[libcamera-devel] [PATCH] ipa: ipu3: agc: Remove the threshold for the histogram calculation

[Laurent Pinchart]

Hello,

On Tue, Nov 16, 2021 at 02:41:14PM +0000, Kieran Bingham wrote:
> Quoting Jean-Michel Hautbois (2021-11-16 08:09:56)
> > Until f8f07f94 (ipa: ipu3: agc: Improve gain calculation, 2021-11-04)

s/Until/Until commit/

I'd drop the date, and expand the commit ID to 12 characters as usual.

> > the gain to apply on the exposure value was only using the histogram.
> > Now that the global brightness of the frame is estimated too, we don't
> > need to remove part of the saturated pixels from the equation anymore.

I'm not sure to understand why the above commit changed the situation,
but I also don't understand why we needed to take saturation into
account here in the first place, so the change looks good to me overall.

Commit f8f07f94 also added a histogram empty check. Do we still need it
?

> > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> 
> This is still working quite well here. No perceived regression as far
> as I can tell, and it's still coping well with backlights and skylights.
> 
> Tested-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> > ---
> >  src/ipa/ipu3/algorithms/agc.cpp | 24 ++++++++----------------
> >  1 file changed, 8 insertions(+), 16 deletions(-)
> > 
> > diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> > index 4e424857..f67a79d3 100644
> > --- a/src/ipa/ipu3/algorithms/agc.cpp
> > +++ b/src/ipa/ipu3/algorithms/agc.cpp
> > @@ -58,12 +58,6 @@ 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;
> > -
> >  /* Number of frames to wait before calculating stats on minimum exposure */
> >  static constexpr uint32_t kNumStartupFrames = 10;
> >  
> > @@ -133,16 +127,14 @@ void Agc::measureBrightness(const ipu3_uapi_stats_3a *stats,
> >                                         &stats->awb_raw_buffer.meta_data[cellPosition]
> >                                 );
> >  
> > -                       if (cell->sat_ratio <= kMinCellsPerZoneRatio) {
> > -                               uint8_t gr = cell->Gr_avg;
> > -                               uint8_t gb = cell->Gb_avg;
> > -                               /*
> > -                                * Store the average green value to estimate the
> > -                                * brightness. Even the overexposed pixels are
> > -                                * taken into account.
> > -                                */
> > -                               hist[(gr + gb) / 2]++;
> > -                       }
> > +                       uint8_t gr = cell->Gr_avg;
> > +                       uint8_t gb = cell->Gb_avg;
> > +                       /*
> > +                        * Store the average green value to estimate the
> > +                        * brightness. Even the overexposed pixels are
> > +                        * taken into account.
> > +                        */
> > +                       hist[(gr + gb) / 2]++;
> >                 }
> >         }
> >  

-- 
Regards,

Laurent Pinchart