| /* |
| * 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 |