[libcamera-devel,v4,07/14] ipa: ipu3: agc: Improve gain calculation
diff mbox series

Message ID 20211111140928.136111-8-jeanmichel.hautbois@ideasonboard.com
State Accepted
Headers show
Series
  • IPA: IPU3: Introduce per-frame controls
Related show

Commit Message

Jean-Michel Hautbois Nov. 11, 2021, 2:09 p.m. UTC
When an image is partially saturated, its brightness is not increasing
linearly when the shutter time or gain increases. It is a big issue with
a backlight as the algorithm is fading to darkness right now.

Introduce a function to estimate the brightness of the frame, based on
the current exposure/gain and loop on it several times to estimate it
again and approach the non linear function.

Inspired-by: 7de5506c30b3 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
---
 src/ipa/ipu3/algorithms/agc.cpp | 104 +++++++++++++++++++++++++++++++-
 src/ipa/ipu3/algorithms/agc.h   |   6 +-
 2 files changed, 106 insertions(+), 4 deletions(-)

Comments

Paul Elder Nov. 12, 2021, 11:12 p.m. UTC | #1
Hi Jean-Michel,

On Thu, Nov 11, 2021 at 03:09:21PM +0100, Jean-Michel Hautbois wrote:
> When an image is partially saturated, its brightness is not increasing
> linearly when the shutter time or gain increases. It is a big issue with
> a backlight as the algorithm is fading to darkness right now.
> 
> Introduce a function to estimate the brightness of the frame, based on
> the current exposure/gain and loop on it several times to estimate it
> again and approach the non linear function.
> 
> Inspired-by: 7de5506c30b3 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
> ---
>  src/ipa/ipu3/algorithms/agc.cpp | 104 +++++++++++++++++++++++++++++++-
>  src/ipa/ipu3/algorithms/agc.h   |   6 +-
>  2 files changed, 106 insertions(+), 4 deletions(-)
> 
> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> index 61ca8b3f..a34f070e 100644
> --- a/src/ipa/ipu3/algorithms/agc.cpp
> +++ b/src/ipa/ipu3/algorithms/agc.cpp
> @@ -67,6 +67,17 @@ static constexpr uint32_t kMinCellsPerZoneRatio = 255 * 20 / 100;
>  /* Number of frames to wait before calculating stats on minimum exposure */
>  static constexpr uint32_t kNumStartupFrames = 10;
>  
> +/* Maximum luminance used for brightness normalization */
> +static constexpr uint32_t kMaxLuminance = 255;
> +
> +/*
> + * Normalized luma value target.
> + *
> + * It's a number that's chosen so that, when the camera points at a grey
> + * target, the resulting image brightness is considered right.
> + */
> +static constexpr double kNormalizedLumaTarget = 0.16;
> +
>  Agc::Agc()
>  	: frameCount_(0), iqMean_(0.0), lineDuration_(0s), minExposureLines_(0),
>  	  maxExposureLines_(0), filteredExposure_(0s), currentExposure_(0s),
> @@ -185,14 +196,20 @@ void Agc::filterExposure()
>  /**
>   * \brief Estimate the new exposure and gain values
>   * \param[inout] frameContext The shared IPA frame Context
> + * \param[in] currentYGain The gain calculated on the current brightness level
>   */
> -void Agc::computeExposure(IPAFrameContext &frameContext)
> +void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
>  {
>  	/* Get the effective exposure and gain applied on the sensor. */
>  	uint32_t exposure = frameContext.sensor.exposure;
>  	double analogueGain = frameContext.sensor.gain;
>  
> -	/* Estimate the gain needed to have the proportion wanted */
> +	/*
> +	 * Estimate the gain needed to have the proportion of pixels in a given
> +	 * range wanted. iqMean_ returns the mean value of the top 2% of the
> +	 * cumulative histogram, and we want it to be as close as possible to a
> +	 * configured target.
> +	 */
>  	double evGain = kEvGainTarget * knumHistogramBins / iqMean_;
>  
>  	if (std::abs(evGain - 1.0) < 0.01) {
> @@ -202,6 +219,7 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>  	}
>  
>  	/* extracted from Rpi::Agc::computeTargetExposure */
> +
>  	/* Calculate the shutter time in seconds */
>  	utils::Duration currentShutter = exposure * lineDuration_;
>  	LOG(IPU3Agc, Debug) << "Actual total exposure " << currentShutter * analogueGain
> @@ -209,6 +227,14 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>  			    << " Gain " << analogueGain
>  			    << " Needed ev gain " << evGain;
>  
> +	if (evGain < currentYGain)
> +		evGain = currentYGain;
> +
> +	/* Consider within 1% of the target as correctly exposed */
> +	if (std::abs(evGain - 1.0) < 0.01)
> +		LOG(IPU3Agc, Debug) << "We are well exposed (iqMean = "
> +				    << iqMean_ << ")";

I thought we saw this stanza in the last patch, right before this hunk?

> +
>  	/*
>  	 * Calculate the current exposure value for the scene as the latest
>  	 * exposure value applied multiplied by the new estimated gain.
> @@ -257,6 +283,56 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>  	prevExposureValue_ = shutterTime * analogueGain;
>  }
>  
> +/**
> + * \brief Estimate the average brightness of the frame
> + * \param[in] frameContext The shared IPA frame context
> + * \param[in] grid The grid used to store the statistics in the IPU3
> + * \param[in] stats The IPU3 statistics and ISP results
> + * \param[in] currentYGain The gain calculated on the current brightness level
> + * \return The normalized luma
> + *
> + * Luma is the weighted sum of gamma-compressed R′G′B′ components of a color
> + * video. The luma values are normalized as 0.0 to 1.0, with 1.0 being a
> + * theoretical perfect reflector of 100% reference white. We use the Rec. 601
> + * luma here.
> + *
> + * More detailed information can be found in:
> + * https://en.wikipedia.org/wiki/Luma_(video)
> + */
> +double Agc::computeInitialY(IPAFrameContext &frameContext,
> +			    const ipu3_uapi_grid_config &grid,
> +			    const ipu3_uapi_stats_3a *stats,
> +			    double currentYGain)
> +{
> +	double redSum = 0, greenSum = 0, blueSum = 0;
> +
> +	for (unsigned int cellY = 0; cellY < grid.height; cellY++) {
> +		for (unsigned int cellX = 0; cellX < grid.width; cellX++) {
> +			uint32_t cellPosition = cellY * stride_ + cellX;
> +
> +			const ipu3_uapi_awb_set_item *cell =
> +				reinterpret_cast<const ipu3_uapi_awb_set_item *>(
> +					&stats->awb_raw_buffer.meta_data[cellPosition]
> +				);
> +
> +			redSum += cell->R_avg * currentYGain;
> +			greenSum += (cell->Gr_avg + cell->Gb_avg) / 2 * currentYGain;
> +			blueSum += cell->B_avg * currentYGain;
> +		}
> +	}
> +
> +	/*
> +	 * Estimate the sum of the brightness values, weighted with the gains
> +	 * applied on the channels in AWB as the Rec. 601 luma.
> +	 */
> +	double Y_sum = redSum * frameContext.awb.gains.red * .299 +
> +		       greenSum * frameContext.awb.gains.green * .587 +
> +		       blueSum * frameContext.awb.gains.blue * .114;
> +
> +	/* Return the normalized relative luminance. */
> +	return Y_sum / (grid.height * grid.width) / kMaxLuminance;
> +}
> +
>  /**
>   * \brief Process IPU3 statistics, and run AGC operations
>   * \param[in] context The shared IPA context
> @@ -268,7 +344,29 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>  void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
>  {
>  	measureBrightness(stats, context.configuration.grid.bdsGrid);
> -	computeExposure(context.frameContext);
> +
> +	double currentYGain = 1.0;
> +	double targetY = kNormalizedLumaTarget;
> +
> +	/*
> +	 * Do this calculation a few times as brightness increase can be
> +	 * non-linear when there are saturated regions.
> +	 */
> +	for (int i = 0; i < 8; i++) {
> +		double initialY = computeInitialY(context.frameContext,
> +						  context.configuration.grid.bdsGrid,
> +						  stats, currentYGain);
> +		double extra_gain = std::min(10.0, targetY / (initialY + .001));
> +
> +		currentYGain *= extra_gain;
> +		LOG(IPU3Agc, Debug) << "Initial Y " << initialY
> +				    << " target " << targetY
> +				    << " gives gain " << currentYGain;
> +		if (extra_gain < 1.01)
> +			break;
> +	}
> +
> +	computeExposure(context.frameContext, currentYGain);
>  	frameCount_++;
>  }
>  
> diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
> index f0db25ee..79736283 100644
> --- a/src/ipa/ipu3/algorithms/agc.h
> +++ b/src/ipa/ipu3/algorithms/agc.h
> @@ -34,7 +34,11 @@ private:
>  	void measureBrightness(const ipu3_uapi_stats_3a *stats,
>  			       const ipu3_uapi_grid_config &grid);
>  	void filterExposure();
> -	void computeExposure(IPAFrameContext &frameContext);
> +	void computeExposure(IPAFrameContext &frameContext, double currentYGain);
> +	double computeInitialY(IPAFrameContext &frameContext,
> +			       const ipu3_uapi_grid_config &grid,
> +			       const ipu3_uapi_stats_3a *stats,
> +			       double currentYGain);

The rest looks good.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

>  
>  	uint64_t frameCount_;
>  	uint64_t lastFrame_;
> -- 
> 2.32.0
>
Jean-Michel Hautbois Nov. 13, 2021, 8:19 a.m. UTC | #2
Hi Paul,

On 13/11/2021 00:12, Paul Elder wrote:
> Hi Jean-Michel,
> 
> On Thu, Nov 11, 2021 at 03:09:21PM +0100, Jean-Michel Hautbois wrote:
>> When an image is partially saturated, its brightness is not increasing
>> linearly when the shutter time or gain increases. It is a big issue with
>> a backlight as the algorithm is fading to darkness right now.
>>
>> Introduce a function to estimate the brightness of the frame, based on
>> the current exposure/gain and loop on it several times to estimate it
>> again and approach the non linear function.
>>
>> Inspired-by: 7de5506c30b3 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
>> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
>> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
>> ---
>>   src/ipa/ipu3/algorithms/agc.cpp | 104 +++++++++++++++++++++++++++++++-
>>   src/ipa/ipu3/algorithms/agc.h   |   6 +-
>>   2 files changed, 106 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
>> index 61ca8b3f..a34f070e 100644
>> --- a/src/ipa/ipu3/algorithms/agc.cpp
>> +++ b/src/ipa/ipu3/algorithms/agc.cpp
>> @@ -67,6 +67,17 @@ static constexpr uint32_t kMinCellsPerZoneRatio = 255 * 20 / 100;
>>   /* Number of frames to wait before calculating stats on minimum exposure */
>>   static constexpr uint32_t kNumStartupFrames = 10;
>>   
>> +/* Maximum luminance used for brightness normalization */
>> +static constexpr uint32_t kMaxLuminance = 255;
>> +
>> +/*
>> + * Normalized luma value target.
>> + *
>> + * It's a number that's chosen so that, when the camera points at a grey
>> + * target, the resulting image brightness is considered right.
>> + */
>> +static constexpr double kNormalizedLumaTarget = 0.16;
>> +
>>   Agc::Agc()
>>   	: frameCount_(0), iqMean_(0.0), lineDuration_(0s), minExposureLines_(0),
>>   	  maxExposureLines_(0), filteredExposure_(0s), currentExposure_(0s),
>> @@ -185,14 +196,20 @@ void Agc::filterExposure()
>>   /**
>>    * \brief Estimate the new exposure and gain values
>>    * \param[inout] frameContext The shared IPA frame Context
>> + * \param[in] currentYGain The gain calculated on the current brightness level
>>    */
>> -void Agc::computeExposure(IPAFrameContext &frameContext)
>> +void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
>>   {
>>   	/* Get the effective exposure and gain applied on the sensor. */
>>   	uint32_t exposure = frameContext.sensor.exposure;
>>   	double analogueGain = frameContext.sensor.gain;
>>   
>> -	/* Estimate the gain needed to have the proportion wanted */
>> +	/*
>> +	 * Estimate the gain needed to have the proportion of pixels in a given
>> +	 * range wanted. iqMean_ returns the mean value of the top 2% of the
>> +	 * cumulative histogram, and we want it to be as close as possible to a
>> +	 * configured target.
>> +	 */
>>   	double evGain = kEvGainTarget * knumHistogramBins / iqMean_;
>>   
>>   	if (std::abs(evGain - 1.0) < 0.01) {
>> @@ -202,6 +219,7 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>>   	}
>>   
>>   	/* extracted from Rpi::Agc::computeTargetExposure */
>> +
>>   	/* Calculate the shutter time in seconds */
>>   	utils::Duration currentShutter = exposure * lineDuration_;
>>   	LOG(IPU3Agc, Debug) << "Actual total exposure " << currentShutter * analogueGain
>> @@ -209,6 +227,14 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>>   			    << " Gain " << analogueGain
>>   			    << " Needed ev gain " << evGain;
>>   
>> +	if (evGain < currentYGain)
>> +		evGain = currentYGain;
>> +
>> +	/* Consider within 1% of the target as correctly exposed */
>> +	if (std::abs(evGain - 1.0) < 0.01)
>> +		LOG(IPU3Agc, Debug) << "We are well exposed (iqMean = "
>> +				    << iqMean_ << ")";
> 
> I thought we saw this stanza in the last patch, right before this hunk?

Good catch ! This is indeed a bug :-).
I will send v5, with all the reviewed-by and commit messages changed, 
with this bug fixed.

> 
>> +
>>   	/*
>>   	 * Calculate the current exposure value for the scene as the latest
>>   	 * exposure value applied multiplied by the new estimated gain.
>> @@ -257,6 +283,56 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>>   	prevExposureValue_ = shutterTime * analogueGain;
>>   }
>>   
>> +/**
>> + * \brief Estimate the average brightness of the frame
>> + * \param[in] frameContext The shared IPA frame context
>> + * \param[in] grid The grid used to store the statistics in the IPU3
>> + * \param[in] stats The IPU3 statistics and ISP results
>> + * \param[in] currentYGain The gain calculated on the current brightness level
>> + * \return The normalized luma
>> + *
>> + * Luma is the weighted sum of gamma-compressed R′G′B′ components of a color
>> + * video. The luma values are normalized as 0.0 to 1.0, with 1.0 being a
>> + * theoretical perfect reflector of 100% reference white. We use the Rec. 601
>> + * luma here.
>> + *
>> + * More detailed information can be found in:
>> + * https://en.wikipedia.org/wiki/Luma_(video)
>> + */
>> +double Agc::computeInitialY(IPAFrameContext &frameContext,
>> +			    const ipu3_uapi_grid_config &grid,
>> +			    const ipu3_uapi_stats_3a *stats,
>> +			    double currentYGain)
>> +{
>> +	double redSum = 0, greenSum = 0, blueSum = 0;
>> +
>> +	for (unsigned int cellY = 0; cellY < grid.height; cellY++) {
>> +		for (unsigned int cellX = 0; cellX < grid.width; cellX++) {
>> +			uint32_t cellPosition = cellY * stride_ + cellX;
>> +
>> +			const ipu3_uapi_awb_set_item *cell =
>> +				reinterpret_cast<const ipu3_uapi_awb_set_item *>(
>> +					&stats->awb_raw_buffer.meta_data[cellPosition]
>> +				);
>> +
>> +			redSum += cell->R_avg * currentYGain;
>> +			greenSum += (cell->Gr_avg + cell->Gb_avg) / 2 * currentYGain;
>> +			blueSum += cell->B_avg * currentYGain;
>> +		}
>> +	}
>> +
>> +	/*
>> +	 * Estimate the sum of the brightness values, weighted with the gains
>> +	 * applied on the channels in AWB as the Rec. 601 luma.
>> +	 */
>> +	double Y_sum = redSum * frameContext.awb.gains.red * .299 +
>> +		       greenSum * frameContext.awb.gains.green * .587 +
>> +		       blueSum * frameContext.awb.gains.blue * .114;
>> +
>> +	/* Return the normalized relative luminance. */
>> +	return Y_sum / (grid.height * grid.width) / kMaxLuminance;
>> +}
>> +
>>   /**
>>    * \brief Process IPU3 statistics, and run AGC operations
>>    * \param[in] context The shared IPA context
>> @@ -268,7 +344,29 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
>>   void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
>>   {
>>   	measureBrightness(stats, context.configuration.grid.bdsGrid);
>> -	computeExposure(context.frameContext);
>> +
>> +	double currentYGain = 1.0;
>> +	double targetY = kNormalizedLumaTarget;
>> +
>> +	/*
>> +	 * Do this calculation a few times as brightness increase can be
>> +	 * non-linear when there are saturated regions.
>> +	 */
>> +	for (int i = 0; i < 8; i++) {
>> +		double initialY = computeInitialY(context.frameContext,
>> +						  context.configuration.grid.bdsGrid,
>> +						  stats, currentYGain);
>> +		double extra_gain = std::min(10.0, targetY / (initialY + .001));
>> +
>> +		currentYGain *= extra_gain;
>> +		LOG(IPU3Agc, Debug) << "Initial Y " << initialY
>> +				    << " target " << targetY
>> +				    << " gives gain " << currentYGain;
>> +		if (extra_gain < 1.01)
>> +			break;
>> +	}
>> +
>> +	computeExposure(context.frameContext, currentYGain);
>>   	frameCount_++;
>>   }
>>   
>> diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
>> index f0db25ee..79736283 100644
>> --- a/src/ipa/ipu3/algorithms/agc.h
>> +++ b/src/ipa/ipu3/algorithms/agc.h
>> @@ -34,7 +34,11 @@ private:
>>   	void measureBrightness(const ipu3_uapi_stats_3a *stats,
>>   			       const ipu3_uapi_grid_config &grid);
>>   	void filterExposure();
>> -	void computeExposure(IPAFrameContext &frameContext);
>> +	void computeExposure(IPAFrameContext &frameContext, double currentYGain);
>> +	double computeInitialY(IPAFrameContext &frameContext,
>> +			       const ipu3_uapi_grid_config &grid,
>> +			       const ipu3_uapi_stats_3a *stats,
>> +			       double currentYGain);
> 
> The rest looks good.
> 
> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
> 
>>   
>>   	uint64_t frameCount_;
>>   	uint64_t lastFrame_;
>> -- 
>> 2.32.0
>>

Patch
diff mbox series

diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
index 61ca8b3f..a34f070e 100644
--- a/src/ipa/ipu3/algorithms/agc.cpp
+++ b/src/ipa/ipu3/algorithms/agc.cpp
@@ -67,6 +67,17 @@  static constexpr uint32_t kMinCellsPerZoneRatio = 255 * 20 / 100;
 /* Number of frames to wait before calculating stats on minimum exposure */
 static constexpr uint32_t kNumStartupFrames = 10;
 
+/* Maximum luminance used for brightness normalization */
+static constexpr uint32_t kMaxLuminance = 255;
+
+/*
+ * Normalized luma value target.
+ *
+ * It's a number that's chosen so that, when the camera points at a grey
+ * target, the resulting image brightness is considered right.
+ */
+static constexpr double kNormalizedLumaTarget = 0.16;
+
 Agc::Agc()
 	: frameCount_(0), iqMean_(0.0), lineDuration_(0s), minExposureLines_(0),
 	  maxExposureLines_(0), filteredExposure_(0s), currentExposure_(0s),
@@ -185,14 +196,20 @@  void Agc::filterExposure()
 /**
  * \brief Estimate the new exposure and gain values
  * \param[inout] frameContext The shared IPA frame Context
+ * \param[in] currentYGain The gain calculated on the current brightness level
  */
-void Agc::computeExposure(IPAFrameContext &frameContext)
+void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
 {
 	/* Get the effective exposure and gain applied on the sensor. */
 	uint32_t exposure = frameContext.sensor.exposure;
 	double analogueGain = frameContext.sensor.gain;
 
-	/* Estimate the gain needed to have the proportion wanted */
+	/*
+	 * Estimate the gain needed to have the proportion of pixels in a given
+	 * range wanted. iqMean_ returns the mean value of the top 2% of the
+	 * cumulative histogram, and we want it to be as close as possible to a
+	 * configured target.
+	 */
 	double evGain = kEvGainTarget * knumHistogramBins / iqMean_;
 
 	if (std::abs(evGain - 1.0) < 0.01) {
@@ -202,6 +219,7 @@  void Agc::computeExposure(IPAFrameContext &frameContext)
 	}
 
 	/* extracted from Rpi::Agc::computeTargetExposure */
+
 	/* Calculate the shutter time in seconds */
 	utils::Duration currentShutter = exposure * lineDuration_;
 	LOG(IPU3Agc, Debug) << "Actual total exposure " << currentShutter * analogueGain
@@ -209,6 +227,14 @@  void Agc::computeExposure(IPAFrameContext &frameContext)
 			    << " Gain " << analogueGain
 			    << " Needed ev gain " << evGain;
 
+	if (evGain < currentYGain)
+		evGain = currentYGain;
+
+	/* Consider within 1% of the target as correctly exposed */
+	if (std::abs(evGain - 1.0) < 0.01)
+		LOG(IPU3Agc, Debug) << "We are well exposed (iqMean = "
+				    << iqMean_ << ")";
+
 	/*
 	 * Calculate the current exposure value for the scene as the latest
 	 * exposure value applied multiplied by the new estimated gain.
@@ -257,6 +283,56 @@  void Agc::computeExposure(IPAFrameContext &frameContext)
 	prevExposureValue_ = shutterTime * analogueGain;
 }
 
+/**
+ * \brief Estimate the average brightness of the frame
+ * \param[in] frameContext The shared IPA frame context
+ * \param[in] grid The grid used to store the statistics in the IPU3
+ * \param[in] stats The IPU3 statistics and ISP results
+ * \param[in] currentYGain The gain calculated on the current brightness level
+ * \return The normalized luma
+ *
+ * Luma is the weighted sum of gamma-compressed R′G′B′ components of a color
+ * video. The luma values are normalized as 0.0 to 1.0, with 1.0 being a
+ * theoretical perfect reflector of 100% reference white. We use the Rec. 601
+ * luma here.
+ *
+ * More detailed information can be found in:
+ * https://en.wikipedia.org/wiki/Luma_(video)
+ */
+double Agc::computeInitialY(IPAFrameContext &frameContext,
+			    const ipu3_uapi_grid_config &grid,
+			    const ipu3_uapi_stats_3a *stats,
+			    double currentYGain)
+{
+	double redSum = 0, greenSum = 0, blueSum = 0;
+
+	for (unsigned int cellY = 0; cellY < grid.height; cellY++) {
+		for (unsigned int cellX = 0; cellX < grid.width; cellX++) {
+			uint32_t cellPosition = cellY * stride_ + cellX;
+
+			const ipu3_uapi_awb_set_item *cell =
+				reinterpret_cast<const ipu3_uapi_awb_set_item *>(
+					&stats->awb_raw_buffer.meta_data[cellPosition]
+				);
+
+			redSum += cell->R_avg * currentYGain;
+			greenSum += (cell->Gr_avg + cell->Gb_avg) / 2 * currentYGain;
+			blueSum += cell->B_avg * currentYGain;
+		}
+	}
+
+	/*
+	 * Estimate the sum of the brightness values, weighted with the gains
+	 * applied on the channels in AWB as the Rec. 601 luma.
+	 */
+	double Y_sum = redSum * frameContext.awb.gains.red * .299 +
+		       greenSum * frameContext.awb.gains.green * .587 +
+		       blueSum * frameContext.awb.gains.blue * .114;
+
+	/* Return the normalized relative luminance. */
+	return Y_sum / (grid.height * grid.width) / kMaxLuminance;
+}
+
 /**
  * \brief Process IPU3 statistics, and run AGC operations
  * \param[in] context The shared IPA context
@@ -268,7 +344,29 @@  void Agc::computeExposure(IPAFrameContext &frameContext)
 void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
 {
 	measureBrightness(stats, context.configuration.grid.bdsGrid);
-	computeExposure(context.frameContext);
+
+	double currentYGain = 1.0;
+	double targetY = kNormalizedLumaTarget;
+
+	/*
+	 * Do this calculation a few times as brightness increase can be
+	 * non-linear when there are saturated regions.
+	 */
+	for (int i = 0; i < 8; i++) {
+		double initialY = computeInitialY(context.frameContext,
+						  context.configuration.grid.bdsGrid,
+						  stats, currentYGain);
+		double extra_gain = std::min(10.0, targetY / (initialY + .001));
+
+		currentYGain *= extra_gain;
+		LOG(IPU3Agc, Debug) << "Initial Y " << initialY
+				    << " target " << targetY
+				    << " gives gain " << currentYGain;
+		if (extra_gain < 1.01)
+			break;
+	}
+
+	computeExposure(context.frameContext, currentYGain);
 	frameCount_++;
 }
 
diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
index f0db25ee..79736283 100644
--- a/src/ipa/ipu3/algorithms/agc.h
+++ b/src/ipa/ipu3/algorithms/agc.h
@@ -34,7 +34,11 @@  private:
 	void measureBrightness(const ipu3_uapi_stats_3a *stats,
 			       const ipu3_uapi_grid_config &grid);
 	void filterExposure();
-	void computeExposure(IPAFrameContext &frameContext);
+	void computeExposure(IPAFrameContext &frameContext, double currentYGain);
+	double computeInitialY(IPAFrameContext &frameContext,
+			       const ipu3_uapi_grid_config &grid,
+			       const ipu3_uapi_stats_3a *stats,
+			       double currentYGain);
 
 	uint64_t frameCount_;
 	uint64_t lastFrame_;