[PATCH v2 8/9] libipa: agc_mean_luminance: Add exposure compensation support

[Kieran Bingham]

Quoting Stefan Klug (2025-04-11 13:36:36)
> Exposure compensation allows to over- or under-expose an
> image by a given value. Add support for that in agc_mean_luminance.
> 
> The added exposure compensation can lead to luminance target values that
> are close or above saturation and are therefore never reachable. Add a
> fix for that by limiting the maximum luminance target to 0.95.
> 
> Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
> Reviewed-by: Daniel Scally <dan.scally at ideasonboard.com>
> 
> ---
> 
> Changes in v2:
> - Fixed compiler error that slipped through in v1
> - Improved commit message to explain the luminance target limitation
> - Collected tag
> ---
>  src/ipa/libipa/agc_mean_luminance.cpp | 23 +++++++++++++++++++++--
>  src/ipa/libipa/agc_mean_luminance.h   |  6 ++++++
>  2 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
> index 9154f083a510..f6cb7144b31a 100644
> --- a/src/ipa/libipa/agc_mean_luminance.cpp
> +++ b/src/ipa/libipa/agc_mean_luminance.cpp
> @@ -44,6 +44,15 @@ static constexpr uint32_t kNumStartupFrames = 10;
>   */
>  static constexpr double kDefaultRelativeLuminanceTarget = 0.16;
>  
> +/*
> + * Maximum relative luminance target
> + *
> + * This value limits the relative luminance target after applying the exposure
> + * compensation. Targeting a value above this limit results in saturation
> + * and the inability to regulate properly.
> + */
> +static constexpr double kMaxRelativeLuminanceTarget = 0.95;
> +
>  /**
>   * \struct AgcMeanLuminance::AgcConstraint
>   * \brief The boundaries and target for an AeConstraintMode constraint
> @@ -134,7 +143,7 @@ static constexpr double kDefaultRelativeLuminanceTarget = 0.16;
>   */
>  
>  AgcMeanLuminance::AgcMeanLuminance()
> -       : frameCount_(0), filteredExposure_(0s), relativeLuminanceTarget_(0)
> +       : exposureCompensation_(1.0), frameCount_(0), filteredExposure_(0s), relativeLuminanceTarget_(0)
>  {
>  }
>  
> @@ -369,6 +378,15 @@ int AgcMeanLuminance::parseTuningData(const YamlObject &tuningData)
>         return parseExposureModes(tuningData);
>  }
>  
> +/**
> + * \fn AgcMeanLuminance::setExposureCompensation()
> + * \brief Set the exposure compensation value
> + * \param[in] gain The exposure compensation gain
> + *
> + * This function sets the exposure compensation value to be used in the
> + * AGC calculations. It is expressed as gain instead of EV.
> + */
> +
>  /**
>   * \brief Set the ExposureModeHelper limits for this class
>   * \param[in] minExposureTime Minimum exposure time to allow
> @@ -425,7 +443,8 @@ void AgcMeanLuminance::setLimits(utils::Duration minExposureTime,
>   */
>  double AgcMeanLuminance::estimateInitialGain() const
>  {
> -       double yTarget = relativeLuminanceTarget_;
> +       double yTarget = std::min(relativeLuminanceTarget_ * exposureCompensation_,
> +                                 kMaxRelativeLuminanceTarget);
>         double yGain = 1.0;
>  
>         /*
> diff --git a/src/ipa/libipa/agc_mean_luminance.h b/src/ipa/libipa/agc_mean_luminance.h
> index c41391cb0b73..cad7ef845487 100644
> --- a/src/ipa/libipa/agc_mean_luminance.h
> +++ b/src/ipa/libipa/agc_mean_luminance.h
> @@ -44,6 +44,11 @@ public:
>  
>         int parseTuningData(const YamlObject &tuningData);
>  
> +       void setExposureCompensation(double gain)
> +       {
> +               exposureCompensation_ = gain;
> +       }
> +
>         void setLimits(utils::Duration minExposureTime, utils::Duration maxExposureTime,
>                        double minGain, double maxGain);
>  
> @@ -84,6 +89,7 @@ private:
>                                    double gain);
>         utils::Duration filterExposure(utils::Duration exposureValue);
>  
> +       double exposureCompensation_;

This seems like a 'streaming session' configuration parameter, but
storing this here means it will persist even between
stop/start/reconfigure cycles.

Should it be reset at configure() time or such ?

Or do you expect each helper user to reset it at configure time ? Maybe
this means we need to bring the same basic IPA interface to the helpers
directly to maintain the same call hierarchy/events through the whole
architecture (and I think even the metadata/control handling could then
be de-duplicated when the controls modify the helpers rather than
platform specific parameters ...)



>         uint64_t frameCount_;
>         utils::Duration filteredExposure_;
>         double relativeLuminanceTarget_;
> -- 
> 2.43.0
>