[libcamera-devel] [PATCH v4 07/14] ipa: ipu3: agc: Improve gain calculation

[Paul Elder]

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 at ideasonboard.com>
> Reviewed-by: Kieran Bingham <kieran.bingham at 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 at ideasonboard.com>

>  
>  	uint64_t frameCount_;
>  	uint64_t lastFrame_;
> -- 
> 2.32.0
>