sdk/fidl/fuchsia.media/timeline_function.fidl

// 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;
};