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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; };