1 // Copyright 2015 The Chromium Authors
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_
6 #define MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_
14 #include "base/time/time.h"
15 #include "media/base/media_export.h"
16 #include "third_party/abseil-cpp/absl/types/optional.h"
20 // Estimates whether a given frame duration and render interval length have a
21 // render cadence which would allow for optimal uniformity of displayed frame
22 // durations over time.
24 // Cadence is the ideal repeating frame pattern for a group of frames; currently
25 // VideoCadenceEstimator supports N-frame ([a1:a2:..:aN]) cadences where N <= 5.
26 // Details on what this means are below.
28 // The perfect cadence of a set of frames is the ratio of the frame duration to
29 // render interval length. I.e. for 30fps in 60Hz the cadence would be (1/30) /
30 // (1/60) = 60 / 30 = 2. It's common that this is not an exact integer, e.g.,
31 // 29.974fps in 60Hz which would have a cadence of (1/29.974) / (1/60) =
34 // The perfect cadence is always a real number. All N-cadences [a1:a2:..:aN]
35 // where aK is an integer are an approximation of the perfect cadence; i.e. the
36 // average of [a1:..:aN] will approximately equal the perfect cadence. When N=1
37 // we have a 1-frame cadence, when N=2, we have a 2-frame cadence, etc.
39 // For single frame cadence we just round the perfect cadence (~2.0029 in the
40 // previous example) to the nearest integer value (2 in this case; which is
41 // denoted as a cadence of [2]). If the delta between those values is small we
42 // can choose to render frames for the integer number of render intervals;
43 // shortening or lengthening the actual rendered frame duration. Doing so
44 // ensures each frame gets an optimal amount of display time.
46 // For N-frame cadence, the idea is similar, we just round the perfect cadence
47 // to some K/N, where K is an integer, and distribute [floor(K/N), floor(K/N)+1]
48 // into the cadence vector as evenly as possible. For example, 23.97fps in
49 // 60Hz, the perfect cadence is 2.50313, we can round it to 2.5 = 5/2, and we
50 // can then construct the cadence vector as [2:3].
52 // The delta between the perfect cadence and the rounded cadence leads to drift
53 // over time of the actual VideoFrame timestamp relative to its rendered time,
54 // so we perform some calculations to ensure we only use a cadence when it will
55 // take some time to drift an undesirable amount; see CalculateCadence() for
56 // details on how this calculation is made.
58 // In practice this works out to the following for common setups if we use
59 // cadence based selection:
61 // 29.5fps in 60Hz, ~17ms max drift => exhausted in ~1 second.
62 // 29.9fps in 60Hz, ~17ms max drift => exhausted in ~16.4 seconds.
63 // 24fps in 59.9Hz, ~21ms max drift => exhausted in ~12.6 seconds.
64 // 24.9fps in 60Hz, ~20ms max drift => exhausted in ~4.0 seconds.
65 // 59.9fps in 60Hz, ~8.3ms max drift => exhausted in ~8.2 seconds.
66 // 24.9fps in 50Hz, ~20ms max drift => exhausted in ~20.5 seconds.
67 // 120fps in 59.9Hz, ~8.3ms max drift => exhausted in ~8.2 seconds.
69 class MEDIA_EXPORT VideoCadenceEstimator {
71 using Cadence = std::vector<int>;
73 // As mentioned in the introduction, the determination of whether to clamp to
74 // a given cadence is based on how long it takes before a frame would have to
75 // be dropped or repeated to compensate for reaching the maximum acceptable
76 // drift; this time length is controlled by |minimum_time_until_max_drift|.
77 explicit VideoCadenceEstimator(base::TimeDelta minimum_time_until_max_drift);
79 VideoCadenceEstimator(const VideoCadenceEstimator&) = delete;
80 VideoCadenceEstimator& operator=(const VideoCadenceEstimator&) = delete;
82 ~VideoCadenceEstimator();
84 // Clears stored cadence information.
87 // Updates the estimates for |cadence_| based on the given values as described
88 // in the introduction above.
90 // Clients should call this and then update the cadence for all frames via the
91 // GetCadenceForFrame() method if the cadence changes.
93 // Cadence changes will not take affect until enough render intervals have
94 // elapsed. For the purposes of hysteresis, each UpdateCadenceEstimate() call
95 // is assumed to elapse one |render_interval| worth of time.
97 // Returns true if the cadence has changed since the last call.
98 bool UpdateCadenceEstimate(base::TimeDelta render_interval,
99 base::TimeDelta frame_duration,
100 base::TimeDelta frame_duration_deviation,
101 base::TimeDelta max_acceptable_drift);
103 // Returns true if a useful cadence was found.
104 bool has_cadence() const { return !cadence_.empty(); }
106 // Given a |frame_number|, where zero is the most recently rendered frame,
107 // returns the ideal cadence for that frame.
109 // Note: Callers must track the base |frame_number| relative to all frames
110 // rendered or removed after the first frame for which cadence is detected.
111 // The first frame after cadence is detected has a |frame_number| of 0.
113 // Frames which come in before the last rendered frame should be ignored in
114 // terms of impact to the base |frame_number|.
115 int GetCadenceForFrame(uint64_t frame_number) const;
117 void set_cadence_hysteresis_threshold_for_testing(base::TimeDelta threshold) {
118 cadence_hysteresis_threshold_ = threshold;
121 double avg_cadence_for_testing() const;
122 size_t cadence_size_for_testing() const { return cadence_.size(); }
123 std::string GetCadenceForTesting() const { return CadenceToString(cadence_); }
125 // Determines whether a simple (single-valued) integer cadence exists for
126 // |render_interval| and |frame_duration| that won't drift more than
127 // |render_interval| within |minimum_time_until_max_drift|.
128 static bool HasSimpleCadence(
129 base::TimeDelta render_interval,
130 base::TimeDelta frame_duration,
131 base::TimeDelta minimum_time_until_max_drift = base::Seconds(8));
134 // Attempts to find an N-frame cadence. Returns the cadence vector if cadence
135 // is found and sets |time_until_max_drift| for the computed cadence. If
136 // multiple cadences satisfying the max drift constraint exist, we are going
137 // to return the one with largest |time_until_max_drift|.
138 // For details on the math and algorithm, see https://goo.gl/QK0vbz
139 Cadence CalculateCadence(base::TimeDelta render_interval,
140 base::TimeDelta frame_duration,
141 base::TimeDelta max_acceptable_drift,
142 base::TimeDelta* time_until_max_drift) const;
144 // Converts a cadence vector into a human readable string of the form
146 std::string CadenceToString(const Cadence& cadence) const;
148 bool UpdateBresenhamCadenceEstimate(base::TimeDelta render_interval,
149 base::TimeDelta frame_duration);
151 // The approximate best N-frame cadence for all frames seen thus far; updated
152 // by UpdateCadenceEstimate(). Empty when no cadence has been detected.
155 // Used as hysteresis to prevent oscillation between cadence approximations
156 // for spurious blips in the render interval or frame duration.
158 // Once a new cadence is detected, |render_intervals_cadence_held_| is
159 // incremented for each UpdateCadenceEstimate() call where |cadence_| matches
160 // |pending_cadence_|. |render_intervals_cadence_held_| is cleared when a
161 // "new" cadence matches |cadence_| or |pending_cadence_|.
163 // Once |kMinimumCadenceDurationMs| is exceeded in render intervals, the
164 // detected cadence is set in |cadence_|.
165 Cadence pending_cadence_;
166 int render_intervals_cadence_held_;
167 base::TimeDelta cadence_hysteresis_threshold_;
169 // Tracks how many times cadence has switched during a given playback, used to
170 // histogram the number of cadence changes in a playback.
171 bool first_update_call_;
172 int cadence_changes_;
174 // The minimum amount of time allowed before a glitch occurs before confirming
175 // cadence for a given render interval and frame duration.
176 const base::TimeDelta minimum_time_until_max_drift_;
178 bool is_variable_frame_rate_;
183 #endif // MEDIA_FILTERS_VIDEO_CADENCE_ESTIMATOR_H_