[libcamera-devel] [PATCH v5 04/14] ipa: ipu3: agc: Limit the number of saturated cells

[Jean-Michel Hautbois]

Hi Laurent,

On 15/11/2021 16:39, Laurent Pinchart wrote:
> Hi Jean-Michel,
> 
> Thank you for the patch.
> 
> On Sat, Nov 13, 2021 at 09:49:37AM +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.
> 
> Do we need this threshold ? For AWB handling saturation is important,
> but for AGC, what will happen if we compute the histogram using all
> cells regardless of saturation ?

I think we need this until the "ipa: ipu3: agc: Improve gain 
calculation" is applied. So, on master, I think we can probably remove 
it now.

> 
>> 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);
>> +	}
>>   }
>>   
>>   /**
>