[PATCH 2/3] ipa: libipa: Adjust for flicker in ExposureModeHelper

[Daniel Scally]

Update the ExposureModeHelper class to compensate for flickering
light sources in the ::splitExposure() function. The adjustment
simply caps exposure time at a multiple of the given flicker period
and compensates for any loss in the effective exposure value by
increasing analogue and then digital gain.

Initially in the one call-site for this function, a flicker period
of 0us is passed, making this a no-op.

Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
---
 src/ipa/libipa/agc_mean_luminance.cpp   |  2 +-
 src/ipa/libipa/exposure_mode_helper.cpp | 20 +++++++++++++++++++-
 src/ipa/libipa/exposure_mode_helper.h   |  2 +-
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
index 02555a44..b5e6afe3 100644
--- a/src/ipa/libipa/agc_mean_luminance.cpp
+++ b/src/ipa/libipa/agc_mean_luminance.cpp
@@ -560,7 +560,7 @@ AgcMeanLuminance::calculateNewEv(uint32_t constraintModeIndex,
 	newExposureValue = filterExposure(newExposureValue);
 
 	frameCount_++;
-	return exposureModeHelper->splitExposure(newExposureValue);
+	return exposureModeHelper->splitExposure(newExposureValue, 0us);
 }
 
 /**
diff --git a/src/ipa/libipa/exposure_mode_helper.cpp b/src/ipa/libipa/exposure_mode_helper.cpp
index f235316d..038aa33c 100644
--- a/src/ipa/libipa/exposure_mode_helper.cpp
+++ b/src/ipa/libipa/exposure_mode_helper.cpp
@@ -121,6 +121,7 @@ double ExposureModeHelper::clampGain(double gain) const
 /**
  * \brief Split exposure into exposure time and gain
  * \param[in] exposure Exposure value
+ * \param[in] flickerPeriod The period of a flickering light source
  *
  * This function divides a given exposure into exposure time, analogue and
  * digital gain by iterating through stages of exposure time and gain limits.
@@ -147,10 +148,15 @@ double ExposureModeHelper::clampGain(double gain) const
  * required exposure, the helper falls-back to simply maximising the exposure
  * time first, followed by analogue gain, followed by digital gain.
  *
+ * Once the exposure time has been determined from the modes, an adjustment is
+ * made to compensate for a flickering light source by fixing the exposure time
+ * to an exact multiple of the flicker period. Any effective exposure value that
+ * is lost is added back via analogue and digital gain.
+ *
  * \return Tuple of exposure time, analogue gain, and digital gain
  */
 std::tuple<utils::Duration, double, double>
-ExposureModeHelper::splitExposure(utils::Duration exposure) const
+ExposureModeHelper::splitExposure(utils::Duration exposure, utils::Duration flickerPeriod) const
 {
 	ASSERT(maxExposureTime_);
 	ASSERT(maxGain_);
@@ -205,6 +211,18 @@ ExposureModeHelper::splitExposure(utils::Duration exposure) const
 	 * exposure time is maxed before gain is touched at all.
 	 */
 	exposureTime = clampExposureTime(exposure / clampGain(stageGain));
+
+	/*
+	 * Finally, if we have been given a flicker period we need to reduce the
+	 * exposure time to be a multiple of that period (if possible). The
+	 * effect of this should be to hide the flicker.
+	 */
+	if (flickerPeriod > 0us && flickerPeriod < exposureTime) {
+		unsigned int flickerPeriods = exposureTime / flickerPeriod;
+		if (flickerPeriods)
+			exposureTime = flickerPeriods * flickerPeriod;
+	}
+
 	gain = clampGain(exposure / exposureTime);
 
 	return { exposureTime, gain, exposure / (exposureTime * gain) };
diff --git a/src/ipa/libipa/exposure_mode_helper.h b/src/ipa/libipa/exposure_mode_helper.h
index c5be1b67..a5fcc366 100644
--- a/src/ipa/libipa/exposure_mode_helper.h
+++ b/src/ipa/libipa/exposure_mode_helper.h
@@ -28,7 +28,7 @@ public:
 		       double minGain, double maxGain);
 
 	std::tuple<utils::Duration, double, double>
-	splitExposure(utils::Duration exposure) const;
+	splitExposure(utils::Duration exposure, utils::Duration flickerPeriod) const;
 
 	utils::Duration minExposureTime() const { return minExposureTime_; }
 	utils::Duration maxExposureTime() const { return maxExposureTime_; }
-- 
2.34.1