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

[Kieran Bingham]

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 ?

>  
>                 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?


>                 };
>  
>                 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
>