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/*
* 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 <limits>
#include "rtc_base/gunit.h"
#include "rtc_base/random.h"
#include "rtc_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));
}
}
class TimestampAlignerForTest : public TimestampAligner {
// Make internal methods accessible to testing.
public:
using TimestampAligner::UpdateOffset;
using TimestampAligner::ClipTimestamp;
};
void TestTimestampFilter(double rel_freq_error) {
TimestampAlignerForTest timestamp_aligner_for_test;
TimestampAligner timestamp_aligner;
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_for_test.UpdateOffset(
camera_time_us, system_measured_us);
int64_t filtered_time_us = camera_time_us + offset_us;
int64_t translated_time_us = timestamp_aligner_for_test.ClipTimestamp(
filtered_time_us, system_measured_us);
// Check that we get identical result from the all-in-one helper method.
ASSERT_EQ(translated_time_us, timestamp_aligner.TranslateTimestamp(
camera_time_us, system_measured_us));
EXPECT_LE(translated_time_us, system_measured_us);
EXPECT_GE(translated_time_us,
prev_translated_time_us + rtc::kNumMicrosecsPerMillisec);
// 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;
}
}
} // Anonymous namespace
TEST(TimestampAlignerTest, AttenuateTimestampJitterNoDrift) {
TestTimestampFilter(0.0);
}
// 100 ppm is a worst case for a reasonable crystal.
TEST(TimestampAlignerTest, AttenuateTimestampJitterSmallPosDrift) {
TestTimestampFilter(0.0001);
}
TEST(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(TimestampAlignerTest, AttenuateTimestampJitterLargePosDrift) {
TestTimestampFilter(0.003);
}
TEST(TimestampAlignerTest, AttenuateTimestampJitterLargeNegDrift) {
TestTimestampFilter(-0.003);
}
// Exhibits a mostly hypothetical problem, where certain inputs to the
// TimestampAligner.UpdateOffset filter result in non-monotonous
// translated timestamps. This test verifies that the ClipTimestamp
// logic handles this case correctly.
TEST(TimestampAlignerTest, ClipToMonotonous) {
TimestampAlignerForTest timestamp_aligner;
// For system time stamps { 0, s1, s1 + s2 }, and camera timestamps
// {0, c1, c1 + c2}, we exhibit non-monotonous behaviour if and only
// if c1 > s1 + 2 s2 + 4 c2.
const int kNumSamples = 3;
const int64_t camera_time_us[kNumSamples] = {0, 80000, 90001};
const int64_t system_time_us[kNumSamples] = {0, 10000, 20000};
const int64_t expected_offset_us[kNumSamples] = {0, -35000, -46667};
// Non-monotonic translated timestamps can happen when only for
// translated timestamps in the future. Which is tolerated if
// |timestamp_aligner.clip_bias_us| is large enough. Instead of
// changing that private member for this test, just add the bias to
// |system_time_us| when calling ClipTimestamp.
const int64_t kClipBiasUs = 100000;
bool did_clip = false;
int64_t prev_timestamp_us = std::numeric_limits<int64_t>::min();
for (int i = 0; i < kNumSamples; i++) {
int64_t offset_us =
timestamp_aligner.UpdateOffset(camera_time_us[i], system_time_us[i]);
EXPECT_EQ(offset_us, expected_offset_us[i]);
int64_t translated_timestamp_us = camera_time_us[i] + offset_us;
int64_t clip_timestamp_us = timestamp_aligner.ClipTimestamp(
translated_timestamp_us, system_time_us[i] + kClipBiasUs);
if (translated_timestamp_us <= prev_timestamp_us) {
did_clip = true;
EXPECT_EQ(clip_timestamp_us,
prev_timestamp_us + rtc::kNumMicrosecsPerMillisec);
} else {
// No change from clipping.
EXPECT_EQ(clip_timestamp_us, translated_timestamp_us);
}
prev_timestamp_us = clip_timestamp_us;
}
EXPECT_TRUE(did_clip);
}
} // namespace rtc
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