[PATCH 3/3] ipa: ipu3: Use a Y histogram instead of green

[Jean-Michel Hautbois]

Hi Daniel,

On 18/04/2024 14:46, Daniel Scally wrote:
> The IPU3 IPA module uses a green histogram for the AGC algorithm's
> constraint mode calculations. Switch instead to a luminance histogram
> calculated using the Rec.601 coefficients.
> 
> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
> ---
>   src/ipa/ipu3/algorithms/agc.cpp | 15 ++++++++-------
>   1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> index a59b73fb..76d2bb60 100644
> --- a/src/ipa/ipu3/algorithms/agc.cpp
> +++ b/src/ipa/ipu3/algorithms/agc.cpp
> @@ -152,18 +152,19 @@ Histogram Agc::parseStatistics(const ipu3_uapi_stats_3a *stats,
>   				reinterpret_cast<const ipu3_uapi_awb_set_item *>(
>   					&stats->awb_raw_buffer.meta_data[cellPosition]);
>   
> -			rgbTriples_.push_back({
> -				cell->R_avg,
> -				(cell->Gr_avg + cell->Gb_avg) / 2,
> -				cell->B_avg
> -			});
> +			uint8_t G_avg = (cell->Gr_avg + cell->Gb_avg) / 2;
> +
> +			rgbTriples_.push_back({cell->R_avg, G_avg, cell->B_avg});
>   
>   			/*
> -			 * Store the average green value to estimate the
> +			 * Store the average luminance value to estimate the
>   			 * brightness. Even the overexposed pixels are
>   			 * taken into account.
>   			 */
> -			hist[(cell->Gr_avg + cell->Gb_avg) / 2]++;
> +			uint8_t y = (cell->R_avg * .299) +
> +				    (G_avg * .587) +
> +				    (cell->B_avg * .114);
> +			hist[y]++;

Is there a benefit to have the "real" Y value and not only the green 
parts (which reflect this Y value quite nicely as it is ~60% of the 
value) ? Could you measure it ? No increase in CPU time for big stats 
grids ?

JM