[PATCH v3 3/8] ipa: libipa: Add ExposureModeHelper

[Jacopo Mondi]

Hi Dan

On Wed, Apr 24, 2024 at 01:49:12PM +0100, Daniel Scally wrote:
> From: Paul Elder <paul.elder at ideasonboard.com>
>
> Add a helper for managing exposure modes and splitting exposure times
> into shutter and gain values.
>
> Reviewed-by: Paul Elder <paul.elder at ideasonboard.com>
> Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
> ---
> Changes in v3:
>
> 	- Referred to "shutter time" instead of "shutter"
> 	- Removed ::init() and swapped its functionality to the constructor
> 	- Replaced the parameter to the constructor (ex-::init()) with a Span
> 	  instead of a vector.
> 	- Lots of documentation updates and function renaming
> Changes in v2:
>
> 	- Expanded the documentation
> 	- Dropped the overloads for fixed shutter / gain - the same
> 	  functionality is instead done by setting min and max shutter and gain
> 	  to the same value
> 	- Changed ::init() to consume a vector of pairs instead of two separate
> 	  vectors
> 	- Reworked splitExposure()
>
>  src/ipa/libipa/exposure_mode_helper.cpp | 246 ++++++++++++++++++++++++
>  src/ipa/libipa/exposure_mode_helper.h   |  53 +++++
>  src/ipa/libipa/meson.build              |   2 +
>  3 files changed, 301 insertions(+)
>  create mode 100644 src/ipa/libipa/exposure_mode_helper.cpp
>  create mode 100644 src/ipa/libipa/exposure_mode_helper.h
>
> diff --git a/src/ipa/libipa/exposure_mode_helper.cpp b/src/ipa/libipa/exposure_mode_helper.cpp
> new file mode 100644
> index 00000000..47a300f3
> --- /dev/null
> +++ b/src/ipa/libipa/exposure_mode_helper.cpp
> @@ -0,0 +1,246 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2024, Paul Elder <paul.elder at ideasonboard.com>
> + *
> + * exposure_mode_helper.cpp - Helper class that performs computations relating to exposure
> + */
> +#include "exposure_mode_helper.h"
> +
> +#include <algorithm>
> +
> +#include <libcamera/base/log.h>
> +
> +/**
> + * \file exposure_mode_helper.h
> + * \brief Helper class that performs computations relating to exposure
> + *
> + * AEGC algorithms have a need to split exposure between shutter time, analogue
> + * and digital gain. Multiple implementations do so based on paired stages of
> + * shutter time and gain limits; provide a helper to avoid duplicating the code.
> + */
> +
> +namespace libcamera {
> +
> +using namespace std::literals::chrono_literals;
> +
> +LOG_DEFINE_CATEGORY(ExposureModeHelper)
> +
> +namespace ipa {
> +
> +/**
> + * \class ExposureModeHelper
> + * \brief Class for splitting exposure into shutter time and total gain
> + *
> + * The ExposureModeHelper class provides a standard interface through which an
> + * AEGC algorithm can divide exposure between shutter time and gain. It is
> + * configured with a set of shutter time and gain pairs and works by initially
> + * fixing gain at 1.0 and increasing shutter time up to the shutter time value
> + * from the first pair in the set in an attempt to meet the required exposure
> + * value.
> + *
> + * If the required exposure is not achievable by the first shutter time value
> + * alone it ramps gain up to the value from the first pair in the set. If the
> + * required exposure is still not met it then allows shutter time to ramp up to
> + * the shutter time value from the second pair in the set, and continues in this
> + * vein until either the required exposure time is met, or else the hardware's
> + * shutter time or gain limits are reached.
> + *
> + * This method allows users to strike a balance between a well-exposed image and
> + * an acceptable frame-rate, as opposed to simply maximising shutter time
> + * followed by gain. The same helpers can be used to perform the latter
> + * operation if needed by passing an empty set of pairs to the initialisation
> + * function.
> + *
> + * The gain values may exceed a camera sensor's analogue gain limits if either
> + * it or the IPA is also capable of digital gain. The configure() function must
> + * be called with the hardware's limits to inform the helper of those
> + * constraints. Any gain that is needed will be applied as analogue gain first
> + * until the hardware's limit is reached, following which digital gain will be
> + * used.
> + */
> +
> +/**
> + * \brief Construct an ExposureModeHelper instance
> + * \param[in] stages The vector of paired shutter time and gain limits
> + *
> + * The input stages are shutter time and _total_ gain pairs; the gain
> + * encompasses both analogue and digital gain.
> + *
> + * The vector of stages may be empty. In that case, the helper will simply use
> + * the runtime limits set through setShutterGainLimits() instead.
> + */
> +ExposureModeHelper::ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages)
> +{
> +	minShutter_ = 0us;
> +	maxShutter_ = 0us;
> +	minGain_ = 0;
> +	maxGain_ = 0;
> +
> +	for (const auto &[s, g] : stages) {
> +		shutters_.push_back(s);
> +		gains_.push_back(g);
> +	}
> +}
> +
> +/**
> + * \brief Set the shutter time and gain limits
> + * \param[in] minShutter The minimum shutter time supported
> + * \param[in] maxShutter The maximum shutter time supported
> + * \param[in] minGain The minimum analogue gain supported
> + * \param[in] maxGain The maximum analogue gain supported
> + *
> + * This function configures the shutter time and analogue gain limits that need
> + * to be adhered to as the helper divides up exposure. Note that this function
> + * *must* be called whenever those limits change and before splitExposure() is
> + * used.
> + *
> + * If the algorithm using the helpers needs to indicate that either shutter time
> + * or analogue gain or both should be fixed it can do so by setting both the
> + * minima and maxima to the same value.
> + */
> +void ExposureModeHelper::setLimits(utils::Duration minShutter,
> +				   utils::Duration maxShutter,
> +				   double minGain, double maxGain)
> +{
> +	minShutter_ = minShutter;
> +	maxShutter_ = maxShutter;
> +	minGain_ = minGain;
> +	maxGain_ = maxGain;
> +}
> +
> +utils::Duration ExposureModeHelper::clampShutter(utils::Duration shutter) const
> +{
> +	return std::clamp(shutter, minShutter_, maxShutter_);
> +}
> +
> +double ExposureModeHelper::clampGain(double gain) const
> +{
> +	return std::clamp(gain, minGain_, maxGain_);
> +}
> +
> +/**
> + * \brief Split exposure time into shutter time and gain

Ok, I'll accept that what you pass in as exposure value (in
agc_mean_value.cpp) is here an exposure time

> + * \param[in] exposure Exposure time
> + *
> + * This function divides a given exposure time into shutter time, analogue and
> + * digital gain by iterating through stages of shutter time and gain limits. At
> + * each stage the current stage's shutter time limit is multiplied by the
> + * previous stage's gain limit (or 1.0 initially) to see if the combination of
> + * the two can meet the required exposure time. If they cannot then the current
> + * stage's shutter time limit is multiplied by the same stage's gain limit to
> + * see if that combination can meet the required exposure time. If they cannot
> + * then the function moves to consider the next stage.
> + *
> + * When a combination of shutter time and gain _stage_ limits are found that are
> + * sufficient to meet the required exposure time, the function attempts to
> + * reduce shutter time as much as possible whilst fixing gain and still meeting
> + * the exposure time. If a _runtime_ limit prevents shutter time from being
> + * lowered enough to meet the exposure time with gain fixed at the stage limit,
> + * gain is also lowered to compensate.
> + *
> + * Once the shutter time and gain values are ascertained, gain is assigned as
> + * analogue gain as much as possible, with digital gain only in use if the
> + * maximum analogue gain runtime limit is unable to accomodate the exposure

accommodate ?

Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>

Thanks
  j

> + * value.
> + *
> + * If no combination of shutter time and gain limits is found that meets the
> + * required exposure time, the helper falls-back to simply maximising the
> + * shutter time first, followed by analogue gain, followed by digital gain.
> + *
> + * \return Tuple of shutter time, analogue gain, and digital gain
> + */
> +std::tuple<utils::Duration, double, double>
> +ExposureModeHelper::splitExposure(utils::Duration exposure) const
> +{
> +	ASSERT(maxShutter_);
> +	ASSERT(maxGain_);
> +
> +	bool gainFixed = minGain_ == maxGain_;
> +	bool shutterFixed = minShutter_ == maxShutter_;
> +
> +	/*
> +	 * There's no point entering the loop if we cannot change either gain
> +	 * nor shutter anyway.
> +	 */
> +	if (shutterFixed && gainFixed)
> +		return { minShutter_, minGain_, exposure / (minShutter_ * minGain_) };
> +
> +	utils::Duration shutter;
> +	double stageGain;
> +	double gain;
> +
> +	for (unsigned int stage = 0; stage < gains_.size(); stage++) {
> +		double lastStageGain = stage == 0 ? 1.0 : clampGain(gains_[stage - 1]);
> +		utils::Duration stageShutter = clampShutter(shutters_[stage]);
> +		stageGain = clampGain(gains_[stage]);
> +
> +		/*
> +		 * We perform the clamping on both shutter and gain in case the
> +		 * helper has had limits set that prevent those values being
> +		 * lowered beyond a certain minimum...this can happen at runtime
> +		 * for various reasons and so would not be known when the stage
> +		 * limits are initialised.
> +		 */
> +
> +		if (stageShutter * lastStageGain >= exposure) {
> +			shutter = clampShutter(exposure / clampGain(lastStageGain));
> +			gain = clampGain(exposure / shutter);
> +
> +			return { shutter, gain, exposure / (shutter * gain) };
> +		}
> +
> +		if (stageShutter * stageGain >= exposure) {
> +			shutter = clampShutter(exposure / clampGain(stageGain));
> +			gain = clampGain(exposure / shutter);
> +
> +			return { shutter, gain, exposure / (shutter * gain) };
> +		}
> +	}
> +
> +	/*
> +	 * From here on all we can do is max out the shutter time, followed by
> +	 * the analogue gain. If we still haven't achieved the target we send
> +	 * the rest of the exposure time to digital gain. If we were given no
> +	 * stages to use then set stageGain to 1.0 so that shutter time is maxed
> +	 * before gain touched at all.
> +	 */
> +	if (gains_.empty())
> +		stageGain = 1.0;
> +
> +	shutter = clampShutter(exposure / clampGain(stageGain));
> +	gain = clampGain(exposure / shutter);
> +
> +	return { shutter, gain, exposure / (shutter * gain) };
> +}
> +
> +/**
> + * \fn ExposureModeHelper::minShutter()
> + * \brief Retrieve the configured minimum shutter time limit set through
> + * setShutterGainLimits()
> + * \return The minShutter_ value
> + */
> +
> +/**
> + * \fn ExposureModeHelper::maxShutter()
> + * \brief Retrieve the configured maximum shutter time set through
> + * setShutterGainLimits()
> + * \return The maxShutter_ value
> + */
> +
> +/**
> + * \fn ExposureModeHelper::minGain()
> + * \brief Retrieve the configured minimum gain set through
> + * setShutterGainLimits()
> + * \return The minGain_ value
> + */
> +
> +/**
> + * \fn ExposureModeHelper::maxGain()
> + * \brief Retrieve the configured maximum gain set through
> + * setShutterGainLimits()
> + * \return The maxGain_ value
> + */
> +
> +} /* namespace ipa */
> +
> +} /* namespace libcamera */
> diff --git a/src/ipa/libipa/exposure_mode_helper.h b/src/ipa/libipa/exposure_mode_helper.h
> new file mode 100644
> index 00000000..0ffc164e
> --- /dev/null
> +++ b/src/ipa/libipa/exposure_mode_helper.h
> @@ -0,0 +1,53 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2024, Paul Elder <paul.elder at ideasonboard.com>
> + *
> + * exposure_mode_helper.h - Helper class that performs computations relating to exposure
> + */
> +
> +#pragma once
> +
> +#include <tuple>
> +#include <utility>
> +#include <vector>
> +
> +#include <libcamera/base/span.h>
> +#include <libcamera/base/utils.h>
> +
> +namespace libcamera {
> +
> +namespace ipa {
> +
> +class ExposureModeHelper
> +{
> +public:
> +	ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages);
> +	~ExposureModeHelper() = default;
> +
> +	void setLimits(utils::Duration minShutter, utils::Duration maxShutter,
> +		       double minGain, double maxGain);
> +
> +	std::tuple<utils::Duration, double, double>
> +	splitExposure(utils::Duration exposure) const;
> +
> +	utils::Duration minShutter() const { return minShutter_; }
> +	utils::Duration maxShutter() const { return maxShutter_; }
> +	double minGain() const { return minGain_; }
> +	double maxGain() const { return maxGain_; }
> +
> +private:
> +	utils::Duration clampShutter(utils::Duration shutter) const;
> +	double clampGain(double gain) const;
> +
> +	std::vector<utils::Duration> shutters_;
> +	std::vector<double> gains_;
> +
> +	utils::Duration minShutter_;
> +	utils::Duration maxShutter_;
> +	double minGain_;
> +	double maxGain_;
> +};
> +
> +} /* namespace ipa */
> +
> +} /* namespace libcamera */
> diff --git a/src/ipa/libipa/meson.build b/src/ipa/libipa/meson.build
> index 016b8e0e..37fbd177 100644
> --- a/src/ipa/libipa/meson.build
> +++ b/src/ipa/libipa/meson.build
> @@ -3,6 +3,7 @@
>  libipa_headers = files([
>      'algorithm.h',
>      'camera_sensor_helper.h',
> +    'exposure_mode_helper.h',
>      'fc_queue.h',
>      'histogram.h',
>      'module.h',
> @@ -11,6 +12,7 @@ libipa_headers = files([
>  libipa_sources = files([
>      'algorithm.cpp',
>      'camera_sensor_helper.cpp',
> +    'exposure_mode_helper.cpp',
>      'fc_queue.cpp',
>      'histogram.cpp',
>      'module.cpp',
> --
> 2.34.1
>