libcamera: software_isp: Update black level only on exposure changes

The black level is likely to get updated, if ever, only after exposure
or gain changes.  Don't compute its possible updates if exposure and
gain are unchanged.

It's probably not worth trying to implement something more
sophisticated.  Better to spend the effort on supporting tuning files.

Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/ipa/simple/algorithms/blc.cpp

@@ -30,10 +30,15 @@ int BlackLevel::configure(IPAContext &context,
 
 void BlackLevel::process(IPAContext &context,
 			 [[maybe_unused]] const uint32_t frame,
-			 [[maybe_unused]] IPAFrameContext &frameContext,
+			 IPAFrameContext &frameContext,
 			 const SwIspStats *stats,
 			 [[maybe_unused]] ControlList &metadata)
 {
+	if (frameContext.sensor.exposure == exposure_ &&
+	    frameContext.sensor.gain == gain_) {
+		return;
+	}
+
 	const SwIspStats::Histogram &histogram = stats->yHistogram;
 
 	/*
@@ -54,6 +59,8 @@ void BlackLevel::process(IPAContext &context,
 		seen += histogram[i];
 		if (seen >= pixelThreshold) {
 			context.activeState.blc.level = i * histogramRatio;
+			exposure_ = frameContext.sensor.exposure;
+			gain_ = frameContext.sensor.gain;
 			LOG(IPASoftBL, Debug)
 				<< "Auto-set black level: "
 				<< i << "/" << SwIspStats::kYHistogramSize

src/ipa/simple/algorithms/blc.h

@@ -24,6 +24,10 @@ public:
 		     IPAFrameContext &frameContext,
 		     const SwIspStats *stats,
 		     ControlList &metadata) override;
+
+private:
+	uint32_t exposure_;
+	double gain_;
 };
 
 } /* namespace ipa::soft::algorithms */