainB = 1024;
-	else
-		params_->gainB = 256 * stats_->sumG_ / stats_->sumB_;
+		if (sumB <= sumG / 4)
+			params_->gainB = 1024;
+		else
+			params_->gainB = 256 * sumG / sumB;
+	}
 
 	/* Green gain and gamma values are fixed */
 	params_->gainG = 256;
 	params_->gamma = 0.5;
 
-	if (ignoreUpdates_ > 0)
-		blackLevel_.update(stats_->yHistogram);
-	params_->blackLevel = blackLevel_.get();
-
 	setIspParams.emit();
 
 	/* \todo Switch to the libipa/algorithm.h API someday. */
-- 
2.42.0