[libcamera-devel] [PATCH v2 10/12] ipa: ipu3: awb: Introduce Black Level Correction

[Jean-Michel Hautbois]

The pixels output by the camera normally include a black level, because
sensors do not always report a signal level of '0' for black. Pixels at
or below this level should be considered black and to achieve that, we
need to substract an offset to all the pixels. This can be taken into
account by reading the lowest value of a special region on sensors which
is not exposed to light. This provides a substracting factor to be
able to adjust the expected black levels in the resultant images.

For a camera outputting 10-bit pixel values (in the range 0 to 1023) a
typical black level might be 64. It is a fixed value, obtained by
capturing a raw frame with minimum exposure and gain fixed to 1.0 while
covering the sensor (the darker the better). We consider it good enough
as a very first approximation, until we measure it during a tuning
process and include it in a configuration file

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
---
 src/ipa/ipu3/algorithms/black_correction.cpp | 67 ++++++++++++++++++++
 src/ipa/ipu3/algorithms/black_correction.h   | 28 ++++++++
 src/ipa/ipu3/algorithms/meson.build          |  1 +
 src/ipa/ipu3/ipu3.cpp                        |  2 +
 4 files changed, 98 insertions(+)
 create mode 100644 src/ipa/ipu3/algorithms/black_correction.cpp
 create mode 100644 src/ipa/ipu3/algorithms/black_correction.h

diff --git a/src/ipa/ipu3/algorithms/black_correction.cpp b/src/ipa/ipu3/algorithms/black_correction.cpp
new file mode 100644
index 00000000..5b5b5d31
--- /dev/null
+++ b/src/ipa/ipu3/algorithms/black_correction.cpp
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google inc.
+ *
+ * black_correction.cpp - IPU3 Optical Black Correction control
+ */
+
+#include "black_correction.h"
+
+#include <string.h>
+
+/**
+ * \file black_correction.h
+ */
+
+namespace libcamera {
+
+namespace ipa::ipu3::algorithms {
+
+/**
+ * \class BlackCorrection
+ * \brief A class to handle optical black correction
+ *
+ * The pixels output by the camera normally include a black level, because
+ * sensors do not always report a signal level of '0' for black. Pixels at or
+ * below this level should be considered black and to achieve that, we need to
+ * substract an offset to all the pixels.
+ *
+ * This can be taken into account by reading the lowest value of a special
+ * region on sensors which is not exposed to light. This provides a substracting
+ * factor to be able to adjust the expected black levels in the resultant
+ * images.
+ */
+
+BlackCorrection::BlackCorrection()
+{
+}
+
+/**
+ * \brief Fill in the parameter structure, and enable optical black correction
+ * \param context The shared IPA context
+ * \param params The IPU3 parameters
+ *
+ * Populate the IPU3 parameter structure with the correction values for each
+ * channel and enable the corresponding ImgU block processing.
+ */
+void BlackCorrection::prepare([[maybe_unused]] IPAContext &context,
+			      ipu3_uapi_params *params)
+{
+	/*
+	 * The Optical Black Level correction values
+	 * \todo The correction values should come from sensor specific
+	 * tuning processes. This is a first approximation
+	 */
+	params->obgrid_param.gr = 64;
+	params->obgrid_param.r  = 64;
+	params->obgrid_param.b  = 64;
+	params->obgrid_param.gb = 64;
+
+	/* Enable the custom optical black correction processing */
+	params->use.obgrid = 1;
+	params->use.obgrid_param = 1;
+}
+
+} /* namespace ipa::ipu3::algorithms */
+
+} /* namespace libcamera */
diff --git a/src/ipa/ipu3/algorithms/black_correction.h b/src/ipa/ipu3/algorithms/black_correction.h
new file mode 100644
index 00000000..6486ac5b
--- /dev/null
+++ b/src/ipa/ipu3/algorithms/black_correction.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google inc.
+ *
+ * black_correction.h - IPU3 Optical Black Correction control
+ */
+#ifndef __LIBCAMERA_IPU3_ALGORITHMS_BLACK_CORRECTION_H__
+#define __LIBCAMERA_IPU3_ALGORITHMS_BLACK_CORRECTION_H__
+
+#include "algorithm.h"
+
+namespace libcamera {
+
+namespace ipa::ipu3::algorithms {
+
+class BlackCorrection : public Algorithm
+{
+public:
+	BlackCorrection();
+
+	void prepare(IPAContext &context, ipu3_uapi_params *params) override;
+};
+
+} /* namespace ipa::ipu3::algorithms */
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_IPU3_ALGORITHMS_BLACK_CORRECTION_H__ */
diff --git a/src/ipa/ipu3/algorithms/meson.build b/src/ipa/ipu3/algorithms/meson.build
index deae225b..66877c1b 100644
--- a/src/ipa/ipu3/algorithms/meson.build
+++ b/src/ipa/ipu3/algorithms/meson.build
@@ -4,5 +4,6 @@ ipu3_ipa_algorithms = files([
     'agc.cpp',
     'algorithm.cpp',
     'awb.cpp',
+    'black_correction.cpp',
     'tone_mapping.cpp',
 ])
diff --git a/src/ipa/ipu3/ipu3.cpp b/src/ipa/ipu3/ipu3.cpp
index b33e8c77..7cc0fb89 100644
--- a/src/ipa/ipu3/ipu3.cpp
+++ b/src/ipa/ipu3/ipu3.cpp
@@ -33,6 +33,7 @@
 #include "algorithms/agc.h"
 #include "algorithms/algorithm.h"
 #include "algorithms/awb.h"
+#include "algorithms/black_correction.h"
 #include "algorithms/tone_mapping.h"
 #include "libipa/camera_sensor_helper.h"
 
@@ -279,6 +280,7 @@ int IPAIPU3::init(const IPASettings &settings,
 	/* Construct our Algorithms */
 	algorithms_.push_back(std::make_unique<algorithms::Agc>());
 	algorithms_.push_back(std::make_unique<algorithms::Awb>());
+	algorithms_.push_back(std::make_unique<algorithms::BlackCorrection>());
 	algorithms_.push_back(std::make_unique<algorithms::ToneMapping>());
 
 	return 0;
-- 
2.30.2