4 files changed, 142 insertions, 11 deletions
diff --git a/src/ipa/simple/black_level.cpp b/src/ipa/simple/black_level.cpp
new file mode 100644
index 00000000..c7e8d8b7
--- /dev/null
+++ b/src/ipa/simple/black_level.cpp
@@ -0,0 +1,88 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Red Hat Inc.
+ *
+ * black_level.cpp - black level handling
+ */
+
+#include "black_level.h"
+
+#include <numeric>
+
+#include <libcamera/base/log.h>
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(IPASoftBL)
+
+/**
+ * \class BlackLevel
+ * \brief Object providing black point level for software ISP
+ *
+ * Black level can be provided in hardware tuning files or, if no tuning file is
+ * available for the given hardware, guessed automatically, with less accuracy.
+ * As tuning files are not yet implemented for software ISP, BlackLevel
+ * currently provides only guessed black levels.
+ *
+ * This class serves for tracking black level as a property of the underlying
+ * hardware, not as means of enhancing a particular scene or image.
+ *
+ * The class is supposed to be instantiated for the given camera stream.
+ * The black level can be retrieved using BlackLevel::get() method. It is
+ * initially 0 and may change when updated using BlackLevel::update() method.
+ */
+
+BlackLevel::BlackLevel()
+	: blackLevel_(255), blackLevelSet_(false)
+{
+}
+
+/**
+ * \brief Return the current black level
+ *
+ * \return The black level, in the range from 0 (minimum) to 255 (maximum).
+ * If the black level couldn't be determined yet, return 0.
+ */
+unsigned int BlackLevel::get() const
+{
+	return blackLevelSet_ ? blackLevel_ : 0;
+}
+
+/**
+ * \brief Update black level from the provided histogram
+ * \param[in] yHistogram The histogram to be used for updating black level
+ *
+ * The black level is property of the given hardware, not image. It is updated
+ * only if it has not been yet set or if it is lower than the lowest value seen
+ * so far.
+ */
+void BlackLevel::update(SwIspStats::Histogram &yHistogram)
+{
+	/*
+	 * The constant is selected to be "good enough", not overly conservative or
+	 * aggressive. There is no magic about the given value.
+	 */
+	constexpr float ignoredPercentage_ = 0.02;
+	const unsigned int total =
+		std::accumulate(begin(yHistogram), end(yHistogram), 0);
+	const unsigned int pixelThreshold = ignoredPercentage_ * total;
+	const unsigned int histogramRatio = 256 / SwIspStats::kYHistogramSize;
+	const unsigned int currentBlackIdx = blackLevel_ / histogramRatio;
+
+	for (unsigned int i = 0, seen = 0;
+	     i < currentBlackIdx && i < SwIspStats::kYHistogramSize;
+	     i++) {
+		seen += yHistogram[i];
+		if (seen >= pixelThreshold) {
+			blackLevel_ = i * histogramRatio;
+			blackLevelSet_ = true;
+			LOG(IPASoftBL, Debug)
+				<< "Auto-set black level: "
+				<< i << "/" << SwIspStats::kYHistogramSize
+				<< " (" << 100 * (seen - yHistogram[i]) / total << "% below, "
+				<< 100 * seen / total << "% at or below)";
+			break;
+		}
+	};
+}
+} /* namespace libcamera */
diff --git a/src/ipa/simple/black_level.h b/src/ipa/simple/black_level.h
new file mode 100644
index 00000000..7e37757e
--- /dev/null
+++ b/src/ipa/simple/black_level.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Red Hat Inc.
+ *
+ * black_level.h - black level handling
+ */
+
+#pragma once
+
+#include <array>
+
+#include "libcamera/internal/software_isp/swisp_stats.h"
+
+namespace libcamera {
+
+class BlackLevel
+{
+public:
+	BlackLevel();
+	unsigned int get() const;
+	void update(SwIspStats::Histogram &yHistogram);
+
+private:
+	unsigned int blackLevel_;
+	bool blackLevelSet_;
+};
+
+} /* namespace libcamera */
diff --git a/src/ipa/simple/meson.build b/src/ipa/simple/meson.build
index 3e863db7..44b5f1d7 100644
--- a/src/ipa/simple/meson.build
+++ b/src/ipa/simple/meson.build
@@ -2,8 +2,13 @@
 
 ipa_name = 'ipa_soft_simple'
 
+soft_simple_sources = files([
+    'soft_simple.cpp',
+    'black_level.cpp',
+])
+
 mod = shared_module(ipa_name,
-                    ['soft_simple.cpp', libcamera_generated_ipa_headers],
+                    [soft_simple_sources, libcamera_generated_ipa_headers],
                     name_prefix : '',
                     include_directories : [ipa_includes, libipa_includes],
                     dependencies : libcamera_private,
diff --git a/src/ipa/simple/soft_simple.cpp b/src/ipa/simple/soft_simple.cpp
index ff1d8e0c..b9fb58b5 100644
--- a/src/ipa/simple/soft_simple.cpp
+++ b/src/ipa/simple/soft_simple.cpp
@@ -26,8 +26,9 @@
 
 #include "libipa/camera_sensor_helper.h"
 
-namespace libcamera {
+#include "black_level.h"
 
+namespace libcamera {
 LOG_DEFINE_CATEGORY(IPASoft)
 
 namespace ipa::soft {
@@ -54,7 +55,8 @@ class IPASoftSimple : public ipa::soft::IPASoftInterface
 {
 public:
 	IPASoftSimple()
-		: params_(nullptr), stats_(nullptr), ignoreUpdates_(0)
+		: params_(nullptr), stats_(nullptr), blackLevel_(BlackLevel()),
+		  ignoreUpdates_(0)
 	{
 	}
 
@@ -78,6 +80,7 @@ private:
 	SwIspStats *stats_;
 	std::unique_ptr<CameraSensorHelper> camHelper_;
 	ControlInfoMap sensorInfoMap_;
+	BlackLevel blackLevel_;
 
 	int32_t exposureMin_, exposureMax_;
 	int32_t exposure_;
@@ -255,6 +258,10 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
 	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. */
@@ -273,18 +280,20 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
 	 * Calculate Mean Sample Value (MSV) according to formula from:
 	 * https://www.araa.asn.au/acra/acra2007/papers/paper84final.pdf
 	 */
-	constexpr unsigned int yHistValsPerBin =
-		SwIspStats::kYHistogramSize / kExposureBinsCount;
-	constexpr unsigned int yHistValsPerBinMod =
-		SwIspStats::kYHistogramSize /
-		(SwIspStats::kYHistogramSize % kExposureBinsCount + 1);
+	const unsigned int blackLevelHistIdx =
+		params_->blackLevel / (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 < SwIspStats::kYHistogramSize; i++) {
+	for (unsigned int i = 0; i < histogramSize; i++) {
 		unsigned int idx = (i - (i / yHistValsPerBinMod)) / yHistValsPerBin;
-		exposureBins[idx] += stats_->yHistogram[i];
+		exposureBins[idx] += stats_->yHistogram[blackLevelHistIdx + i];
 	}
 
 	for (unsigned int i = 0; i < kExposureBinsCount; i++) {
@@ -320,7 +329,8 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
 
 	LOG(IPASoft, Debug) << "exposureMSV " << exposureMSV
 			    << " exp " << exposure_ << " again " << again_
-			    << " gain R/B " << params_->gainR << "/" << params_->gainB;
+			    << " gain R/B " << params_->gainR << "/" << params_->gainB
+			    << " black level " << params_->blackLevel;
 }
 
 void IPASoftSimple::updateExposure(double exposureMSV)