[PATCH 2/5] libcamera: Add ClockRecovery class to generate wallclock timestamps
David Plowman
david.plowman at raspberrypi.com
Fri Dec 13 10:21:32 CET 2024
Hi Barnabas
Thanks for looking at this!
On Thu, 12 Dec 2024 at 17:56, Barnabás Pőcze <pobrn at protonmail.com> wrote:
>
> Hi
>
>
> 2024. december 6., péntek 15:27 keltezéssel, David Plowman <david.plowman at raspberrypi.com> írta:
>
> > The ClockRecovery class takes pairs of timestamps from two different
> > clocks, and models the second ("output") clock from the first ("input")
> > clock.
> >
> > We can use it, in particular, to get a good wallclock estimate for a
> > frame's SensorTimestamp.
> >
> > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > ---
> > include/libcamera/internal/clock_recovery.h | 72 +++++++
> > include/libcamera/internal/meson.build | 1 +
> > src/libcamera/clock_recovery.cpp | 207 ++++++++++++++++++++
> > src/libcamera/meson.build | 1 +
> > 4 files changed, 281 insertions(+)
> > create mode 100644 include/libcamera/internal/clock_recovery.h
> > create mode 100644 src/libcamera/clock_recovery.cpp
> >
> > diff --git a/include/libcamera/internal/clock_recovery.h b/include/libcamera/internal/clock_recovery.h
> > new file mode 100644
> > index 00000000..c874574e
> > --- /dev/null
> > +++ b/include/libcamera/internal/clock_recovery.h
> > @@ -0,0 +1,72 @@
> > +/* SPDX-License-Identifier: BSD-2-Clause */
> > +/*
> > + * Copyright (C) 2024, Raspberry Pi Ltd
> > + *
> > + * Camera recovery algorithm
> > + */
> > +#pragma once
> > +
> > +#include <stdint.h>
> > +
> > +namespace libcamera {
> > +
> > +class ClockRecovery
> > +{
> > +public:
> > + ClockRecovery();
> > +
> > + /* Set configuration parameters. */
> > + void configure(unsigned int numPts = 100, unsigned int maxJitter = 2000, unsigned int minPts = 10,
> > + unsigned int errorThreshold = 50000);
> > + /* Erase all history and restart the fitting process. */
> > + void reset();
> > +
> > + /*
> > + * Add a new input clock / output clock sample, taking the input from the Linux
> > + * CLOCK_BOOTTIME and the output from the CLOCK_REALTIME.
> > + */
> > + void addSample();
> > + /*
> > + * Add a new input clock / output clock sample, specifying the clock times exactly. Use this
> > + * when you want to use clocks other than the ones described above.
> > + */
> > + void addSample(uint64_t input, uint64_t output);
> > + /* Calculate the output clock value for this input. */
> > + uint64_t getOutput(uint64_t input);
> > +
> > +private:
> > + unsigned int numPts_; /* how many samples contribute to the history */
> > + unsigned int maxJitter_; /* smooth out any jitter larger than this immediately */
> > + unsigned int minPts_; /* number of samples below which we treat clocks as 1:1 */
> > + unsigned int errorThreshold_; /* reset everything when the error exceeds this */
> > +
> > + unsigned int count_; /* how many samples seen (up to numPts_) */
> > + uint64_t inputBase_; /* subtract this from all input values, just to make the numbers easier */
> > + uint64_t outputBase_; /* as above, for the output */
> > +
> > + uint64_t lastInput_; /* the previous input sample */
> > + uint64_t lastOutput_; /* the previous output sample */
> > +
> > + /*
> > + * We do a linear regression of y against x, where:
> > + * x is the value input - inputBase_, and
> > + * y is the value output - outputBase_ - x.
> > + * We additionally subtract x from y so that y "should" be zero, again making the numnbers easier.
> > + */
> > + double xAve_; /* average x value seen so far */
> > + double yAve_; /* average y value seen so far */
> > + double x2Ave_; /* average x^2 value seen so far */
> > + double xyAve_; /* average x*y value seen so far */
> > +
> > + /*
> > + * Once we've seen more than minPts_ samples, we recalculate the slope and offset according
> > + * to the linear regression normal equations.
> > + */
> > + double slope_; /* latest slope value */
> > + double offset_; /* latest offset value */
> > +
> > + /* We use this cumulative error to monitor spontaneous system clock updates. */
> > + double error_;
> > +};
> > +
> > +} /* namespace libcamera */
> > diff --git a/include/libcamera/internal/meson.build b/include/libcamera/internal/meson.build
> > index 7d6aa8b7..41500636 100644
> > --- a/include/libcamera/internal/meson.build
> > +++ b/include/libcamera/internal/meson.build
> > @@ -11,6 +11,7 @@ libcamera_internal_headers = files([
> > 'camera_manager.h',
> > 'camera_sensor.h',
> > 'camera_sensor_properties.h',
> > + 'clock_recovery.h',
> > 'control_serializer.h',
> > 'control_validator.h',
> > 'converter.h',
> > diff --git a/src/libcamera/clock_recovery.cpp b/src/libcamera/clock_recovery.cpp
> > new file mode 100644
> > index 00000000..966599ee
> > --- /dev/null
> > +++ b/src/libcamera/clock_recovery.cpp
> > @@ -0,0 +1,207 @@
> > +/* SPDX-License-Identifier: BSD-2-Clause */
> > +/*
> > + * Copyright (C) 2024, Raspberry Pi Ltd
> > + *
> > + * Clock recovery algorithm
> > + */
> > +
> > +#include "libcamera/internal/clock_recovery.h"
> > +
> > +#include <time.h>
> > +
> > +#include <libcamera/base/log.h>
> > +
> > +/**
> > + * \file clock_recovery.h
> > + * \brief Clock recovery - deriving one clock from another independent clock
> > + */
> > +
> > +namespace libcamera {
> > +
> > +LOG_DEFINE_CATEGORY(ClockRec)
> > +
> > +/**
> > + * \class ClockRecovery
> > + * \brief Recover an output clock from an input clock
> > + *
> > + * The ClockRecovery class derives an output clock from an input clock,
> > + * modelling the output clock as being linearly related to the input clock.
> > + * For example, we may use it to derive wall clock timestamps from timestamps
> > + * measured by the internal system clock which counts local time since boot.
> > + *
> > + * When pairs of corresponding input and output timestamps are available,
> > + * they should be submitted to the model with addSample(). The model will
> > + * update, and output clock values for known input clock values can be
> > + * obtained using getOutput().
> > + *
> > + * As a convenience, if the input clock is indeed the time since boot, and the
> > + * output clock represents a real wallclock time, then addSample() can be
> > + * called with no arguments, and a pair of timestamps will be captured at
> > + * that moment.
> > + *
> > + * The configure() function accepts some configuration parameters to control
> > + * the linear fitting process.
> > + */
> > +
> > +/**
> > + * \brief Construct a ClockRecovery
> > + */
> > +ClockRecovery::ClockRecovery()
> > +{
> > + configure();
> > + reset();
> > +}
> > +
> > +/**
> > + * \brief Set configuration parameters
> > + * \param[in] numPts The approximate duration for which the state of the model
> > + * is persistent, measured in samples
> > + * \param[in] maxJitter New output samples are clamped to no more than this
> > + * amount of jitter, to prevent sudden swings from having a large effect
> > + * \param[in] minPts The fitted clock model is not used to generate outputs
> > + * until this many samples have been received
> > + * \param[in] errorThreshold If the accumulated differences between input and
> > + * output clocks reaches this amount over a few frames, the model is reset
> > + */
> > +void ClockRecovery::configure(unsigned int numPts, unsigned int maxJitter, unsigned int minPts,
> > + unsigned int errorThreshold)
> > +{
> > + LOG(ClockRec, Debug)
> > + << "configure " << numPts << " " << maxJitter << " " << minPts << " " << errorThreshold;
> > +
> > + numPts_ = numPts;
> > + maxJitter_ = maxJitter;
> > + minPts_ = minPts;
> > + errorThreshold_ = errorThreshold;
> > +}
> > +
> > +/**
> > + * \brief Reset the clock recovery model and start again from scratch
> > + */
> > +void ClockRecovery::reset()
> > +{
> > + LOG(ClockRec, Debug) << "reset";
> > +
> > + lastInput_ = 0;
> > + lastOutput_ = 0;
> > + xAve_ = 0;
> > + yAve_ = 0;
> > + x2Ave_ = 0;
> > + xyAve_ = 0;
> > + count_ = 0;
> > + slope_ = 0.0;
> > + offset_ = 0.0;
> > + error_ = 0.0;
> > +}
> > +
> > +/**
> > + * \brief Add a sample point to the clock recovery model, for recovering a wall
> > + * clock value from the internal system time since boot
> > + *
> > + * This is a convenience function to make it easy to derive a wall clock value
> > + * (using the Linux CLOCK_REALTIME) from the time since the system started
> > + * (measured by CLOCK_BOOTTIME).
> > + */
> > +void ClockRecovery::addSample()
> > +{
> > + LOG(ClockRec, Debug) << "addSample";
> > +
> > + struct timespec bootTime;
> > + struct timespec wallTime;
> > +
> > + /* Get boot and wall clocks in microseconds. */
> > + clock_gettime(CLOCK_BOOTTIME, &bootTime);
> > + clock_gettime(CLOCK_REALTIME, &wallTime);
> > + uint64_t boot = bootTime.tv_sec * 1000000ULL + bootTime.tv_nsec / 1000;
> > + uint64_t wall = wallTime.tv_sec * 1000000ULL + wallTime.tv_nsec / 1000;
>
> It could be that I am missing something that accounts for this, but I am wondering
> if it would make sense to sample one of the clocks twice, and average the two samples.
> i.e.
>
> x1 = clock_boottime()
> y = clock_realtime()
> x2 = clock_boottime()
>
> addSample(midpoint(x1, x2), y)
>
> Otherwise I'd expect a constant offset to be present in the mapping, although
> I am not sure if that makes an appreciable difference.
>
>
> Regards,
> Barnabás Pőcze
That's a good suggestion, so thanks for that, I'll give it a try.
I think you're also right that, in reality, there will be no
meaningful difference - we're only counting microseconds - and for the
purposes of camera synchronisation, fixed offsets don't matter. But
it's still a good thought, so I'll try it!
Thanks
David
>
>
> > +
> > + addSample(boot, wall);
> > +}
> > +
> > +/**
> > + * \brief Add a sample point to the clock recovery model, specifying the exact
> > + * input and output clock values
> > + *
> > + * This function should be used for corresponding clocks other than the Linux
> > + * BOOTTIME and REALTIME clocks.
> > + */
> > +void ClockRecovery::addSample(uint64_t input, uint64_t output)
> > +{
> > + LOG(ClockRec, Debug) << "addSample " << input << " " << output;
> > +
> > + if (count_ == 0) {
> > + inputBase_ = input;
> > + outputBase_ = output;
> > + }
> > +
> > + /*
> > + * We keep an eye on cumulative drift over the last several frames. If this exceeds a
> > + * threshold, then probably the system clock has been updated and we're going to have to
> > + * reset everything and start over.
> > + */
> > + if (lastOutput_) {
> > + int64_t inputDiff = getOutput(input) - getOutput(lastInput_);
> > + int64_t outputDiff = output - lastOutput_;
> > + error_ = error_ * 0.95 + (outputDiff - inputDiff);
> > + if (std::abs(error_) > errorThreshold_) {
> > + reset();
> > + inputBase_ = input;
> > + outputBase_ = output;
> > + }
> > + }
> > + lastInput_ = input;
> > + lastOutput_ = output;
> > +
> > + /*
> > + * Never let the new output value be more than maxJitter_ away from what we would have expected.
> > + * This is just to reduce the effect of sudden large delays in the measured output.
> > + */
> > + uint64_t expectedOutput = getOutput(input);
> > + output = std::clamp(output, expectedOutput - maxJitter_, expectedOutput + maxJitter_);
> > +
> > + /*
> > + * We use x, y, x^2 and x*y sums to calculate the best fit line. Here we update them by
> > + * pretending we have count_ samples at the previous fit, and now one new one. Gradually
> > + * the effect of the older values gets lost. This is a very simple way of updating the
> > + * fit (there are much more complicated ones!), but it works well enough. Using averages
> > + * instead of sums makes the relative effect of old values and the new sample clearer.
> > + */
> > + double x = static_cast<int64_t>(input - inputBase_);
> > + double y = static_cast<int64_t>(output - outputBase_) - x;
> > + unsigned int count1 = count_ + 1;
> > + xAve_ = (count_ * xAve_ + x) / count1;
> > + yAve_ = (count_ * yAve_ + y) / count1;
> > + x2Ave_ = (count_ * x2Ave_ + x * x) / count1;
> > + xyAve_ = (count_ * xyAve_ + x * y) / count1;
> > +
> > + /* Don't update slope and offset until we've seen "enough" sample points. */
> > + if (count_ > minPts_) {
> > + /* These are the standard equations for least squares linear regression. */
> > + slope_ = (count1 * count1 * xyAve_ - count1 * xAve_ * count1 * yAve_) /
> > + (count1 * count1 * x2Ave_ - count1 * xAve_ * count1 * xAve_);
> > + offset_ = yAve_ - slope_ * xAve_;
> > + }
> > +
> > + /* Don't increase count_ above numPts_, as this controls the long-term amount of the residual fit. */
> > + if (count1 < numPts_)
> > + count_++;
> > +}
> > +
> > +/**
> > + * \brief Calculate the output clock value according to the model from an input
> > + * clock value
> > + *
> > + * \return Output clock value
> > + */
> > +uint64_t ClockRecovery::getOutput(uint64_t input)
> > +{
> > + double x = static_cast<int64_t>(input - inputBase_);
> > + double y = slope_ * x + offset_;
> > + uint64_t output = y + x + outputBase_;
> > +
> > + LOG(ClockRec, Debug) << "getOutput " << input << " " << output;
> > +
> > + return output;
> > +}
> > +
> > +} /* namespace libcamera */
> > diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> > index 57fde8a8..4eaa1c8e 100644
> > --- a/src/libcamera/meson.build
> > +++ b/src/libcamera/meson.build
> > @@ -21,6 +21,7 @@ libcamera_internal_sources = files([
> > 'byte_stream_buffer.cpp',
> > 'camera_controls.cpp',
> > 'camera_lens.cpp',
> > + 'clock_recovery.cpp',
> > 'control_serializer.cpp',
> > 'control_validator.cpp',
> > 'converter.cpp',
> > --
> > 2.39.5
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