From f077c58e087ef90644621f4039bd7922ced0bc57 Mon Sep 17 00:00:00 2001
From: Stefan Klug <stefan.klug@ideasonboard.com>
Date: Fri, 19 Sep 2025 11:40:19 +0200
Subject: [PATCH] libipa: exposure_mode_helper: Take exposure/gain quantization
 into account

In ExposureModeHelper::splitExposure() the quantization of exposure time
and gain is not taken into account. This can lead to visible flicker
when the quantization steps are too big. As a preparation to fixing
that, add a function to set the sensor line length and the current
sensor mode helper and extend the clampXXX functions to return the
quantization error.

By default the exposure time quantization is assumed to be 1us and gain
is assumed to not be quantized at all.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
---
 src/ipa/libipa/exposure_mode_helper.cpp | 49 ++++++++++++++++++++-----
 src/ipa/libipa/exposure_mode_helper.h   | 10 ++++-
 2 files changed, 48 insertions(+), 11 deletions(-)

diff --git a/src/ipa/libipa/exposure_mode_helper.cpp b/src/ipa/libipa/exposure_mode_helper.cpp
index 0c1e99e3..b776a031 100644
--- a/src/ipa/libipa/exposure_mode_helper.cpp
+++ b/src/ipa/libipa/exposure_mode_helper.cpp
@@ -70,18 +70,37 @@ namespace ipa {
  * the runtime limits set through setLimits() instead.
  */
 ExposureModeHelper::ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages)
+	: lineDuration_(1us), minExposureTime_(0us), maxExposureTime_(0us),
+	  minGain_(0), maxGain_(0), sensorHelper_(nullptr)
 {
-	minExposureTime_ = 0us;
-	maxExposureTime_ = 0us;
-	minGain_ = 0;
-	maxGain_ = 0;
-
 	for (const auto &[s, g] : stages) {
 		exposureTimes_.push_back(s);
 		gains_.push_back(g);
 	}
 }
 
+/**
+ * \brief Configure sensor details
+ * \param[in] lineDuration The current line length of the sensor
+ * \param[in] sensorHelper The sensor helper
+ *
+ * This function sets the line length and sensor helper. These are used in
+ * splitExposure() to take the quantization of the exposure and gain into
+ * account.
+ *
+ * When this has not been called, it is assumed that exposure is in micro second
+ * granularity and gain has no quantization at all.
+ *
+ * ExposureModeHelper keeps a pointer to the CameraSensorHelper, so the caller
+ * has to ensure that sensorHelper is valid until the next call to configure().
+ */
+void ExposureModeHelper::configure(utils::Duration lineDuration,
+				   const CameraSensorHelper *sensorHelper)
+{
+	lineDuration_ = lineDuration;
+	sensorHelper_ = sensorHelper;
+}
+
 /**
  * \brief Set the exposure time and gain limits
  * \param[in] minExposureTime The minimum exposure time supported
@@ -108,14 +127,26 @@ void ExposureModeHelper::setLimits(utils::Duration minExposureTime,
 	maxGain_ = maxGain;
 }
 
-utils::Duration ExposureModeHelper::clampExposureTime(utils::Duration exposureTime) const
+utils::Duration ExposureModeHelper::clampExposureTime(utils::Duration exposureTime,
+						      double *quantizationGain) const
 {
-	return std::clamp(exposureTime, minExposureTime_, maxExposureTime_);
+	utils::Duration clamped;
+	utils::Duration exp;
+
+	clamped = std::clamp(exposureTime, minExposureTime_, maxExposureTime_);
+	exp = static_cast<long>(clamped / lineDuration_) * lineDuration_;
+	if (quantizationGain)
+		*quantizationGain = clamped / exp;
+
+	return exp;
 }
 
-double ExposureModeHelper::clampGain(double gain) const
+double ExposureModeHelper::clampGain(double gain, double *quantizationGain) const
 {
-	return std::clamp(gain, minGain_, maxGain_);
+	double clamped = std::clamp(gain, minGain_, maxGain_);
+	if (!sensorHelper_)
+		return clamped;
+	return sensorHelper_->quantizeGain(clamped, quantizationGain);
 }
 
 /**
diff --git a/src/ipa/libipa/exposure_mode_helper.h b/src/ipa/libipa/exposure_mode_helper.h
index c5be1b67..ac7e8da9 100644
--- a/src/ipa/libipa/exposure_mode_helper.h
+++ b/src/ipa/libipa/exposure_mode_helper.h
@@ -14,6 +14,8 @@
 #include <libcamera/base/span.h>
 #include <libcamera/base/utils.h>
 
+#include "camera_sensor_helper.h"
+
 namespace libcamera {
 
 namespace ipa {
@@ -24,6 +26,7 @@ public:
 	ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages);
 	~ExposureModeHelper() = default;
 
+	void configure(utils::Duration lineLength, const CameraSensorHelper *sensorHelper);
 	void setLimits(utils::Duration minExposureTime, utils::Duration maxExposureTime,
 		       double minGain, double maxGain);
 
@@ -36,16 +39,19 @@ public:
 	double maxGain() const { return maxGain_; }
 
 private:
-	utils::Duration clampExposureTime(utils::Duration exposureTime) const;
-	double clampGain(double gain) const;
+	utils::Duration clampExposureTime(utils::Duration exposureTime,
+					  double *quantizationGain = nullptr) const;
+	double clampGain(double gain, double *quantizationGain = nullptr) const;
 
 	std::vector<utils::Duration> exposureTimes_;
 	std::vector<double> gains_;
 
+	utils::Duration lineDuration_;
 	utils::Duration minExposureTime_;
 	utils::Duration maxExposureTime_;
 	double minGain_;
 	double maxGain_;
+	const CameraSensorHelper *sensorHelper_;
 };
 
 } /* namespace ipa */