1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
|
/*
* 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.
*/
#ifndef RTC_BASE_TIMESTAMP_ALIGNER_H_
#define RTC_BASE_TIMESTAMP_ALIGNER_H_
#include <stdint.h>
#include "rtc_base/system/rtc_export.h"
#include "rtc_base/time_utils.h"
namespace rtc {
// The TimestampAligner class helps translating timestamps of a capture system
// into the same timescale as is used by rtc::TimeMicros(). Some capture systems
// provide timestamps, which comes from the capturing hardware (camera or sound
// card) or stamped close to the capturing hardware. Such timestamps are more
// accurate (less jittery) than reading the system clock, but may have a
// different epoch and unknown clock drift. Frame timestamps in webrtc should
// use rtc::TimeMicros (system monotonic time), and this class provides a filter
// which lets us use the rtc::TimeMicros timescale, and at the same time take
// advantage of higher accuracy of the capturer's clock.
// This class is not thread safe, so all calls to it must be synchronized
// externally.
class RTC_EXPORT TimestampAligner {
public:
TimestampAligner();
~TimestampAligner();
TimestampAligner(const TimestampAligner&) = delete;
TimestampAligner& operator=(const TimestampAligner&) = delete;
public:
// Minimum difference of two timestamps generated by
// "TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us)"
// This avoids the caller from getting two timestamps with the same
// millisecond.
static constexpr int64_t kMinFrameIntervalUs = rtc::kNumMicrosecsPerMillisec;
// Translates timestamps of a capture system to the same timescale as is used
// by rtc::TimeMicros(). `capturer_time_us` is assumed to be accurate, but
// with an unknown epoch and clock drift. `system_time_us` is
// time according to rtc::TimeMicros(), preferably read as soon as
// possible when the frame is captured. It may have poor accuracy
// due to poor resolution or scheduling delays. Returns the
// translated timestamp.
int64_t TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us);
// Returns the translated timestamp without updating the states. This is to
// allow TimestampAligner to translate capturer time into system clock based
// on earlier observations. It won't guarantee monotonicity.
int64_t TranslateTimestamp(int64_t capturer_time_us) const;
protected:
// Update the estimated offset between capturer's time and system monotonic
// time.
int64_t UpdateOffset(int64_t capturer_time_us, int64_t system_time_us);
// Clip timestamp, return value is always
// <= `system_time_us`, and
// >= min(`prev_translated_time_us_` + `kMinFrameIntervalUs`,
// `system_time_us`).
int64_t ClipTimestamp(int64_t filtered_time_us, int64_t system_time_us);
private:
// State for the timestamp translation.
int frames_seen_;
// Estimated offset between capturer's time and system monotonic time.
int64_t offset_us_;
// State for the ClipTimestamp method, applied after the filter.
// A large negative clock drift of the capturer tends to push translated
// timestamps into the future. `clip_bias_us_` is subtracted from the
// translated timestamps, to get them back from the future.
int64_t clip_bias_us_;
// Used to ensure that translated timestamps are monotonous.
int64_t prev_translated_time_us_;
// Offset between `prev_translated_time_us_` and the corresponding capturer
// time.
int64_t prev_time_offset_us_;
};
} // namespace rtc
#endif // RTC_BASE_TIMESTAMP_ALIGNER_H_
|
