webrtc/modules/video_coding/inter_frame_delay.cc

/*
 *  Copyright (c) 2011 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 "webrtc/modules/video_coding/inter_frame_delay.h"

namespace webrtc {

VCMInterFrameDelay::VCMInterFrameDelay(int64_t currentWallClock)
{
    Reset(currentWallClock);
}

// Resets the delay estimate
void
VCMInterFrameDelay::Reset(int64_t currentWallClock)
{
    _zeroWallClock = currentWallClock;
    _wrapArounds = 0;
    _prevWallClock = 0;
    _prevTimestamp = 0;
    _dTS = 0;
}

// Calculates the delay of a frame with the given timestamp.
// This method is called when the frame is complete.
bool
VCMInterFrameDelay::CalculateDelay(uint32_t timestamp,
                                int64_t *delay,
                                int64_t currentWallClock)
{
    if (_prevWallClock == 0)
    {
        // First set of data, initialization, wait for next frame
        _prevWallClock = currentWallClock;
        _prevTimestamp = timestamp;
        *delay = 0;
        return true;
    }

    int32_t prevWrapArounds = _wrapArounds;
    CheckForWrapArounds(timestamp);

    // This will be -1 for backward wrap arounds and +1 for forward wrap arounds
    int32_t wrapAroundsSincePrev = _wrapArounds - prevWrapArounds;

    // Account for reordering in jitter variance estimate in the future?
    // Note that this also captures incomplete frames which are grabbed
    // for decoding after a later frame has been complete, i.e. real
    // packet losses.
    if ((wrapAroundsSincePrev == 0 && timestamp < _prevTimestamp) || wrapAroundsSincePrev < 0)
    {
        *delay = 0;
        return false;
    }

    // Compute the compensated timestamp difference and convert it to ms and
    // round it to closest integer.
    _dTS = static_cast<int64_t>((timestamp + wrapAroundsSincePrev *
                (static_cast<int64_t>(1)<<32) - _prevTimestamp) / 90.0 + 0.5);

    // frameDelay is the difference of dT and dTS -- i.e. the difference of
    // the wall clock time difference and the timestamp difference between
    // two following frames.
    *delay = static_cast<int64_t>(currentWallClock - _prevWallClock - _dTS);

    _prevTimestamp = timestamp;
    _prevWallClock = currentWallClock;

    return true;
}

// Returns the current difference between incoming timestamps
uint32_t VCMInterFrameDelay::CurrentTimeStampDiffMs() const
{
    if (_dTS < 0)
    {
        return 0;
    }
    return static_cast<uint32_t>(_dTS);
}

// Investigates if the timestamp clock has overflowed since the last timestamp and
// keeps track of the number of wrap arounds since reset.
void
VCMInterFrameDelay::CheckForWrapArounds(uint32_t timestamp)
{
    if (timestamp < _prevTimestamp)
    {
        // This difference will probably be less than -2^31 if we have had a wrap around
        // (e.g. timestamp = 1, _previousTimestamp = 2^32 - 1). Since it is cast to a Word32,
        // it should be positive.
        if (static_cast<int32_t>(timestamp - _prevTimestamp) > 0)
        {
            // Forward wrap around
            _wrapArounds++;
        }
    }
    // This difference will probably be less than -2^31 if we have had a backward wrap around.
    // Since it is cast to a Word32, it should be positive.
    else if (static_cast<int32_t>(_prevTimestamp - timestamp) > 0)
    {
        // Backward wrap around
        _wrapArounds--;
    }
}

}