Implement timestamp translation/filter in VideoCapturer.

webrtc/base/timestampaligner_unittest.cc

+/*
+ *  Copyright 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 <math.h>
+
+#include <algorithm>
+
+#include "webrtc/base/gunit.h"
+#include "webrtc/base/random.h"
+#include "webrtc/base/timestampaligner.h"
+
+namespace rtc {
+
+namespace {
+// Computes the difference x_k - mean(x), when x_k is the linear sequence x_k =
+// k, and the "mean" is plain mean for the first |window_size| samples, followed
+// by exponential averaging with weight 1 / |window_size| for each new sample.
+// This is needed to predict the effect of camera clock drift on the timestamp
+// translation. See the comment on TimestampAligner::UpdateOffset for more
+// context.
+double MeanTimeDifference(int nsamples, int window_size) {
+  if (nsamples <= window_size) {
+    // Plain averaging.
+    return nsamples / 2.0;
+  } else {
+    // Exponential convergence towards
+    // interval_error * (window_size - 1)
+    double alpha = 1.0 - 1.0 / window_size;
+
+    return ((window_size - 1) -
+            (window_size / 2.0 - 1) * pow(alpha, nsamples - window_size));
+  }
+}
+
+}  // Anonymous namespace
+
+class TimestampAlignerTest : public testing::Test {
+ protected:
+  void TestTimestampFilter(double rel_freq_error) {
+    const int64_t kEpoch = 10000;
+    const int64_t kJitterUs = 5000;
+    const int64_t kIntervalUs = 33333;  // 30 FPS
+    const int kWindowSize = 100;
+    const int kNumFrames = 3 * kWindowSize;
+
+    int64_t interval_error_us = kIntervalUs * rel_freq_error;
+    int64_t system_start_us = rtc::TimeMicros();
+    webrtc::Random random(17);
+
+    int64_t prev_translated_time_us = system_start_us;
+
+    for (int i = 0; i < kNumFrames; i++) {
+      // Camera time subject to drift.
+      int64_t camera_time_us = kEpoch + i * (kIntervalUs + interval_error_us);
+      int64_t system_time_us = system_start_us + i * kIntervalUs;
+      // And system time readings are subject to jitter.
+      int64_t system_measured_us = system_time_us + random.Rand(kJitterUs);
+
+      int64_t offset_us =
+          timestamp_aligner_.UpdateOffset(camera_time_us, system_measured_us);
+
+      int64_t filtered_time_us = camera_time_us + offset_us;
+      int64_t translated_time_us = timestamp_aligner_.ClipTimestamp(
+          filtered_time_us, system_measured_us);
+
+      EXPECT_LE(translated_time_us, system_measured_us);
+      EXPECT_GE(translated_time_us, prev_translated_time_us);
+
+      // The relative frequency error contributes to the expected error
+      // by a factor which is the difference between the current time
+      // and the average of earlier sample times.
+      int64_t expected_error_us =
+          kJitterUs / 2 +
+          rel_freq_error * kIntervalUs * MeanTimeDifference(i, kWindowSize);
+
+      int64_t bias_us = filtered_time_us - translated_time_us;
+      EXPECT_GE(bias_us, 0);
+
+      if (i == 0) {
+        EXPECT_EQ(translated_time_us, system_measured_us);
+      } else {
+        EXPECT_NEAR(filtered_time_us, system_time_us + expected_error_us,
+                    2.0 * kJitterUs / sqrt(std::max(i, kWindowSize)));
+      }
+      // If the camera clock runs too fast (rel_freq_error > 0.0), The
+      // bias is expected to roughly cancel the expected error from the
+      // clock drift, as this grows. Otherwise, it reflects the
+      // measurement noise. The tolerances here were selected after some
+      // trial and error.
+      if (i < 10 || rel_freq_error <= 0.0) {
+        EXPECT_LE(bias_us, 3000);
+      } else {
+        EXPECT_NEAR(bias_us, expected_error_us, 1500);
+      }
+      prev_translated_time_us = translated_time_us;
+    }
+  }
+
+ private:
+  TimestampAligner timestamp_aligner_;
+};
+
+TEST_F(TimestampAlignerTest, AttenuateTimestampJitterNoDrift) {
+  TestTimestampFilter(0.0);
+}
+
+// 100 ppm is a worst case for a reasonable crystal.
+TEST_F(TimestampAlignerTest, AttenuateTimestampJitterSmallPosDrift) {
+  TestTimestampFilter(0.0001);
+}
+
+TEST_F(TimestampAlignerTest, AttenuateTimestampJitterSmallNegDrift) {
+  TestTimestampFilter(-0.0001);
+}
+
+// 3000 ppm, 3 ms / s, is the worst observed drift, see
+// https://bugs.chromium.org/p/webrtc/issues/detail?id=5456
+TEST_F(TimestampAlignerTest, AttenuateTimestampJitterLargePosDrift) {
+  TestTimestampFilter(0.003);
+}
+
+TEST_F(TimestampAlignerTest, AttenuateTimestampJitterLargeNegDrift) {
+  TestTimestampFilter(-0.003);
+}
+
+}  // namespace rtc