[libcamera-devel] [PATCH v3 09/14] ipa: ipu3: agc: Simplify division of exposure/gain

Laurent Pinchart laurent.pinchart at ideasonboard.com
Fri Oct 22 06:08:01 CEST 2021


On Thu, Oct 21, 2021 at 10:08:37PM +0100, Kieran Bingham wrote:
> Quoting Jean-Michel Hautbois (2021-10-21 17:43:56)
> > Until now, the algorithm makes complex assumptions when dividing the
> > exposure and analogue gain values. Instead, use a simpler clamping of
> > the shutter speed first, and then of the analogue gain, based on the
> > limits configured.
> > 
> > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> > Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/ipa/ipu3/algorithms/agc.cpp | 36 +++++++++++++++++----------------
> >  1 file changed, 19 insertions(+), 17 deletions(-)
> > 
> > diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> > index f5bb3328..6f5b6a45 100644
> > --- a/src/ipa/ipu3/algorithms/agc.cpp
> > +++ b/src/ipa/ipu3/algorithms/agc.cpp
> > @@ -147,7 +147,9 @@ void Agc::lockExposureGain(uint32_t &exposure, double &gain)
> >                                     << " Gain " << gain;
> >  
> >                 currentExposure_ = currentExposureNoDg_ * newGain;
> > +               utils::Duration minShutterSpeed = minExposureLines_ * lineDuration_;
> >                 utils::Duration maxShutterSpeed = maxExposureLines_ * lineDuration_;
> > +
> >                 utils::Duration maxTotalExposure = maxShutterSpeed * kMaxGain;
> >                 currentExposure_ = std::min(currentExposure_, maxTotalExposure);
> >                 LOG(IPU3Agc, Debug) << "Target total exposure " << currentExposure_
> > @@ -156,23 +158,23 @@ void Agc::lockExposureGain(uint32_t &exposure, double &gain)
> >                 /* \todo: estimate if we need to desaturate */
> >                 filterExposure();
> >  
> > -               utils::Duration newExposure = 0.0s;
> > -               if (currentShutter < maxShutterSpeed) {
> > -                       exposure = std::clamp<uint32_t>(exposure * filteredExposure_ / currentExposureNoDg_,
> > -                                                       minExposureLines_,
> > -                                                       maxExposureLines_);
> > -                       newExposure = filteredExposure_ / exposure;
> > -                       gain = std::clamp(gain * filteredExposure_ / newExposure,
> > -                                         kMinGain, kMaxGain);
> > -               } else {
> > -                       gain = std::clamp(gain * filteredExposure_ / currentExposureNoDg_,
> > -                                         kMinGain, kMaxGain);
> > -                       newExposure = filteredExposure_ / gain;
> > -                       exposure = std::clamp<uint32_t>(exposure * filteredExposure_ / newExposure,
> 
> Makes the previous patch a bit redundant, but at least we removed code
> that was more 'correct' ;-)

The two used to be combined in a single patch, and I've pointed out
twice that they were separate changes :-) The switch to
filteredExposure_ was easy to overlook in previous versions.

> > -                                                       minExposureLines_,
> > -                                                       maxExposureLines_);
> > -               }
> > -               LOG(IPU3Agc, Debug) << "Adjust exposure " << exposure * lineDuration_ << " and gain " << gain;
> > +               utils::Duration exposureValue = filteredExposure_;
> > +               utils::Duration shutterTime = minShutterSpeed;
> > +
> > +               /*
> > +                * Push the shutter time up to the maximum first, and only then
> > +                * increase the gain.
> > +                */
> > +               shutterTime = std::clamp<utils::Duration>(exposureValue / kMinGain,
> > +                                                         minShutterSpeed, maxShutterSpeed);
> > +               double stepGain = std::clamp(exposureValue / shutterTime,
> > +                                            kMinGain, kMaxGain);
> > +               LOG(IPU3Agc, Debug) << "Divided up shutter and gain are "
> > +                                   << shutterTime << " and "
> > +                                   << stepGain;
> > +
> > +               exposure = shutterTime / lineDuration_;
> > +               gain = stepGain;
> 
> This looks a lot simpler indeed. I haven't gone through and fully
> validated all the equations, but as I believe I've seen this working,
> locally on my device, I think I can trust that it's functional.
> 
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> >         }
> >         lastFrame_ = frameCount_;
> >  }

-- 
Regards,

Laurent Pinchart


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