[libcamera-devel] [PATCH 2/5] ipa: ipu3: agc: Standardize vocabulary on "relative luminance"
Jean-Michel Hautbois
jeanmichel.hautbois at ideasonboard.com
Wed Nov 17 11:30:14 CET 2021
On 17/11/2021 11:26, Laurent Pinchart wrote:
> Hi Jean-Michel,
>
> On Wed, Nov 17, 2021 at 11:15:23AM +0100, Jean-Michel Hautbois wrote:
>> On 16/11/2021 17:26, Laurent Pinchart wrote:
>>> The AGC computes the average relative luminance of the frame and calls
>>> the value "normalized luma", "brightness" or "initialY". The latter is
>>> the most accurate term, as the relative luminance is abbreviated Y, but
>>> the "initial" prefix isn't accurate.
>>>
>>> Standardize the vocabulary on "relative luminance" in code and comments,
>>> abbreviating it to Y when needed.
>>>
>>> While at it, rename variables to match the libcamera coding style.
>>>
>>> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>>> ---
>>> src/ipa/ipu3/algorithms/agc.cpp | 44 ++++++++++++++++-----------------
>>> src/ipa/ipu3/algorithms/agc.h | 8 +++---
>>> 2 files changed, 26 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
>>> index 43a39ffd57d6..d5840fb0aa97 100644
>>> --- a/src/ipa/ipu3/algorithms/agc.cpp
>>> +++ b/src/ipa/ipu3/algorithms/agc.cpp
>>> @@ -62,12 +62,12 @@ static constexpr double kEvGainTarget = 0.5;
>>> static constexpr uint32_t kNumStartupFrames = 10;
>>>
>>> /*
>>> - * Normalized luma value target.
>>> + * Relative luminance 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;
>>> +static constexpr double kRelativeLuminanceTarget = 0.16;
>>>
>>> Agc::Agc()
>>> : frameCount_(0), iqMean_(0.0), lineDuration_(0s), minShutterSpeed_(0s),
>>> @@ -250,12 +250,12 @@ void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
>>> }
>>>
>>> /**
>>> - * \brief Estimate the average brightness of the frame
>>> + * \brief Estimate the relative luminance of the frame with a given gain
>>> * \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
>>> + * \return The relative luminance
>>> *
>>> * 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
>>> @@ -263,12 +263,12 @@ void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
>>> * luma here.
>>> *
>>> * More detailed information can be found in:
>>> - * https://en.wikipedia.org/wiki/Luma_(video)
>>> + * https://en.wikipedia.org/wiki/Relative_luminance
>>> */
>>> -double Agc::computeInitialY(IPAFrameContext &frameContext,
>>> - const ipu3_uapi_grid_config &grid,
>>> - const ipu3_uapi_stats_3a *stats,
>>> - double currentYGain)
>>> +double Agc::estimateLuminance(IPAFrameContext &frameContext,
>>> + const ipu3_uapi_grid_config &grid,
>>> + const ipu3_uapi_stats_3a *stats,
>>> + double currentYGain)
>>
>> Shouldn't it be named estimateRelativeLuminance then ? Or is it too long
>> maybe...
>
> I found it a bit long. If you think it's better, I'm ok with it.
>
>>> {
>>> double redSum = 0, greenSum = 0, blueSum = 0;
>>>
>>> @@ -288,14 +288,14 @@ double Agc::computeInitialY(IPAFrameContext &frameContext,
>>> }
>>>
>>> /*
>>> - * Estimate the sum of the brightness values, weighted with the gains
>>> - * applied on the channels in AWB as the Rec. 601 luma.
>>> + * Compute the relative luminance using the Rec. 601 formula, and
>>> + * normalize it.
>>> */
>>
>> Relative luminance is normalized, as it is between 0.0 and 1.0 :-).
>> Why did you remove the gain weights mention though ?
>
> Those are not weights but gains. It should be mentioned though, I'll
> update it.
>
> Why do we multiply by the AWB gains here ?
Because we want to estimate the brightness based on what the scene
really is (or as much as possible) so we need corrected channels for
that to be accurate.
>
>>> - double Y_sum = redSum * frameContext.awb.gains.red * .299 +
>>> - greenSum * frameContext.awb.gains.green * .587 +
>>> - blueSum * frameContext.awb.gains.blue * .114;
>>> + double ySum = redSum * frameContext.awb.gains.red * .299 +
>>> + greenSum * frameContext.awb.gains.green * .587 +
>>> + blueSum * frameContext.awb.gains.blue * .114;
>>>
>>> - return Y_sum / (grid.height * grid.width) / 255;
>>> + return ySum / (grid.height * grid.width) / 255;
>>
>> How could we let that one... :-/
>>
>>> }
>>>
>>> /**
>>> @@ -311,22 +311,22 @@ void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
>>> measureBrightness(stats, context.configuration.grid.bdsGrid);
>>>
>>> double currentYGain = 1.0;
>>> - double targetY = kNormalizedLumaTarget;
>>> + double yTarget = kRelativeLuminanceTarget;
>>>
>>> /*
>>> * 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,
>>> + for (unsigned int i = 0; i < 8; i++) {
>>> + double yValue = estimateLuminance(context.frameContext,
>>> context.configuration.grid.bdsGrid,
>>> stats, currentYGain);
>>> - double extra_gain = std::min(10.0, targetY / (initialY + .001));
>>> + double extra_gain = std::min(10.0, yTarget / (yValue + .001));
>>>
>>> currentYGain *= extra_gain;
>>> - LOG(IPU3Agc, Debug) << "Initial Y " << initialY
>>> - << " target " << targetY
>>> - << " gives gain " << currentYGain;
>>> + LOG(IPU3Agc, Debug) << "Y value: " << yValue
>>> + << ", Y target: " << yTarget
>>> + << ", gives gain " << currentYGain;
>>> if (extra_gain < 1.01)
>>> break;
>>> }
>>> diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
>>> index 31c5a6e519d4..943c354a820e 100644
>>> --- a/src/ipa/ipu3/algorithms/agc.h
>>> +++ b/src/ipa/ipu3/algorithms/agc.h
>>> @@ -35,10 +35,10 @@ private:
>>> const ipu3_uapi_grid_config &grid);
>>> void filterExposure();
>>> void computeExposure(IPAFrameContext &frameContext, double currentYGain);
>>> - double computeInitialY(IPAFrameContext &frameContext,
>>> - const ipu3_uapi_grid_config &grid,
>>> - const ipu3_uapi_stats_3a *stats,
>>> - double currentYGain);
>>> + double estimateLuminance(IPAFrameContext &frameContext,
>>> + const ipu3_uapi_grid_config &grid,
>>> + const ipu3_uapi_stats_3a *stats,
>>> + double currentYGain);
>>>
>>> uint64_t frameCount_;
>>>
>>>
>
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