[PATCH 2/3] ipa: libipa: Adjust for flicker in ExposureModeHelper

[Laurent Pinchart]

Hi Dan,

Thank you for the patch.

On Fri, Jan 17, 2025 at 02:34:09PM +0000, Daniel Scally wrote:
> Update the ExposureModeHelper class to compensate for flickering
> light sources in the ::splitExposure() function. The adjustment

ExposureModeHelper::splitExposure() or just splitExposure()

> simply caps exposure time at a multiple of the given flicker period
> and compensates for any loss in the effective exposure value by
> increasing analogue and then digital gain.
> 
> Initially in the one call-site for this function, a flicker period
> of 0us is passed, making this a no-op.
> 
> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
> ---
>  src/ipa/libipa/agc_mean_luminance.cpp   |  2 +-
>  src/ipa/libipa/exposure_mode_helper.cpp | 20 +++++++++++++++++++-
>  src/ipa/libipa/exposure_mode_helper.h   |  2 +-
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
> index 02555a44..b5e6afe3 100644
> --- a/src/ipa/libipa/agc_mean_luminance.cpp
> +++ b/src/ipa/libipa/agc_mean_luminance.cpp
> @@ -560,7 +560,7 @@ AgcMeanLuminance::calculateNewEv(uint32_t constraintModeIndex,
>  	newExposureValue = filterExposure(newExposureValue);
>  
>  	frameCount_++;
> -	return exposureModeHelper->splitExposure(newExposureValue);
> +	return exposureModeHelper->splitExposure(newExposureValue, 0us);
>  }
>  
>  /**
> diff --git a/src/ipa/libipa/exposure_mode_helper.cpp b/src/ipa/libipa/exposure_mode_helper.cpp
> index f235316d..038aa33c 100644
> --- a/src/ipa/libipa/exposure_mode_helper.cpp
> +++ b/src/ipa/libipa/exposure_mode_helper.cpp
> @@ -121,6 +121,7 @@ double ExposureModeHelper::clampGain(double gain) const
>  /**
>   * \brief Split exposure into exposure time and gain
>   * \param[in] exposure Exposure value
> + * \param[in] flickerPeriod The period of a flickering light source
>   *
>   * This function divides a given exposure into exposure time, analogue and
>   * digital gain by iterating through stages of exposure time and gain limits.
> @@ -147,10 +148,15 @@ double ExposureModeHelper::clampGain(double gain) const
>   * required exposure, the helper falls-back to simply maximising the exposure
>   * time first, followed by analogue gain, followed by digital gain.
>   *
> + * Once the exposure time has been determined from the modes, an adjustment is
> + * made to compensate for a flickering light source by fixing the exposure time
> + * to an exact multiple of the flicker period. Any effective exposure value that
> + * is lost is added back via analogue and digital gain.
> + *
>   * \return Tuple of exposure time, analogue gain, and digital gain
>   */
>  std::tuple<utils::Duration, double, double>
> -ExposureModeHelper::splitExposure(utils::Duration exposure) const
> +ExposureModeHelper::splitExposure(utils::Duration exposure, utils::Duration flickerPeriod) const

We could avoid magic values by passing a std::optional<utils::Duration>.
What do you think ?

>  {
>  	ASSERT(maxExposureTime_);
>  	ASSERT(maxGain_);
> @@ -205,6 +211,18 @@ ExposureModeHelper::splitExposure(utils::Duration exposure) const
>  	 * exposure time is maxed before gain is touched at all.
>  	 */
>  	exposureTime = clampExposureTime(exposure / clampGain(stageGain));
> +
> +	/*
> +	 * Finally, if we have been given a flicker period we need to reduce the
> +	 * exposure time to be a multiple of that period (if possible). The
> +	 * effect of this should be to hide the flicker.
> +	 */
> +	if (flickerPeriod > 0us && flickerPeriod < exposureTime) {
> +		unsigned int flickerPeriods = exposureTime / flickerPeriod;
> +		if (flickerPeriods)
> +			exposureTime = flickerPeriods * flickerPeriod;
> +	}

This means that the exposure time could become lower than
minExposureTime_. Is that an issue ? Should we define what controls take
precedence in that case ? For instance, when manual exposure time is
enabled, should the flicker period be ignored ?

> +
>  	gain = clampGain(exposure / exposureTime);
>  
>  	return { exposureTime, gain, exposure / (exposureTime * gain) };
> diff --git a/src/ipa/libipa/exposure_mode_helper.h b/src/ipa/libipa/exposure_mode_helper.h
> index c5be1b67..a5fcc366 100644
> --- a/src/ipa/libipa/exposure_mode_helper.h
> +++ b/src/ipa/libipa/exposure_mode_helper.h
> @@ -28,7 +28,7 @@ public:
>  		       double minGain, double maxGain);
>  
>  	std::tuple<utils::Duration, double, double>
> -	splitExposure(utils::Duration exposure) const;
> +	splitExposure(utils::Duration exposure, utils::Duration flickerPeriod) const;
>  
>  	utils::Duration minExposureTime() const { return minExposureTime_; }
>  	utils::Duration maxExposureTime() const { return maxExposureTime_; }

-- 
Regards,

Laurent Pinchart