Implement timestamp translation/filter in VideoCapturer.
Use in AndroidVideoCapturer.
BUG=webrtc:5740
Review-Url: https://codereview.webrtc.org/2017443003
Cr-Commit-Position: refs/heads/master@{#13254}
diff --git a/webrtc/base/timestampaligner.cc b/webrtc/base/timestampaligner.cc
new file mode 100644
index 0000000..e03e4fb
--- /dev/null
+++ b/webrtc/base/timestampaligner.cc
@@ -0,0 +1,107 @@
+/*
+ * 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