0bade0df3b
WebRTC no longer has any restriction on what thread frames should be delivered on. One possible problem with this CL is that NV21->I420 conversion and scaling is done on the thread that delivers frames, which might cause fps regressions. R=nisse@webrtc.org, perkj@webrtc.org, tkchin@webrtc.org Review URL: https://codereview.webrtc.org/2137503003 . Cr-Commit-Position: refs/heads/master@{#14021}
109 lines
4.1 KiB
C++
109 lines
4.1 KiB
C++
/*
|
|
* Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
|
|
*
|
|
* Use of this source code is governed by a BSD-style license
|
|
* that can be found in the LICENSE file in the root of the source
|
|
* tree. An additional intellectual property rights grant can be found
|
|
* in the file PATENTS. All contributing project authors may
|
|
* be found in the AUTHORS file in the root of the source tree.
|
|
*/
|
|
|
|
#include "webrtc/base/logging.h"
|
|
#include "webrtc/base/timestampaligner.h"
|
|
|
|
namespace rtc {
|
|
|
|
TimestampAligner::TimestampAligner() : frames_seen_(0), offset_us_(0) {}
|
|
|
|
TimestampAligner::~TimestampAligner() {}
|
|
|
|
int64_t TimestampAligner::UpdateOffset(int64_t camera_time_us,
|
|
int64_t system_time_us) {
|
|
// Estimate the offset between system monotonic time and the capture
|
|
// time from the camera. The camera is assumed to provide more
|
|
// accurate timestamps than we get from the system time. But the
|
|
// camera may use its own free-running clock with a large offset and
|
|
// a small drift compared to the system clock. So the model is
|
|
// basically
|
|
//
|
|
// y_k = c_0 + c_1 * x_k + v_k
|
|
//
|
|
// where x_k is the camera timestamp, believed to be accurate in its
|
|
// own scale. y_k is our reading of the system clock. v_k is the
|
|
// measurement noise, i.e., the delay from frame capture until the
|
|
// system clock was read.
|
|
//
|
|
// It's possible to do (weighted) least-squares estimation of both
|
|
// c_0 and c_1. Then we get the constants as c_1 = Cov(x,y) /
|
|
// Var(x), and c_0 = mean(y) - c_1 * mean(x). Substituting this c_0,
|
|
// we can rearrange the model as
|
|
//
|
|
// y_k = mean(y) + (x_k - mean(x)) + (c_1 - 1) * (x_k - mean(x)) + v_k
|
|
//
|
|
// Now if we use a weighted average which gradually forgets old
|
|
// values, x_k - mean(x) is bounded, of the same order as the time
|
|
// constant (and close to constant for a steady frame rate). In
|
|
// addition, the frequency error |c_1 - 1| should be small. Cameras
|
|
// with a frequency error up to 3000 ppm (3 ms drift per second)
|
|
// have been observed, but frequency errors below 100 ppm could be
|
|
// expected of any cheap crystal.
|
|
//
|
|
// Bottom line is that we ignore the c_1 term, and use only the estimator
|
|
//
|
|
// x_k + mean(y-x)
|
|
//
|
|
// where mean is plain averaging for initial samples, followed by
|
|
// exponential averaging.
|
|
|
|
// The input for averaging, y_k - x_k in the above notation.
|
|
int64_t diff_us = system_time_us - camera_time_us;
|
|
// The deviation from the current average.
|
|
int64_t error_us = diff_us - offset_us_;
|
|
|
|
// If the current difference is far from the currently estimated
|
|
// offset, the filter is reset. This could happen, e.g., if the
|
|
// camera clock is reset, or cameras are plugged in and out, or if
|
|
// the application process is temporarily suspended. The limit of
|
|
// 300 ms should make this unlikely in normal operation, and at the
|
|
// same time, converging gradually rather than resetting the filter
|
|
// should be tolerable for jumps in camera time below this
|
|
// threshold.
|
|
static const int64_t kResetLimitUs = 300000;
|
|
if (std::abs(error_us) > kResetLimitUs) {
|
|
LOG(LS_INFO) << "Resetting timestamp translation after averaging "
|
|
<< frames_seen_ << " frames. Old offset: " << offset_us_
|
|
<< ", new offset: " << diff_us;
|
|
frames_seen_ = 0;
|
|
prev_translated_time_us_ = rtc::Optional<int64_t>();
|
|
}
|
|
|
|
static const int kWindowSize = 100;
|
|
if (frames_seen_ < kWindowSize) {
|
|
++frames_seen_;
|
|
}
|
|
offset_us_ += error_us / frames_seen_;
|
|
return offset_us_;
|
|
}
|
|
|
|
int64_t TimestampAligner::ClipTimestamp(int64_t time_us,
|
|
int64_t system_time_us) {
|
|
// Make timestamps monotonic.
|
|
if (!prev_translated_time_us_) {
|
|
// Initialize.
|
|
clip_bias_us_ = 0;
|
|
} else if (time_us < *prev_translated_time_us_) {
|
|
time_us = *prev_translated_time_us_;
|
|
}
|
|
|
|
// Clip to make sure we don't produce time stamps in the future.
|
|
time_us -= clip_bias_us_;
|
|
if (time_us > system_time_us) {
|
|
clip_bias_us_ += time_us - system_time_us;
|
|
time_us = system_time_us;
|
|
}
|
|
prev_translated_time_us_ = rtc::Optional<int64_t>(time_us);
|
|
return time_us;
|
|
}
|
|
|
|
} // namespace rtc
|