[libcamera-devel] [PATCH v3 03/14] ipa: ipu3: awb: Use saturation under 90%

[Kieran Bingham]

Quoting Jean-Michel Hautbois (2021-10-21 17:43:50)
> The AWB grey world algorithm tries to find a grey value and it can't do
> it on over-exposed images. To exclude those, the saturation ratio is
> used for each cell, and the cell is included only if this ratio is 0.
> 
> Now that we have changed the threshold, more cells may be considered as
> partially saturated and excluded, preventing the algorithm from running
> efficiently.
> 
> Change that behaviour, and consider 90% as a good enough ratio.
> 
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/ipa/ipu3/algorithms/awb.cpp | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
> index 4364928c..ce01791b 100644
> --- a/src/ipa/ipu3/algorithms/awb.cpp
> +++ b/src/ipa/ipu3/algorithms/awb.cpp
> @@ -19,6 +19,18 @@ LOG_DEFINE_CATEGORY(IPU3Awb)
>  
>  static constexpr uint32_t kMinGreenLevelInZone = 32;
>  
> +/*
> + * Minimum proportion of non-saturated cells in a zone for the zone to be used
> + * by the AWB algorithm.
> + */
> +static constexpr double kMinRelevantCellsRatio = 0.8;
> +
> +/*
> + * Maximum ratio of saturated pixels in a cell for the cell to be considered
> + * non-saturated and counted by the AWB algorithm.
> + */
> +static constexpr uint32_t kSaturationThreshold = 255 * 90 / 100;
> +
>  /**
>   * \struct Accumulator
>   * \brief RGB statistics for a given zone
> @@ -160,7 +172,8 @@ int Awb::configure(IPAContext &context,
>          * for it to be relevant for the grey world algorithm.
>          * \todo This proportion could be configured.
>          */
> -       cellsPerZoneThreshold_ = cellsPerZoneX_ * cellsPerZoneY_ * 80 / 100;
> +       cellsPerZoneThreshold_ = cellsPerZoneX_ * cellsPerZoneY_ * kMinRelevantCellsRatio;
> +       LOG(IPU3Awb, Debug) << "Threshold for AWB is set to " << cellsPerZoneThreshold_;
>  
>         return 0;
>  }
> @@ -234,7 +247,18 @@ void Awb::generateAwbStats(const ipu3_uapi_stats_3a *stats)
>                                 reinterpret_cast<const ipu3_uapi_awb_set_item *>(
>                                         &stats->awb_raw_buffer.meta_data[cellPosition]
>                                 );
> -                       if (currentCell->sat_ratio == 0) {
> +
> +                       /*
> +                        * Use cells which have less than 90%
> +                        * saturation as an initial means to include
> +                        * otherwise bright cells which are not fully
> +                        * saturated.
> +                        *
> +                        * \todo The 90% saturation rate may require
> +                        * further empirical measurements and
> +                        * optimisation during camera tuning phases.
> +                        */

I'm a little wary of the comments stating the value of the constants
which are declared elsewhere, (and the chance of them getting out of
sync), but I think this is fine for now. If we ever change this, we're
likely moving away from a constant, to some tuned/config file value so
we'll be forced to update everything anyway.


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

> +                       if (currentCell->sat_ratio <= kSaturationThreshold) {
>                                 /* The cell is not saturated, use the current cell */
>                                 awbStats_[awbZonePosition].counted++;
>                                 uint32_t greenValue = currentCell->Gr_avg + currentCell->Gb_avg;
> -- 
> 2.32.0
>