[libcamera-devel] [PATCH v2 07/14] ipa: ipu3: agc: Improve gain calculation
Kieran Bingham
kieran.bingham at ideasonboard.com
Wed Nov 10 23:29:16 CET 2021
Quoting Jean-Michel Hautbois (2021-11-10 19:58:54)
> When an image is partially saturated, its brightness is not increasing
> linearly when the shutter time or gain increases. It is a big issue with
> a backlight as the algorithm is fading to darkness right now.
>
> Introduce a function to estimate the brightness of the frame, based on
> the current exposure/gain and loop on it several times to estimate it
> again and approach the non linear function.
>
> Inspired-by: 7de5506c30b3 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> ---
> src/ipa/ipu3/algorithms/agc.cpp | 103 +++++++++++++++++++++++++++++++-
> src/ipa/ipu3/algorithms/agc.h | 6 +-
> 2 files changed, 105 insertions(+), 4 deletions(-)
>
> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
> index 119a7938..ee37a9d5 100644
> --- a/src/ipa/ipu3/algorithms/agc.cpp
> +++ b/src/ipa/ipu3/algorithms/agc.cpp
> @@ -67,6 +67,9 @@ static constexpr uint32_t kMinCellsPerZoneRatio = 255 * 20 / 100;
> /* Number of frames to wait before calculating stats on minimum exposure */
> static constexpr uint32_t kNumStartupFrames = 10;
>
> +/* Maximum luminance used for brightness normalization */
> +static constexpr uint32_t kMaxLuminance = 255;
> +
> Agc::Agc()
> : frameCount_(0), iqMean_(0.0), lineDuration_(0s), minExposureLines_(0),
> maxExposureLines_(0), filteredExposure_(0s), currentExposure_(0s),
> @@ -186,10 +189,16 @@ void Agc::filterExposure()
> * \brief Estimate the new exposure and gain values
> * \param[inout] exposure The exposure value reference as a number of lines
> * \param[inout] gain The gain reference to be updated
> + * \param[in] currentYGain The gain calculated on the current brightness level
> */
> -void Agc::computeExposure(uint32_t &exposure, double &analogueGain)
> +void Agc::computeExposure(uint32_t &exposure, double &analogueGain, double currentYGain)
> {
> - /* Estimate the gain needed to have the proportion wanted */
> + /*
> + * Estimate the gain needed to have the proportion of pixels in a given
> + * range wanted. iqMean_ returns the mean value of the top 2% of the
> + * cumulative histogram, and we want it to be as close as possible to a
> + * configured target.
> + */
> double evGain = kEvGainTarget * knumHistogramBins / iqMean_;
>
> if (std::abs(evGain - 1.0) < 0.01) {
> @@ -199,6 +208,7 @@ void Agc::computeExposure(uint32_t &exposure, double &analogueGain)
> }
>
> /* extracted from Rpi::Agc::computeTargetExposure */
> +
> /* Calculate the shutter time in seconds */
> utils::Duration currentShutter = exposure * lineDuration_;
> LOG(IPU3Agc, Debug) << "Actual total exposure " << currentShutter * analogueGain
> @@ -206,6 +216,14 @@ void Agc::computeExposure(uint32_t &exposure, double &analogueGain)
> << " Gain " << analogueGain
> << " Needed ev gain " << evGain;
>
> + if (evGain < currentYGain)
> + evGain = currentYGain;
> +
> + /* Consider within 1% of the target as correctly exposed */
> + if (std::abs(evGain - 1.0) < 0.01)
> + LOG(IPU3Agc, Debug) << "We are well exposed (iqMean = "
> + << iqMean_ << ")";
> +
> /*
> * Calculate the current exposure value for the scene as the latest
> * exposure value applied multiplied by the new estimated gain.
> @@ -253,6 +271,57 @@ void Agc::computeExposure(uint32_t &exposure, double &analogueGain)
> prevExposureValue_ = shutterTime * analogueGain;
> }
>
> +/**
> + * \brief Estimate the average brightness of the frame
> + * \param[in] context The shared IPA context
frameContext?
> + * \param[in] grid The grid used to store the statistics in the IPU3
> + * \param[in] stats The IPU3 statistics and ISP results
> + * \param[in] currentYGain The gain calculated on the current brightness level
> + * \return The normalized luma
> + *
> + * Luma is the weighted sum of gamma-compressed R′G′B′ components of a color
> + * video. The luma values are normalized as 0.0 to 1.0, with 1.0 being a
> + * theoretical perfect reflector of 100% reference white. We use the Rec. 601
> + * luma here.
> + *
> + * More detailed information can be found in:
> + * https://en.wikipedia.org/wiki/Luma_(video)
Thank you - that all helps a lot in making it understandable (and being
explicit about where these conversions are from/represent).
> + */
> +double Agc::computeInitialY(IPAFrameContext &frameContext,
> + const ipu3_uapi_grid_config &grid,
> + const ipu3_uapi_stats_3a *stats,
> + double currentYGain)
> +{
> + double redSum = 0, greenSum = 0, blueSum = 0;
> +
> + for (unsigned int cellY = 0; cellY < grid.height; cellY++) {
> + for (unsigned int cellX = 0; cellX < grid.width; cellX++) {
> + uint32_t cellPosition = cellY * stride_ + cellX;
> +
> + const ipu3_uapi_awb_set_item *cell =
> + reinterpret_cast<const ipu3_uapi_awb_set_item *>(
> + &stats->awb_raw_buffer.meta_data[cellPosition]
> + );
> +
> + redSum += cell->R_avg * currentYGain;
> + greenSum += (cell->Gr_avg + cell->Gb_avg) / 2 * currentYGain;
> + blueSum += cell->B_avg * currentYGain;
> + }
> + }
> +
> + /*
> + * Estimate the sum of the brightness values, weighted with the gains
> + * applied on the channels in AWB as the Rec. 601 luma.
> + */
> + double Y_sum = redSum * frameContext.awb.gains.red * .299 +
> + greenSum * frameContext.awb.gains.green * .587 +
> + blueSum * frameContext.awb.gains.blue * .114;
> +
> + /* Return the normalized relative luminance. */
> + return Y_sum / (grid.height * grid.width) / kMaxLuminance;
> +}
> +
> +
> /**
> * \brief Process IPU3 statistics, and run AGC operations
> * \param[in] context The shared IPA context
> @@ -267,7 +336,35 @@ void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
> uint32_t &exposure = context.frameContext.agc.exposure;
> double &analogueGain = context.frameContext.agc.gain;
> measureBrightness(stats, context.configuration.grid.bdsGrid);
> - computeExposure(exposure, analogueGain);
> +
> + double currentYGain = 1.0;
> + /*
> + * Normalized luma value target.
> + *
> + * It's a number that's chosen so that, when the camera points at a grey
> + * target, the resulting image brightness is considered right.
I think I'd change s/considered right/considered correct/ - but it's not
critical.
> + */
> + double targetY = 0.16;
Wow - this is a big change from the 60 that it was before. Did something
fundamental happen?
Will this be required to come from some sensor tuning profile at a later
date?
If so - I'd add a
\todo targetY should be calibrated by sensor tuning
(Same for any other constant that has been manually tuned at present).
Is targetY ever changed in the code? If not - perhaps it should be a
constexpr to show that it isn't modified.
But aside from those which are all minor.
Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> +
> + /*
> + * Do this calculation a few times as brightness increase can be
> + * non-linear when there are saturated regions.
> + */
> + for (int i = 0; i < 8; i++) {
> + double initialY = computeInitialY(context.frameContext,
> + context.configuration.grid.bdsGrid,
> + stats, currentYGain);
> + double extra_gain = std::min(10.0, targetY / (initialY + .001));
> +
> + currentYGain *= extra_gain;
> + LOG(IPU3Agc, Debug) << "Initial Y " << initialY
> + << " target " << targetY
> + << " gives gain " << currentYGain;
> + if (extra_gain < 1.01)
> + break;
> + }
> +
> + computeExposure(exposure, analogueGain, currentYGain);
> frameCount_++;
> }
>
> diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
> index 69e0b831..0a9152a9 100644
> --- a/src/ipa/ipu3/algorithms/agc.h
> +++ b/src/ipa/ipu3/algorithms/agc.h
> @@ -34,7 +34,11 @@ private:
> void measureBrightness(const ipu3_uapi_stats_3a *stats,
> const ipu3_uapi_grid_config &grid);
> void filterExposure();
> - void computeExposure(uint32_t &exposure, double &gain);
> + void computeExposure(uint32_t &exposure, double &gain, double currentYGain);
> + double computeInitialY(IPAFrameContext &frameContext,
> + const ipu3_uapi_grid_config &grid,
> + const ipu3_uapi_stats_3a *stats,
> + double currentYGain);
>
> uint64_t frameCount_;
> uint64_t lastFrame_;
> --
> 2.32.0
>
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