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 // Copyright 2018 The Fuchsia Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. library fuchsia.media; // A TimelineFunction represents a relationship between a subject timeline and a // reference timeline with a linear relation. // // For example, consider a common use case in which reference time is the // monotonic clock of a system and subject time is intended presentation time // for some media such as a video. // // |reference_time| is the value of the monotonic clock at the beginning of // playback. |subject_time| is 0 assuming playback starts at the beginning of // the media. We then choose a |reference_delta| and |subject_delta| so that // |subject_delta| / |reference_delta| represents the desired playback rate, // e.g. 0/1 for paused and 1/1 for normal playback. // // ## Formulas // // With a function we can determine the subject timeline value |s| in terms of // reference timeline value |r| with this formula (where |reference_delta| > 0): // // s = (r - reference_time) * (subject_delta / reference_delta) + subject_time // // And similarly we can find the reference timeline value |r| in terms of // subject timeline value |s| with this formula (where |subject_delta| > 0): // // r = (s - subject_time) * (reference_delta / subject_delta) + referenc_time // // ## Choosing time values // // Time values can be arbitrary and our linear relation will of course be the // same, but we can use them to represent the bounds of pieces in a piecewise // linear relation. // // For example, if a user performs skip-chapter, we might want to describe // this with a TimelineFunction whose |subject_time| is the time to skip to, // |reference_time| is now plus some epsilon, and delta ratio is 1/1 for normal // playback rate. struct TimelineFunction { // A value from the subject timeline that correlates to reference_time. int64 subject_time = 0; // A value from the reference timeline that correlates to subject_time. int64 reference_time = 0; // The change in the subject timeline corresponding to reference_delta. uint32 subject_delta = 0; // The change in the reference timeline corresponding to subject_delta. // Cannot be zero. uint32 reference_delta = 1; };