86 lines
3.4 KiB
C
86 lines
3.4 KiB
C
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/*
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* Copyright (c) 2016 The WebRTC Project Authors. All rights reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#ifndef RTC_BASE_TIMESTAMP_ALIGNER_H_
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#define RTC_BASE_TIMESTAMP_ALIGNER_H_
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#include <stdint.h>
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#include "rtc_base/constructor_magic.h"
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#include "rtc_base/system/rtc_export.h"
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namespace rtc {
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// The TimestampAligner class helps translating timestamps of a capture system
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// into the same timescale as is used by rtc::TimeMicros(). Some capture systems
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// provide timestamps, which comes from the capturing hardware (camera or sound
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// card) or stamped close to the capturing hardware. Such timestamps are more
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// accurate (less jittery) than reading the system clock, but may have a
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// different epoch and unknown clock drift. Frame timestamps in webrtc should
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// use rtc::TimeMicros (system monotonic time), and this class provides a filter
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// which lets us use the rtc::TimeMicros timescale, and at the same time take
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// advantage of higher accuracy of the capturer's clock.
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// This class is not thread safe, so all calls to it must be synchronized
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// externally.
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class RTC_EXPORT TimestampAligner {
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public:
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TimestampAligner();
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~TimestampAligner();
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public:
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// Translates timestamps of a capture system to the same timescale as is used
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// by rtc::TimeMicros(). |capturer_time_us| is assumed to be accurate, but
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// with an unknown epoch and clock drift. |system_time_us| is
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// time according to rtc::TimeMicros(), preferably read as soon as
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// possible when the frame is captured. It may have poor accuracy
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// due to poor resolution or scheduling delays. Returns the
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// translated timestamp.
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int64_t TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us);
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// Returns the translated timestamp without updating the states. This is to
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// allow TimestampAligner to translate capturer time into system clock based
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// on earlier observations. It won't guarantee monotonicity.
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int64_t TranslateTimestamp(int64_t capturer_time_us) const;
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protected:
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// Update the estimated offset between capturer's time and system monotonic
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// time.
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int64_t UpdateOffset(int64_t capturer_time_us, int64_t system_time_us);
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// Clip timestamp, return value is always
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// <= |system_time_us|, and
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// >= min(|prev_translated_time_us_| + |kMinFrameIntervalUs|,
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// |system_time_us|).
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int64_t ClipTimestamp(int64_t filtered_time_us, int64_t system_time_us);
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private:
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// State for the timestamp translation.
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int frames_seen_;
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// Estimated offset between capturer's time and system monotonic time.
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int64_t offset_us_;
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// State for the ClipTimestamp method, applied after the filter.
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// A large negative clock drift of the capturer tends to push translated
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// timestamps into the future. |clip_bias_us_| is subtracted from the
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// translated timestamps, to get them back from the future.
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int64_t clip_bias_us_;
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// Used to ensure that translated timestamps are monotonous.
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int64_t prev_translated_time_us_;
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// Offset between |prev_translated_time_us_| and the corresponding capturer
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// time.
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int64_t prev_time_offset_us_;
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RTC_DISALLOW_COPY_AND_ASSIGN(TimestampAligner);
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};
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} // namespace rtc
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#endif // RTC_BASE_TIMESTAMP_ALIGNER_H_
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