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ipa: ipu3: agc: Improve gain calculation

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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: 7de5506c30 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
This commit is contained in:
Jean-Michel Hautbois 2021-11-04 10:58:07 +01:00
parent 3d81620b20
commit f8f07f9468
2 changed files with 103 additions and 7 deletions
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104
src/ipa/ipu3/algorithms/agc.cpp
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@ -67,6 +67,17 @@ 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;
/*
* 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.
*/
static constexpr double kNormalizedLumaTarget = 0.16;
Agc::Agc()
: frameCount_(0), iqMean_(0.0), lineDuration_(0s), minExposureLines_(0),
maxExposureLines_(0), filteredExposure_(0s), currentExposure_(0s),
@ -185,23 +196,32 @@ void Agc::filterExposure()
/**
* \brief Estimate the new exposure and gain values
* \param[inout] frameContext The shared IPA frame Context
* \param[in] currentYGain The gain calculated on the current brightness level
*/
void Agc::computeExposure(IPAFrameContext &frameContext)
void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
{
/* Get the effective exposure and gain applied on the sensor. */
uint32_t exposure = frameContext.sensor.exposure;
double analogueGain = frameContext.sensor.gain;
/* Estimate the gain needed to have the proportion wanted */
/*
* Estimate the gain needed to have the proportion of pixels in a given
* desired range. 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) {
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_ << ")";
return;
}
/* extracted from Rpi::Agc::computeTargetExposure */
/* Calculate the shutter time in seconds */
utils::Duration currentShutter = exposure * lineDuration_;
LOG(IPU3Agc, Debug) << "Actual total exposure " << currentShutter * analogueGain
@ -257,6 +277,56 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
prevExposureValue_ = shutterTime * analogueGain;
}
/**
* \brief Estimate the average brightness of the frame
* \param[in] frameContext The shared IPA frame context
* \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)
*/
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
@ -268,7 +338,29 @@ void Agc::computeExposure(IPAFrameContext &frameContext)
void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
{
measureBrightness(stats, context.configuration.grid.bdsGrid);
computeExposure(context.frameContext);
double currentYGain = 1.0;
double targetY = kNormalizedLumaTarget;
/*
* 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(context.frameContext, currentYGain);
frameCount_++;
}