[libcamera-devel] [PATCH v4 04/14] ipa: ipu3: agc: Limit the number of saturated cells

[Paul Elder]

Hi Jean-Michel,

On Thu, Nov 11, 2021 at 03:09:18PM +0100, Jean-Michel Hautbois wrote:
> When the histogram is calculated, we check if a cell is saturated or not
> before cumulating its green value. This is wrong, and it can lead to an
> empty histogram in case of a fully saturated frame.
> 
> Use a constant to limit the amount of pixels within a cell before
> considering it saturated. If at the end of the loop we still have an
> empty histogram, then make it a fully saturated one.
> 
> Bug: https://bugs.libcamera.org/show_bug.cgi?id=84
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

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

> ---
>  src/ipa/ipu3/algorithms/agc.cpp | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> index 5723f6f3..83aa3676 100644
> --- a/src/ipa/ipu3/algorithms/agc.cpp
> +++ b/src/ipa/ipu3/algorithms/agc.cpp
> @@ -63,6 +63,12 @@ static constexpr uint32_t knumHistogramBins = 256;
>  /* Target value to reach for the top 2% of the histogram */
>  static constexpr double kEvGainTarget = 0.5;
>  
> +/*
> + * Maximum ratio of saturated pixels in a cell for the cell to be considered
> + * non-saturated and counted by the AGC algorithm.
> + */
> +static constexpr uint32_t kMinCellsPerZoneRatio = 255 * 20 / 100;
> +
>  Agc::Agc()
>  	: frameCount_(0), lastFrame_(0), iqMean_(0.0), lineDuration_(0s),
>  	  minExposureLines_(0), maxExposureLines_(0), filteredExposure_(0s),
> @@ -124,7 +130,7 @@ void Agc::measureBrightness(const ipu3_uapi_stats_3a *stats,
>  					&stats->awb_raw_buffer.meta_data[cellPosition]
>  				);
>  
> -			if (cell->sat_ratio == 0) {
> +			if (cell->sat_ratio <= kMinCellsPerZoneRatio) {
>  				uint8_t gr = cell->Gr_avg;
>  				uint8_t gb = cell->Gb_avg;
>  				/*
> @@ -137,8 +143,14 @@ void Agc::measureBrightness(const ipu3_uapi_stats_3a *stats,
>  		}
>  	}
>  
> +	Histogram cumulativeHist = Histogram(Span<uint32_t>(hist));
>  	/* Estimate the quantile mean of the top 2% of the histogram */
> -	iqMean_ = Histogram(Span<uint32_t>(hist)).interQuantileMean(0.98, 1.0);
> +	if (cumulativeHist.total() == 0) {
> +		/* Force the value as histogram is empty */
> +		iqMean_ = knumHistogramBins - 0.5;
> +	} else {
> +		iqMean_ = cumulativeHist.interQuantileMean(0.98, 1.0);
> +	}
>  }
>  
>  /**
> -- 
> 2.32.0
>