[PATCH v6 17/18] libcamera: software_isp: Apply black level compensation
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Wed Mar 27 19:55:43 CET 2024
Hi Milan,
(CC'ing David)
Thank you for the patch.
On Tue, Mar 19, 2024 at 01:36:04PM +0100, Milan Zamazal wrote:
> Black may not be represented as 0 pixel value for given hardware, it may be
> higher. If this is not compensated then various problems may occur such as low
> contrast or suboptimal exposure.
>
> The black pixel value can be either retrieved from a tuning file for the given
> hardware, or automatically on fly. The former is the right and correct method,
s/on fly/on the fly/
> while the latter can be used when a tuning file is not available for the given
> hardware. Since there is currently no support for tuning files in software ISP,
> the automatic, hardware independent way, is always used. Support for tuning
> files should be added in future but it will require more work than this patch.
I don't think this is quite right. Strictly speaking, the black level
compensation is about subtracting the black level as produced by the
sensor. This requires tuning, and shouldn't be done on-the-fly.
On the other hand, looking at the histogram to try and stretch it is
called contrast adjustment. There's nothing wrong in doing so, but it's
usually done towards the output of the processing pipeline, and is
associated with manual adjustments of the contrast and brightness. See
src/ipa/rpi/controller/rpi/contrast.cpp for instance.
David may be able to comment further as to why BLC and contrast are
quite different.
As this patch may need more work, I propose not including it in v7 to
avoid delay merging the rest of the implementation.
> The patch looks at the image histogram and assumes that black starts when pixel
> values start occurring on the left. A certain amount of the darkest pixels is
> ignored; it doesn't matter whether they represent various kinds of noise or are
> real, they are better to omit in any case to make the image looking better. It
> also doesn't matter whether the darkest pixels occur around the supposed black
> level or are spread between 0 and the black level, the difference is not
> important.
>
> An arbitrary threshold of 2% darkest pixels is applied; there is no magic about
> that value.
>
> The patch assumes that the black values for different colors are the same and
> doesn't attempt any other non-primitive enhancements. It cannot completely
> replace tuning files and simplicity, while providing visible benefit, is its
> goal. Anything more sophisticated is left for future patches.
>
> A possible cheap enhancement, if needed, could be setting exposure + gain to
> minimum values temporarily, before setting the black level. In theory, the
> black level should be fixed but it may not be reached in all images. For this
> reason, the patch updates black level only if the observed value is lower than
> the current one; it should be never increased.
>
> The purpose of the patch is to compensate for hardware properties. General
> image contrast enhancements are out of scope of this patch.
>
> Stats are still gathered as an uncorrected histogram, to avoid any confusion and
> to represent the raw image data. Exposure must be determined after the black
> level correction -- it has no influence on the sub-black area and must be
> correct after applying the black level correction. The granularity of the
> histogram is increased from 16 to 64 to provide a better precision (there is no
> theory behind either of those numbers).
>
> Reviewed-by: Hans de Goede <hdegoede at redhat.com>
> Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> .../internal/software_isp/debayer_params.h | 4 +
> .../internal/software_isp/swisp_stats.h | 10 ++-
> src/ipa/simple/black_level.cpp | 88 +++++++++++++++++++
> src/ipa/simple/black_level.h | 28 ++++++
> src/ipa/simple/meson.build | 7 +-
> src/ipa/simple/soft_simple.cpp | 28 ++++--
> src/libcamera/software_isp/debayer_cpu.cpp | 13 ++-
> src/libcamera/software_isp/debayer_cpu.h | 1 +
> src/libcamera/software_isp/software_isp.cpp | 2 +-
> 9 files changed, 164 insertions(+), 17 deletions(-)
> create mode 100644 src/ipa/simple/black_level.cpp
> create mode 100644 src/ipa/simple/black_level.h
>
> diff --git a/include/libcamera/internal/software_isp/debayer_params.h b/include/libcamera/internal/software_isp/debayer_params.h
> index 98965fa1..5e38e08b 100644
> --- a/include/libcamera/internal/software_isp/debayer_params.h
> +++ b/include/libcamera/internal/software_isp/debayer_params.h
> @@ -43,6 +43,10 @@ struct DebayerParams {
> * \brief Gamma correction, 1.0 is no correction
> */
> float gamma;
> + /**
> + * \brief Level of the black point, 0..255, 0 is no correction.
> + */
> + unsigned int blackLevel;
> };
>
> } /* namespace libcamera */
> diff --git a/include/libcamera/internal/software_isp/swisp_stats.h b/include/libcamera/internal/software_isp/swisp_stats.h
> index afe42c9a..25cd5abd 100644
> --- a/include/libcamera/internal/software_isp/swisp_stats.h
> +++ b/include/libcamera/internal/software_isp/swisp_stats.h
> @@ -7,6 +7,8 @@
>
> #pragma once
>
> +#include <array>
> +
> namespace libcamera {
>
> /**
> @@ -28,11 +30,15 @@ struct SwIspStats {
> /**
> * \brief Number of bins in the yHistogram.
> */
> - static constexpr unsigned int kYHistogramSize = 16;
> + static constexpr unsigned int kYHistogramSize = 64;
> + /**
> + * \brief Type of the histogram.
> + */
> + using histogram = std::array<unsigned int, kYHistogramSize>;
> /**
> * \brief A histogram of luminance values.
> */
> - std::array<unsigned int, kYHistogramSize> yHistogram;
> + histogram yHistogram;
> };
>
> } /* namespace libcamera */
> diff --git a/src/ipa/simple/black_level.cpp b/src/ipa/simple/black_level.cpp
> new file mode 100644
> index 00000000..9d4aa800
> --- /dev/null
> +++ b/src/ipa/simple/black_level.cpp
> @@ -0,0 +1,88 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2024, Red Hat Inc.
> + *
> + * black_level.cpp - black level handling
> + */
> +
> +#include "black_level.h"
> +
> +#include <numeric>
> +
> +#include <libcamera/base/log.h>
> +
> +namespace libcamera {
> +
> +LOG_DEFINE_CATEGORY(IPASoftBL)
> +
> +/**
> + * \class BlackLevel
> + * \brief Object providing black point level for software ISP
> + *
> + * Black level can be provided in hardware tuning files or, if no tuning file is
> + * available for the given hardware, guessed automatically, with less accuracy.
> + * As tuning files are not yet implemented for software ISP, BlackLevel
> + * currently provides only guessed black levels.
> + *
> + * This class serves for tracking black level as a property of the underlying
> + * hardware, not as means of enhancing a particular scene or image.
> + *
> + * The class is supposed to be instantiated for the given camera stream.
> + * The black level can be retrieved using BlackLevel::get() method. It is
> + * initially 0 and may change when updated using BlackLevel::update() method.
> + */
> +
> +BlackLevel::BlackLevel()
> + : blackLevel_(255), blackLevelSet_(false)
> +{
> +}
> +
> +/**
> + * \brief Return the current black level
> + *
> + * \return The black level, in the range from 0 (minimum) to 255 (maximum).
> + * If the black level couldn't be determined yet, return 0.
> + */
> +unsigned int BlackLevel::get() const
> +{
> + return blackLevelSet_ ? blackLevel_ : 0;
> +}
> +
> +/**
> + * \brief Update black level from the provided histogram
> + * \param[in] yHistogram The histogram to be used for updating black level
> + *
> + * The black level is property of the given hardware, not image. It is updated
> + * only if it has not been yet set or if it is lower than the lowest value seen
> + * so far.
> + */
> +void BlackLevel::update(SwIspStats::histogram &yHistogram)
> +{
> + /*
> + * The constant is selected to be "good enough", not overly conservative or
> + * aggressive. There is no magic about the given value.
> + */
> + constexpr float ignoredPercentage_ = 0.02;
> + const unsigned int total =
> + std::accumulate(begin(yHistogram), end(yHistogram), 0);
> + const unsigned int pixelThreshold = ignoredPercentage_ * total;
> + const unsigned int histogramRatio = 256 / SwIspStats::kYHistogramSize;
> + const unsigned int currentBlackIdx = blackLevel_ / histogramRatio;
> +
> + for (unsigned int i = 0, seen = 0;
> + i < currentBlackIdx && i < SwIspStats::kYHistogramSize;
> + i++) {
> + seen += yHistogram[i];
> + if (seen >= pixelThreshold) {
> + blackLevel_ = i * histogramRatio;
> + blackLevelSet_ = true;
> + LOG(IPASoftBL, Debug)
> + << "Auto-set black level: "
> + << i << "/" << SwIspStats::kYHistogramSize
> + << " (" << 100 * (seen - yHistogram[i]) / total << "% below, "
> + << 100 * seen / total << "% at or below)";
> + break;
> + }
> + };
> +}
> +} // namespace libcamera
} /* namespace libcamera */
Same below.
> diff --git a/src/ipa/simple/black_level.h b/src/ipa/simple/black_level.h
> new file mode 100644
> index 00000000..b3785db0
> --- /dev/null
> +++ b/src/ipa/simple/black_level.h
> @@ -0,0 +1,28 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2024, Red Hat Inc.
> + *
> + * black_level.h - black level handling
> + */
> +
> +#pragma once
> +
> +#include <array>
> +
> +#include "libcamera/internal/software_isp/swisp_stats.h"
> +
> +namespace libcamera {
> +
> +class BlackLevel
Wouldn't it be better to move to using the libipa Algorithm class before
introducing new algorithms ?
> +{
> +public:
> + BlackLevel();
> + unsigned int get() const;
> + void update(std::array<unsigned int, SwIspStats::kYHistogramSize> &yHistogram);
> +
> +private:
> + unsigned int blackLevel_;
> + bool blackLevelSet_;
> +};
> +
> +} // namespace libcamera
> diff --git a/src/ipa/simple/meson.build b/src/ipa/simple/meson.build
> index 3e863db7..44b5f1d7 100644
> --- a/src/ipa/simple/meson.build
> +++ b/src/ipa/simple/meson.build
> @@ -2,8 +2,13 @@
>
> ipa_name = 'ipa_soft_simple'
>
> +soft_simple_sources = files([
> + 'soft_simple.cpp',
> + 'black_level.cpp',
> +])
> +
> mod = shared_module(ipa_name,
> - ['soft_simple.cpp', libcamera_generated_ipa_headers],
> + [soft_simple_sources, libcamera_generated_ipa_headers],
> name_prefix : '',
> include_directories : [ipa_includes, libipa_includes],
> dependencies : libcamera_private,
> diff --git a/src/ipa/simple/soft_simple.cpp b/src/ipa/simple/soft_simple.cpp
> index 312df4ba..ac027568 100644
> --- a/src/ipa/simple/soft_simple.cpp
> +++ b/src/ipa/simple/soft_simple.cpp
> @@ -22,6 +22,8 @@
> #include "libcamera/internal/software_isp/debayer_params.h"
> #include "libcamera/internal/software_isp/swisp_stats.h"
>
> +#include "black_level.h"
> +
> namespace libcamera {
>
> LOG_DEFINE_CATEGORY(IPASoft)
> @@ -33,7 +35,8 @@ class IPASoftSimple : public ipa::soft::IPASoftInterface
> public:
> IPASoftSimple()
> : params_(static_cast<DebayerParams *>(MAP_FAILED)),
> - stats_(static_cast<SwIspStats *>(MAP_FAILED)), ignore_updates_(0)
> + stats_(static_cast<SwIspStats *>(MAP_FAILED)),
> + blackLevel_(BlackLevel()), ignore_updates_(0)
> {
> }
>
> @@ -63,6 +66,7 @@ private:
> SharedFD fdParams_;
> DebayerParams *params_;
> SwIspStats *stats_;
> + BlackLevel blackLevel_;
>
> int32_t exposure_min_, exposure_max_;
> int32_t again_min_, again_max_;
> @@ -196,6 +200,10 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
> params_->gainG = 256;
> params_->gamma = 0.5;
>
> + if (ignore_updates_ > 0)
> + blackLevel_.update(stats_->yHistogram);
> + params_->blackLevel = blackLevel_.get();
> +
> setIspParams.emit(0);
>
> /*
> @@ -211,18 +219,19 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
> * Calculate Mean Sample Value (MSV) according to formula from:
> * https://www.araa.asn.au/acra/acra2007/papers/paper84final.pdf
> */
> - constexpr unsigned int yHistValsPerBin =
> - SwIspStats::kYHistogramSize / kExposureBinsCount;
> - constexpr unsigned int yHistValsPerBinMod =
> - SwIspStats::kYHistogramSize /
> - (SwIspStats::kYHistogramSize % kExposureBinsCount + 1);
> + const unsigned int blackLevelHistIdx =
> + params_->blackLevel / (256 / SwIspStats::kYHistogramSize);
> + const unsigned int histogramSize = SwIspStats::kYHistogramSize - blackLevelHistIdx;
> + const unsigned int yHistValsPerBin = histogramSize / kExposureBinsCount;
> + const unsigned int yHistValsPerBinMod =
> + histogramSize / (histogramSize % kExposureBinsCount + 1);
> int ExposureBins[kExposureBinsCount] = {};
> unsigned int denom = 0;
> unsigned int num = 0;
>
> - for (unsigned int i = 0; i < SwIspStats::kYHistogramSize; i++) {
> + for (unsigned int i = 0; i < histogramSize; i++) {
> unsigned int idx = (i - (i / yHistValsPerBinMod)) / yHistValsPerBin;
> - ExposureBins[idx] += stats_->yHistogram[i];
> + ExposureBins[idx] += stats_->yHistogram[blackLevelHistIdx + i];
> }
>
> for (unsigned int i = 0; i < kExposureBinsCount; i++) {
> @@ -256,7 +265,8 @@ void IPASoftSimple::processStats(const ControlList &sensorControls)
>
> LOG(IPASoft, Debug) << "exposureMSV " << exposureMSV
> << " exp " << exposure_ << " again " << again_
> - << " gain R/B " << params_->gainR << "/" << params_->gainB;
> + << " gain R/B " << params_->gainR << "/" << params_->gainB
> + << " black level " << params_->blackLevel;
> }
>
> void IPASoftSimple::updateExposure(double exposureMSV)
> diff --git a/src/libcamera/software_isp/debayer_cpu.cpp b/src/libcamera/software_isp/debayer_cpu.cpp
> index a1692693..3be3cdfe 100644
> --- a/src/libcamera/software_isp/debayer_cpu.cpp
> +++ b/src/libcamera/software_isp/debayer_cpu.cpp
> @@ -35,7 +35,7 @@ namespace libcamera {
> * \param[in] stats Pointer to the stats object to use.
> */
> DebayerCpu::DebayerCpu(std::unique_ptr<SwStatsCpu> stats)
> - : stats_(std::move(stats)), gamma_correction_(1.0)
> + : stats_(std::move(stats)), gamma_correction_(1.0), blackLevel_(0)
> {
> #ifdef __x86_64__
> enableInputMemcpy_ = false;
> @@ -683,11 +683,16 @@ void DebayerCpu::process(FrameBuffer *input, FrameBuffer *output, DebayerParams
> }
>
> /* Apply DebayerParams */
> - if (params.gamma != gamma_correction_) {
> - for (unsigned int i = 0; i < kGammaLookupSize; i++)
> - gamma_[i] = UINT8_MAX * powf(i / (kGammaLookupSize - 1.0), params.gamma);
> + if (params.gamma != gamma_correction_ || params.blackLevel != blackLevel_) {
> + const unsigned int blackIndex =
> + params.blackLevel * kGammaLookupSize / 256;
> + std::fill(gamma_.begin(), gamma_.begin() + blackIndex, 0);
> + const float divisor = kGammaLookupSize - blackIndex - 1.0;
> + for (unsigned int i = blackIndex; i < kGammaLookupSize; i++)
> + gamma_[i] = UINT8_MAX * powf((i - blackIndex) / divisor, params.gamma);
>
> gamma_correction_ = params.gamma;
> + blackLevel_ = params.blackLevel;
> }
>
> if (swapRedBlueGains_)
> diff --git a/src/libcamera/software_isp/debayer_cpu.h b/src/libcamera/software_isp/debayer_cpu.h
> index 5f44fc65..ea02f909 100644
> --- a/src/libcamera/software_isp/debayer_cpu.h
> +++ b/src/libcamera/software_isp/debayer_cpu.h
> @@ -147,6 +147,7 @@ private:
> bool enableInputMemcpy_;
> bool swapRedBlueGains_;
> float gamma_correction_;
> + unsigned int blackLevel_;
> unsigned int measuredFrames_;
> int64_t frameProcessTime_;
> /* Skip 30 frames for things to stabilize then measure 30 frames */
> diff --git a/src/libcamera/software_isp/software_isp.cpp b/src/libcamera/software_isp/software_isp.cpp
> index 388b4496..9b49be41 100644
> --- a/src/libcamera/software_isp/software_isp.cpp
> +++ b/src/libcamera/software_isp/software_isp.cpp
> @@ -64,7 +64,7 @@ LOG_DEFINE_CATEGORY(SoftwareIsp)
> */
> SoftwareIsp::SoftwareIsp(PipelineHandler *pipe, const ControlInfoMap &sensorControls)
> : debayer_(nullptr),
> - debayerParams_{ DebayerParams::kGain10, DebayerParams::kGain10, DebayerParams::kGain10, 0.5f },
> + debayerParams_{ DebayerParams::kGain10, DebayerParams::kGain10, DebayerParams::kGain10, 0.5f, 0 },
> dmaHeap_(DmaHeap::DmaHeapFlag::Cma | DmaHeap::DmaHeapFlag::System)
> {
> if (!dmaHeap_.isValid()) {
--
Regards,
Laurent Pinchart
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