chromium / chromium / src / 7462ce9906fb2da25c515d3e5b94c173a4c29b9d / . / media / filters / video_cadence_estimator.h

// Copyright 2015 The Chromium Authors. All rights reserved. | |

// Use of this source code is governed by a BSD-style license that can be | |

// found in the LICENSE file. | |

#ifndef MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_ | |

#define MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_ | |

#include <stddef.h> | |

#include <stdint.h> | |

#include <vector> | |

#include "base/macros.h" | |

#include "base/time/time.h" | |

#include "media/base/media_export.h" | |

namespace media { | |

// Estimates whether a given frame duration and render interval length have a | |

// render cadence which would allow for optimal uniformity of displayed frame | |

// durations over time. | |

// | |

// Cadence is the ideal repeating frame pattern for a group of frames; currently | |

// VideoCadenceEstimator supports N-frame ([a1:a2:..:aN]) cadences where N <= 5. | |

// Details on what this means are below. | |

// | |

// The perfect cadence of a set of frames is the ratio of the frame duration to | |

// render interval length. I.e. for 30fps in 60Hz the cadence would be (1/30) / | |

// (1/60) = 60 / 30 = 2. It's common that this is not an exact integer, e.g., | |

// 29.974fps in 60Hz which would have a cadence of (1/29.974) / (1/60) = | |

// ~2.0029. | |

// | |

// The perfect cadence is always a real number. All N-cadences [a1:a2:..:aN] | |

// where aK is an integer are an approximation of the perfect cadence; i.e. the | |

// average of [a1:..:aN] will approximately equal the perfect cadence. When N=1 | |

// we have a 1-frame cadence, when N=2, we have a 2-frame cadence, etc. | |

// | |

// For single frame cadence we just round the perfect cadence (~2.0029 in the | |

// previous example) to the nearest integer value (2 in this case; which is | |

// denoted as a cadence of [2]). If the delta between those values is small we | |

// can choose to render frames for the integer number of render intervals; | |

// shortening or lengthening the actual rendered frame duration. Doing so | |

// ensures each frame gets an optimal amount of display time. | |

// | |

// For N-frame cadence, the idea is similar, we just round the perfect cadence | |

// to some K/N, where K is an integer, and distribute [floor(K/N), floor(K/N)+1] | |

// into the cadence vector as evenly as possible. For example, 23.97fps in | |

// 60Hz, the perfect cadence is 2.50313, we can round it to 2.5 = 5/2, and we | |

// can then construct the cadence vector as [2:3]. | |

// | |

// The delta between the perfect cadence and the rounded cadence leads to drift | |

// over time of the actual VideoFrame timestamp relative to its rendered time, | |

// so we perform some calculations to ensure we only use a cadence when it will | |

// take some time to drift an undesirable amount; see CalculateCadence() for | |

// details on how this calculation is made. | |

// | |

// In practice this works out to the following for common setups if we use | |

// cadence based selection: | |

// | |

// 29.5fps in 60Hz, ~17ms max drift => exhausted in ~1 second. | |

// 29.9fps in 60Hz, ~17ms max drift => exhausted in ~16.4 seconds. | |

// 24fps in 59.9Hz, ~21ms max drift => exhausted in ~12.6 seconds. | |

// 24.9fps in 60Hz, ~20ms max drift => exhausted in ~4.0 seconds. | |

// 59.9fps in 60Hz, ~8.3ms max drift => exhausted in ~8.2 seconds. | |

// 24.9fps in 50Hz, ~20ms max drift => exhausted in ~20.5 seconds. | |

// 120fps in 59.9Hz, ~8.3ms max drift => exhausted in ~8.2 seconds. | |

// | |

class MEDIA_EXPORT VideoCadenceEstimator { | |

public: | |

using Cadence = std::vector<int>; | |

// As mentioned in the introduction, the determination of whether to clamp to | |

// a given cadence is based on how long it takes before a frame would have to | |

// be dropped or repeated to compensate for reaching the maximum acceptable | |

// drift; this time length is controlled by |minimum_time_until_max_drift|. | |

explicit VideoCadenceEstimator(base::TimeDelta minimum_time_until_max_drift); | |

~VideoCadenceEstimator(); | |

// Clears stored cadence information. | |

void Reset(); | |

// Updates the estimates for |cadence_| based on the given values as described | |

// in the introduction above. | |

// | |

// Clients should call this and then update the cadence for all frames via the | |

// GetCadenceForFrame() method if the cadence changes. | |

// | |

// Cadence changes will not take affect until enough render intervals have | |

// elapsed. For the purposes of hysteresis, each UpdateCadenceEstimate() call | |

// is assumed to elapse one |render_interval| worth of time. | |

// | |

// Returns true if the cadence has changed since the last call. | |

bool UpdateCadenceEstimate(base::TimeDelta render_interval, | |

base::TimeDelta frame_duration, | |

base::TimeDelta frame_duration_deviation, | |

base::TimeDelta max_acceptable_drift); | |

// Returns true if a useful cadence was found. | |

bool has_cadence() const { return !cadence_.empty(); } | |

// Given a |frame_number|, where zero is the most recently rendered frame, | |

// returns the ideal cadence for that frame. | |

// | |

// Note: Callers must track the base |frame_number| relative to all frames | |

// rendered or removed after the first frame for which cadence is detected. | |

// The first frame after cadence is detected has a |frame_number| of 0. | |

// | |

// Frames which come in before the last rendered frame should be ignored in | |

// terms of impact to the base |frame_number|. | |

int GetCadenceForFrame(uint64_t frame_number) const; | |

void set_cadence_hysteresis_threshold_for_testing(base::TimeDelta threshold) { | |

cadence_hysteresis_threshold_ = threshold; | |

} | |

size_t cadence_size_for_testing() const { return cadence_.size(); } | |

std::string GetCadenceForTesting() const { return CadenceToString(cadence_); } | |

private: | |

// Attempts to find an N-frame cadence. Returns the cadence vector if cadence | |

// is found and sets |time_until_max_drift| for the computed cadence. If | |

// multiple cadences satisfying the max drift constraint exist, we are going | |

// to return the one with largest |time_until_max_drift|. | |

// For details on the math and algorithm, see https://goo.gl/QK0vbz | |

Cadence CalculateCadence(base::TimeDelta render_interval, | |

base::TimeDelta frame_duration, | |

base::TimeDelta max_acceptable_drift, | |

base::TimeDelta* time_until_max_drift) const; | |

// Converts a cadence vector into a human readable string of the form | |

// "[a: b: ...: z]". | |

std::string CadenceToString(const Cadence& cadence) const; | |

// The approximate best N-frame cadence for all frames seen thus far; updated | |

// by UpdateCadenceEstimate(). Empty when no cadence has been detected. | |

Cadence cadence_; | |

// Used as hysteresis to prevent oscillation between cadence approximations | |

// for spurious blips in the render interval or frame duration. | |

// | |

// Once a new cadence is detected, |render_intervals_cadence_held_| is | |

// incremented for each UpdateCadenceEstimate() call where |cadence_| matches | |

// |pending_cadence_|. |render_intervals_cadence_held_| is cleared when a | |

// "new" cadence matches |cadence_| or |pending_cadence_|. | |

// | |

// Once |kMinimumCadenceDurationMs| is exceeded in render intervals, the | |

// detected cadence is set in |cadence_|. | |

Cadence pending_cadence_; | |

int render_intervals_cadence_held_; | |

base::TimeDelta cadence_hysteresis_threshold_; | |

// Tracks how many times cadence has switched during a given playback, used to | |

// histogram the number of cadence changes in a playback. | |

bool first_update_call_; | |

int cadence_changes_; | |

// The minimum amount of time allowed before a glitch occurs before confirming | |

// cadence for a given render interval and frame duration. | |

const base::TimeDelta minimum_time_until_max_drift_; | |

bool is_variable_frame_rate_; | |

DISALLOW_COPY_AND_ASSIGN(VideoCadenceEstimator); | |

}; | |

} // namespace media | |

#endif // MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_ |