[PATCH v2] ipa: libipa: Change constraint Y target to piecewise linear function

[Paul Elder]

On Mon, Jun 03, 2024 at 01:24:18PM +0100, Kieran Bingham wrote:
> Quoting Paul Elder (2024-05-29 20:33:22)
> > Change the constraint luminance target from a scalar value to a
> > piecewise linear function that needs to be sampled by the estimated lux
> > value.
> > 
> > Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
> > 
> > ---
> > Changes in v2:
> > - s/FPoint/PointF/
> > - construct default Pwl with *two* points so that it actually constructs
> >   properly
> > ---
> >  src/ipa/libipa/agc_mean_luminance.cpp | 15 +++++++++------
> >  src/ipa/libipa/agc_mean_luminance.h   |  4 ++--
> >  2 files changed, 11 insertions(+), 8 deletions(-)
> > 
> > diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
> > index 5b79d5d59..5eaa86c82 100644
> > --- a/src/ipa/libipa/agc_mean_luminance.cpp
> > +++ b/src/ipa/libipa/agc_mean_luminance.cpp
> > @@ -168,8 +168,10 @@ void AgcMeanLuminance::parseConstraint(const YamlObject &modeDict, int32_t id)
> >                 AgcConstraint::Bound bound = static_cast<AgcConstraint::Bound>(idx);
> >                 double qLo = content["qLo"].get<double>().value_or(0.98);
> >                 double qHi = content["qHi"].get<double>().value_or(1.0);
> > -               double yTarget =
> > -                       content["yTarget"].getList<double>().value_or(std::vector<double>{ 0.5 }).at(0);
> > +               Pwl yTarget;
> > +               int ret = yTarget.readYaml(content["yTarget"]);
> > +               if (ret < 0)
> > +                       yTarget = Pwl({ PointF(0, 0.5), PointF(1000, 0.5) });
> 
> Is the 'length' of the Pwl arbitrary? Does it extend if we go above
> '1000' here? From this snipped/hunk - I don't know what units/reference
> 0 and 1000 are? Is it the lux value?
> 
> Can any reasoning for 0...1000 be documented? I assume 0.5 is just 'half
> brightness target' if that's sufficiently self explanatory then fine -
> but even that might warrant a couple of words in why it's chosen?
> 
> Googling lux levels tells me:
> 
> - Direct Sunlight	32,000 to 100,000
> - Ambient Daylight	10,000 to 25,000
> - Overcast Daylight	1000
> - Sunset & Sunrise	400
> 
> Does that impact the choice of '1000' here? As the two points make a
> linear that will always return 0.5 - I assume it doesn't matter here,
> and
> 
>   Pwl({ PointF(0, 0.5), PointF(1, 0.5) });
> 
> would also have been equivalent ?

Yes it would be. Since this is a constant function, the input doesn't
matter and extrapolation and clamping are equivalent.

> 
> >  
> >                 AgcConstraint constraint = { bound, qLo, qHi, yTarget };
> >  
> > @@ -221,7 +223,7 @@ int AgcMeanLuminance::parseConstraintModes(const YamlObject &tuningData)
> >                         AgcConstraint::Bound::lower,
> >                         0.98,
> >                         1.0,
> > -                       0.5
> > +                       Pwl({ PointF(0, 0.5), PointF(1000, 0.5) })
> 
> Should this be some pre-constructed global const if it's going to be
> used in multiple places?

Probably? It's used three times; twice here and once more for
relativeLuminance.


Paul

> 
> 
> >                 };
> >  
> >                 constraintModes_[controls::ConstraintNormal].insert(
> > @@ -471,16 +473,17 @@ double AgcMeanLuminance::estimateInitialGain(double lux) const
> >   * \param[in] constraintModeIndex The index of the constraint to adhere to
> >   * \param[in] hist A histogram over which to calculate inter-quantile means
> >   * \param[in] gain The gain to clamp
> > + * \param[in] lux The lux value at which to sample the constraint luminance target pwl
> >   *
> >   * \return The gain clamped within the constraint bounds
> >   */
> >  double AgcMeanLuminance::constraintClampGain(uint32_t constraintModeIndex,
> >                                              const Histogram &hist,
> > -                                            double gain)
> > +                                            double gain, double lux)
> >  {
> >         std::vector<AgcConstraint> &constraints = constraintModes_[constraintModeIndex];
> >         for (const AgcConstraint &constraint : constraints) {
> > -               double newGain = constraint.yTarget * hist.bins() /
> > +               double newGain = constraint.yTarget.eval(constraint.yTarget.domain().clamp(lux)) * hist.bins() /
> >                                  hist.interQuantileMean(constraint.qLo, constraint.qHi);
> >  
> >                 if (constraint.bound == AgcConstraint::Bound::lower &&
> > @@ -559,7 +562,7 @@ AgcMeanLuminance::calculateNewEv(uint32_t constraintModeIndex,
> >                 exposureModeHelpers_.at(exposureModeIndex);
> >  
> >         double gain = estimateInitialGain(lux);
> > -       gain = constraintClampGain(constraintModeIndex, yHist, gain);
> > +       gain = constraintClampGain(constraintModeIndex, yHist, gain, lux);
> >  
> >         /*
> >          * We don't check whether we're already close to the target, because
> > diff --git a/src/ipa/libipa/agc_mean_luminance.h b/src/ipa/libipa/agc_mean_luminance.h
> > index 6ec2a0dc9..d43f673dd 100644
> > --- a/src/ipa/libipa/agc_mean_luminance.h
> > +++ b/src/ipa/libipa/agc_mean_luminance.h
> > @@ -38,7 +38,7 @@ public:
> >                 Bound bound;
> >                 double qLo;
> >                 double qHi;
> > -               double yTarget;
> > +               Pwl yTarget;
> >         };
> >  
> >         int parseTuningData(const YamlObject &tuningData);
> > @@ -80,7 +80,7 @@ private:
> >         double estimateInitialGain(double lux) const;
> >         double constraintClampGain(uint32_t constraintModeIndex,
> >                                    const Histogram &hist,
> > -                                  double gain);
> > +                                  double gain, double lux);
> >         utils::Duration filterExposure(utils::Duration exposureValue);
> >  
> >         uint64_t frameCount_;
> > -- 
> > 2.39.2
> >