[libcamera-devel] [PATCH v2 07/14] ipa: ipu3: agc: Improve gain calculation
Jean-Michel Hautbois
jeanmichel.hautbois at ideasonboard.com
Thu Nov 11 08:07:30 CET 2021
Hi Kieran,
On 10/11/2021 23:29, Kieran Bingham wrote:
> 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?
I am now returning the normalized luma from computeInitialY() which is
basically dividing it by 255. This is the same as setting targetY = 41
without the division, and this is the value used in RPi too. I have seen
one or two cases where the targetY=60 (so, ~0.25) was a bit high.
>
> 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.
It could be configured, it can certainly be a constexpr :-).
Should the maximum number of loops be a constexpr too ?
>
>
> 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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