libcamera: software_isp: Move exposure+gain to an algorithm module

This is the last step to fully convert software ISP to Algorithm-based
processing.

The newly introduced frameContext.sensor parameters are set, and the
updated code moved, before calling Algorithm::process() to have the
values up-to-date in stats processing.

Resolves software ISP TODO #10.

Signed-off-by: Milan Zamazal <[email protected]>
Reviewed-by: Kieran Bingham <[email protected]>
Reviewed-by: Umang Jain <[email protected]>
Signed-off-by: Kieran Bingham <[email protected]>

src/ipa/simple/algorithms/agc.cpp

@@ -0,0 +1,139 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Red Hat Inc.
+ *
+ * Exposure and gain
+ */
+
+#include "agc.h"
+
+#include <stdint.h>
+
+#include <libcamera/base/log.h>
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(IPASoftExposure)
+
+namespace ipa::soft::algorithms {
+
+/*
+ * The number of bins to use for the optimal exposure calculations.
+ */
+static constexpr unsigned int kExposureBinsCount = 5;
+
+/*
+ * The exposure is optimal when the mean sample value of the histogram is
+ * in the middle of the range.
+ */
+static constexpr float kExposureOptimal = kExposureBinsCount / 2.0;
+
+/*
+ * The below value implements the hysteresis for the exposure adjustment.
+ * It is small enough to have the exposure close to the optimal, and is big
+ * enough to prevent the exposure from wobbling around the optimal value.
+ */
+static constexpr float kExposureSatisfactory = 0.2;
+
+Agc::Agc()
+{
+}
+
+void Agc::updateExposure(IPAContext &context, double exposureMSV)
+{
+	/*
+	 * kExpDenominator of 10 gives ~10% increment/decrement;
+	 * kExpDenominator of 5 - about ~20%
+	 */
+	static constexpr uint8_t kExpDenominator = 10;
+	static constexpr uint8_t kExpNumeratorUp = kExpDenominator + 1;
+	static constexpr uint8_t kExpNumeratorDown = kExpDenominator - 1;
+
+	double next;
+	int32_t &exposure = context.activeState.agc.exposure;
+	double &again = context.activeState.agc.again;
+
+	if (exposureMSV < kExposureOptimal - kExposureSatisfactory) {
+		next = exposure * kExpNumeratorUp / kExpDenominator;
+		if (next - exposure < 1)
+			exposure += 1;
+		else
+			exposure = next;
+		if (exposure >= context.configuration.agc.exposureMax) {
+			next = again * kExpNumeratorUp / kExpDenominator;
+			if (next - again < context.configuration.agc.againMinStep)
+				again += context.configuration.agc.againMinStep;
+			else
+				again = next;
+		}
+	}
+
+	if (exposureMSV > kExposureOptimal + kExposureSatisfactory) {
+		if (exposure == context.configuration.agc.exposureMax &&
+		    again > context.configuration.agc.againMin) {
+			next = again * kExpNumeratorDown / kExpDenominator;
+			if (again - next < context.configuration.agc.againMinStep)
+				again -= context.configuration.agc.againMinStep;
+			else
+				again = next;
+		} else {
+			next = exposure * kExpNumeratorDown / kExpDenominator;
+			if (exposure - next < 1)
+				exposure -= 1;
+			else
+				exposure = next;
+		}
+	}
+
+	exposure = std::clamp(exposure, context.configuration.agc.exposureMin,
+			      context.configuration.agc.exposureMax);
+	again = std::clamp(again, context.configuration.agc.againMin,
+			   context.configuration.agc.againMax);
+
+	LOG(IPASoftExposure, Debug)
+		<< "exposureMSV " << exposureMSV
+		<< " exp " << exposure << " again " << again;
+}
+
+void Agc::process(IPAContext &context,
+		  [[maybe_unused]] const uint32_t frame,
+		  [[maybe_unused]] IPAFrameContext &frameContext,
+		  const SwIspStats *stats,
+		  [[maybe_unused]] ControlList &metadata)
+{
+	/*
+	 * Calculate Mean Sample Value (MSV) according to formula from:
+	 * https://www.araa.asn.au/acra/acra2007/papers/paper84final.pdf
+	 */
+	const auto &histogram = stats->yHistogram;
+	const unsigned int blackLevelHistIdx =
+		context.activeState.blc.level / (256 / SwIspStats::kYHistogramSize);
+	const unsigned int histogramSize =
+		SwIspStats::kYHistogramSize - blackLevelHistIdx;
+	const unsigned int yHistValsPerBin = histogramSize / kExposureBinsCount;
+	const unsigned int yHistValsPerBinMod =
+		histogramSize / (histogramSize % kExposureBinsCount + 1);
+	int exposureBins[kExposureBinsCount] = {};
+	unsigned int denom = 0;
+	unsigned int num = 0;
+
+	for (unsigned int i = 0; i < histogramSize; i++) {
+		unsigned int idx = (i - (i / yHistValsPerBinMod)) / yHistValsPerBin;
+		exposureBins[idx] += histogram[blackLevelHistIdx + i];
+	}
+
+	for (unsigned int i = 0; i < kExposureBinsCount; i++) {
+		LOG(IPASoftExposure, Debug) << i << ": " << exposureBins[i];
+		denom += exposureBins[i];
+		num += exposureBins[i] * (i + 1);
+	}
+
+	float exposureMSV = (denom == 0 ? 0 : static_cast<float>(num) / denom);
+	updateExposure(context, exposureMSV);
+}
+
+REGISTER_IPA_ALGORITHM(Agc, "Agc")
+
+} /* namespace ipa::soft::algorithms */
+
+} /* namespace libcamera */

src/ipa/simple/algorithms/agc.h

@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Red Hat Inc.
+ *
+ * Exposure and gain
+ */
+
+#pragma once
+
+#include "algorithm.h"
+
+namespace libcamera {
+
+namespace ipa::soft::algorithms {
+
+class Agc : public Algorithm
+{
+public:
+	Agc();
+	~Agc() = default;
+
+	void process(IPAContext &context, const uint32_t frame,
+		     IPAFrameContext &frameContext,
+		     const SwIspStats *stats,
+		     ControlList &metadata) override;
+
+private:
+	void updateExposure(IPAContext &context, double exposureMSV);
+};
+
+} /* namespace ipa::soft::algorithms */
+
+} /* namespace libcamera */

src/ipa/simple/algorithms/meson.build

@@ -2,6 +2,7 @@
 
 soft_simple_ipa_algorithms = files([
     'awb.cpp',
+    'agc.cpp',
     'blc.cpp',
     'lut.cpp',
 ])

src/ipa/simple/data/uncalibrated.yaml

@@ -6,4 +6,5 @@ algorithms:
   - BlackLevel:
   - Awb:
   - Lut:
+  - Agc:
 ...

src/ipa/simple/ipa_context.cpp

@@ -77,6 +77,17 @@ namespace libcamera::ipa::soft {
  * \brief Gain of blue color
  */
 
+/**
+ * \var IPAActiveState::agc
+ * \brief Context for the AGC algorithm
+ *
+ * \var IPAActiveState::agc.exposure
+ * \brief Current exposure value
+ *
+ * \var IPAActiveState::agc.again
+ * \brief Current analog gain value
+ */
+
 /**
  * \var IPAActiveState::gamma
  * \brief Context for gamma in the Colors algorithm

src/ipa/simple/ipa_context.h

@@ -18,6 +18,10 @@ namespace ipa::soft {
 
 struct IPASessionConfiguration {
 	float gamma;
+	struct {
+		int32_t exposureMin, exposureMax;
+		double againMin, againMax, againMinStep;
+	} agc;
 };
 
 struct IPAActiveState {
@@ -31,6 +35,11 @@ struct IPAActiveState {
 		double blue;
 	} gains;
 
+	struct {
+		int32_t exposure;
+		double again;
+	} agc;
+
 	static constexpr unsigned int kGammaLookupSize = 1024;
 	struct {
 		std::array<double, kGammaLookupSize> gammaTable;
@@ -39,6 +48,10 @@ struct IPAActiveState {
 };
 
 struct IPAFrameContext : public FrameContext {
+	struct {
+		uint32_t exposure;
+		double gain;
+	} sensor;
 };
 
 struct IPAContext {