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* 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 <stdint.h>
#include "rtc_base/system/rtc_export.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 {
TimestampAligner(const TimestampAligner&) = delete;
TimestampAligner& operator=(const TimestampAligner&) = delete;
// 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;
// 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);
// 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