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<!DOCTYPE html>
<!-- vim: set expandtab ts=2 sw=2 tw=80: -->
<html>
<head>
<title>Web Animations 1.0</title>
<meta charset='utf-8'>
<script src="respec/respec.js" class="remove"></script>
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specStatus: "ED",
// the specification's short name, as in
// http://www.w3.org/TR/short-name/
shortName: "web-animations",
// if your specification has a subtitle that goes below the main
// formal title, define it here
// subtitle : "an excellent document",
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publishDate: "2013-06-25",
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// the start date here:
// copyrightStart: "2012"
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// its YYYY-MM-DD date and its maturity status
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edDraftURI: "http://dev.w3.org/FXTF/web-anim/web-animations.html",
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</head>
<body>
<section id="abstract">
This specification defines a model for synchronization and timing of
changes to the presentation of a Web page.
This specification also defines an application programming interface for
interacting with this model and it is expected that further specifications
will define declarative means for exposing these features.
</section>
<section class="informative">
<h2>Introduction</h2>
<p>
Web Animations defines a model for supporting animation and
synchronization on the Web platform.
It is intended that other specifications will build on this model and
expose its features through declarative means.
In addition, this specification also defines a programming interface to
the model that may be implemented by user agents that provide support
for scripting.
</p>
<section>
<h3>Use cases</h3>
<p>
The Web Animations model aims at two broad areas of application:
</p>
<dl>
<dt>User interface effects</dt>
<dd>
<p>
Animation can be used to give visual clues and feedback to make
a user interface more readily comprehensible.
</p>
<p>
For example, a user action results in a table row being
removed to represent an item being removed from a shopping cart.
In such a case, fading the row to transparent and then shifting
the subsequent rows up to fill the space over a few hundred
milliseconds provides the user with clear feedback as to the
results of their action as opposed to instantly removing the row
from the DOM.
</p>
<p>
To support this scenario not only are the animated effects of
fading and shifting required, but so is synchronization, both
between the animations, and between animations and scripted
actions (removing the table row from the DOM after the animations
have completed).
</p>
</dd>
<dt>Storytelling and visualisation</dt>
<dd>
<p>
Another type of animation uses the animated effect to convey
a story or represent some information.
Unlike user interface effects which are largely a presentational
adjunct to the content, these animations form an essential part of
the content presented to the user.
</p>
<p>
For example, in an animated cartoon two cats fly through space
to another planet leaving a rainbow trail behind them.
After arriving at the planet a change of scene occurs and the user
should decide whether or not the cats enter a magic mountain by
selecting one of two preset destinations in the scene.
</p>
<p>
This scenario requires the following features:
</p>
<ul>
<li>animated effects for moving characters along a path as well as
warping a path (the rainbow trail),
<li>synchronization that allows some actions to happen
simultaneously (the two cats moving) and others in sequence
(the change of scene),
<li>play control to allow rewinding the cartoon, or changing its
playback rate to accommodate particular learning or
accessibility needs,
<li>the ability to trigger animations in response to user input
</ul>
<p>
Similar use cases in this category include visualising physical
phenomena such as spring motion for educational purposes,
or visualising data such as the prevalence of a disease over
a geographical space over a year whereby animation is used to
present the time-based component of the data.
</p>
</dd>
</dl>
</section>
<section>
<h3>Relationship to other specifications</h3>
<p>
CSS Transitions [[CSS3-TRANSITIONS]], CSS Animations
[[CSS3-ANIMATIONS]], and SVG [[SVG112]] all provide mechanisms that
generate animated content on a Web page.
Although the three specifications provide many similar features,
they are described in different terms.
This specification proposes an abstract animation model that
encompasses the common features of all three specifications.
This model is backwards-compatible with the current behavior of these
specifications such that they can be defined in terms of this model
without any observable change.
</p>
<p>
The animation features in SVG 1.1 are defined in terms of SMIL
Animation [[SMIL-ANIMATION]].
It is intended that by defining SVG's animation features in terms of
the Web Animations model, the dependency between SVG and SMIL
Animation can be removed.
</p>
<p>
The programming interface component of this specification makes
some additions to interfaces defined in HTML5 [[HTML5]].
</p>
</section>
<section>
<h3>Overview of this specification</h3>
<p>
This specification begins by defining an abstract model for animation.
This is followed by a programming interface defined in terms of the
abstract model.
The programming interface is defined in terms of the abstract model
and is only relevant to user agents that provide scripting support.
</p>
</section>
</section>
<section class="informative">
<h2>Web Animations model overview</h2>
<p>
At a glance, the Web Animations model consists of two largely
independent pieces, a <em>timing model</em> and an <em>animation
model</em>. The role of these pieces is as follows:
</p>
<dl>
<dt>Timing model</dt>
<dd>
Takes a moment in time and converts it to a proportional distance
within a single iteration of an animation called the <em>time
fraction</em>.
An <em>iteration index</em> is also generated for animations
that vary as they repeat.
</dd>
<dt>Animation model</dt>
<dd>
Takes the <em>time fractions</em> and <em>iteration indices</em>
produced by the timing model and converts them into a series of values
to apply to the target properties and attributes.
</dd>
</dl>
<p>
Graphically, this flow can be represented as follows:
</p>
<div class="figure">
<img src="img/timing-and-animation-models.svg" width="600"
alt='Overview of the operation of the Web Animations model.'>
</div>
<p class="caption">
Overview of the operation of the Web Animations model.<br>
The current time is input to the timing model which produces a time
fraction and an iteration index.<br>
These parameters are used as input to the animation model which produces
the values to apply.<br>
</p>
<p>
For example, consider an animation that:
</p>
<ul>
<li>starts after 3 seconds,</li>
<li>runs twice,</li>
<li>takes 2 seconds every time, and</li>
<li>changes the width of a rectangle from 50 pixels to 100
pixels.</li>
</ul>
<p>
The first three points apply to the timing model.
At a time of 6 seconds, it will calculate that the animation should be
half-way through its second iteration and produces the result 0.5.
The animation model then uses that information to calculate a width
for the rectangle of 75.
</p>
<p>
This specification begins with the timing model and then proceeds to
the animation model.
</p>
</section>
<section>
<h2>Timing model</h2>
<p>
This section describes and defines the behavior of the Web Animations
timing model.
<a>Timing events</a>, however, which are also a feature of the timing
model, are described separately in <a href="#timing-events"
class="sectionRef"></a>.
</p>
<section class="informative">
<h2>The timing model at a glance</h2>
<p>
Two features characterise the Web Animations timing model: it is
<em>stateless</em> and it is <em>hierarchical</em>.
</p>
<section>
<h3>Stateless</h3>
<p>
The Web Animations timing model operates by taking an input time and
producing an output time fraction.
Since the output is based solely on the input time and is independent
of previous inputs, the model may be described as stateless.
This gives the model the following properties:
</p>
<dl>
<dt>Frame-rate independent</dt>
<dd>
Since the output is independent of previous inputs, the rate at
which the model is sampled will not affect its progress.
Provided the input times are proportional to the progress of
real-world time, animations will progress at an identical rate
regardless of the capabilities of the device running them.
</dd>
<dt>Direction-agnostic</dt>
<dd>
Since the sequence of inputs is insignificant, the model is
directionless.
This means that the model can be sampled in reverse or even in
a backwards and forwards pattern without requiring any specialized
handling.
</dd>
<dt>Constant-time seeking</dt>
<dd>
Since each input is independent of the previous input, the
processing required to perform a seek operation, even far into the
future, is at least potentially constant.
</dd>
</dl>
<p>
There are a few apparent exceptions to the stateless behavior of the
timing model.
</p>
<p>
Firstly, <a>timing events</a> are fired when, for example, one sample
falls on the opposite side of an animation's interval boundary to the
previous sample.
This is certainly stative behavior.
However, events should be considered as a layer added on top of the
core timing model.
When no event listeners are registered, the model is stateless.
</p>
<p>
The other exception to this stateless behavior is that a number of
methods defined in the <a href="#script-interface">programming
interface</a> to the model provide play control such as pausing
an item.
These methods are defined in terms of the time at which they are
called and are therefore stative.
These methods are provided primarily for convenience and are not part
of the core timing model but, like events, are layered on top.
</p>
<p>
Finally, each time the model is sampled, it can be considered to
establish a temporary state.
While this temporary state affects the values returned from the <a
href="#script-interface">programming interface</a>, it has no
influence on the subsequent samples and hence does not conflict with
the stateless qualities described above.
</p>
</section>
<section>
<h3>Hierarchical</h3>
<p>
The other characteristic feature of the Web Animations timing model is
that time is inherited.
Time begins with a monotonically increasing time source and cascades
down a number of steps to each animation.
At each step, time may be shifted backwards and forwards, scaled,
reversed, paused, and repeated.
</p>
<div class="figure">
<img src="img/time-hierarchy.svg" width="600"
alt="A hierarchy of timing nodes">
</div>
<p class="caption">
A hierarchy of timing nodes.
Each node in the tree derives its time from its parent node.
At the root of the tree is the global clock.
</p>
<p>
A consequence of this hierarchical arrangement is that complex
animation arrangements can be reversed, scheduled, accelerated and so
on as a whole unit since the manipulations applied to the parent
cascade down to its <a>descendants</a>.
Furthermore, since time has a common source, it is easy to synchronize
animations.
</p>
</section>
</section>
<section>
<h2>Timing model concepts</h2>
<p>
In Web Animations timing is based on a hierarchy of time relationships
between timing nodes.
Parent nodes provide timing information to their child nodes in the form
of <a>time values</a>.
A <dfn>time value</dfn> is a real number which nominally represents
a number of seconds from some moment.
The connection between <a>time values</a> and wall-clock seconds may be
obscured by any number of transformations applied to the value as it
passes through the time hierarchy.
</p>
<p class="annotation">
In the future we may have timelines that are based on UI gestures in
which case the connection between time values and seconds will be
weakened even further.
</p>
<p>
Periodically, the user agent will trigger an update to the timing model
in a process called <dfn>sampling</dfn>.
On each <dfn>sample</dfn> the <a>time values</a> of each timing node are
updated.
</p>
<p class="note">
A more precise definition of when the model is updated when scripting is
involved is provided in <a
href="#script-execution-and-live-updates-to-the-model"
class="sectionRef"></a>.
</p>
</section>
<section>
<h2>The global clock</h2>
<p>
At the root of the Web Animations timing hierarchy is the <a>global
clock</a>.
</p>
<p>
The <dfn>global clock</dfn> is a source of monotonically increasing
<a>time values</a> unaffected by adjustments to the system clock.
The <a>time values</a> produced by the <a>global clock</a> represent
wall-clock seconds from an unspecified historical moment.
The absolute value of the <a>time values</a> produced by the <a>global
clock</a> are insignificant, only their rate of change.
</p>
<p class="note">
Note that the <a>global clock</a> is not exposed in the <a
href="#script-interface">programming interface</a> and nor is it
expected to be exposed by markup.
As a result the moment from which <a>global clock</a> <a>time values</a>
are measured, that is, the zero time of the clock, is allowed to be
implementation-dependent.
One user agent may measure the number of seconds since the the user
agent was loaded whilst another may use the time when the device was
started.
Both approaches are acceptable and produce no observable difference
in the output of the model.
</p>
</section>
<section>
<h2>Timelines</h2>
<p>
A <dfn>timeline</dfn> provides a source of <a>time values</a> for the
purpose of synchronization.
</p>
<p>
Typically, a <a>timeline</a> is tied to the <a>global clock</a> such
that its absolute time is calculated as a fixed offset from the time of
the <a>global clock</a>.
This offset is established by designating some moment as the timeline's
<dfn>zero time</dfn> and recording the <a>time value</a> of the
<a>global clock</a> at that moment.
At subsequent moments, the <a>time value</a> of the timeline is
calculated as the difference between the current <a>time value</a> of
the <a>global clock</a> and the value recorded at the <a>zero time</a>.
</p>
<p class="annotation">
Note that we anticipate that other types of timelines may be introduced
in the future that are not tied to the global clock.
For example, a timeline whose time values correspond to UI gestures.
</p>
<p>
Since a <a>timeline</a> may be defined to relative a moment that has yet
to occur, it may not always be able to return a meaningful <a>time
value</a>.
A <a>timeline</a> is considered to be <dfn>not started</dfn> when it is
in such a state that it cannot produce a <a>time value</a>.
</p>
<section>
<h3>The document timeline</h3>
<p>
Each document has a <a>timeline</a> called the <dfn>document
timeline</dfn> whose <a>time value</a> at a given moment is calculated
as a fixed offset from the <a>global clock</a> such that its <a>zero
time</a> corresponds to the moment immediately prior to dispatching
the load event of the document.
Prior to this moment, the <a>document timeline</a> is <a>not
started</a>.
</p>
<p>
For documents that support the concept of <a
href="http://www.w3.org/TR/html51/dom.html#current-document-readiness
">current document readiness</a>, this is the moment after the <a
href="http://www.w3.org/TR/html51/dom.html#current-document-readiness
">current document readiness</a> has changed to "complete" but
before dispatching the load event.
For user agents that support Navigation Timing [[NAVIGATION-TIMING]],
this occurs between the <em>domComplete</em> and
<em>loadEventStart</em> timings.
</p>
<div class="issue">
This is not correct.
We need a means to start animations prior to document load.
</div>
<p>
Since the <a>document timeline</a> is tied to the <a>global clock</a>
by a fixed offset, <a>time values</a> reported by the <a>document
timeline</a> increase monotonically.
Furthermore, since no scaling is applied, these <a>time values</a>
are proportional to wall-clock seconds.
</p>
</section>
</section>
<section>
<h2>Players</h2>
<div class="informative">
<p>
The children of a <a>timeline</a> are called <em>players</em>.
A player takes a <a>timed item</a> which is a static description of
some timed behavior and binds it to a <a>timeline</a> so that it runs.
A player also allows run-time control of the connection between the
<a>timed item</a> and its <a>timeline</a> by providing pausing,
seeking, and speed control.
The relationship between a player and a <a>timed item</a> is analogous
to that of a DVD player and a DVD.
</p>
</div>
<p>
A <dfn>player</dfn> connects a single <a>timed item</a>, called its
<dfn>source content</dfn>, to a <a>timeline</a> and provides playback
control.
</p>
<p>
A <a>player</a> records the <a>time value</a> of its <a>timeline</a>
at which its <a>source content</a> is scheduled to begin
as the <dfn title="player start time">start time</dfn>.
</p>
<p>
When a <a>player</a> is created, it is assigned a globally unique
sequence number called the <dfn>player sequence number</dfn>.
This number is used to resolve the sort order of players that have the
same <a title="player start time">start time</a> for a variety of
situations such as combining animations, queuing events, and returning
the list of players.
</p>
<p class="annotation">
If we later allow restarting a <a>player</a> (e.g. by adding
a <code>play()</code> method), we might update the sequence number at
this point and rename it to something more appropriate.
</p>
<section>
<h3>The current time of a player</h3>
<p>
<a>Players</a> provide a <a>time value</a> to their <a>source
content</a> called the player's <dfn>current time</dfn>.
The calculation of the <a>current time</a> is as follows:
</p>
<blockquote>
<code><a>current time</a> =
(<var>timeline time</var> - <a title="player start time">start
time</a>) * <a title="player playback rate">playback rate</a>
- <a>time drift</a></code>
</blockquote>
<p>
Where:
</p>
<ul>
<li><var>timeline time</var> is the current <a>time value</a> of the
<a>timeline</a> with which this <a>player</a> is associated.</li>
<li><var>start time</var> is the player's <a
title="player start time">start time</a>.</li>
<li><var>playback rate</var> is the player's <a
title="player playback rate">playback rate</a> value described in <a
href="#speed-control" class="sectionRef"></a>.</li>
<li><var>time drift</var> is the <a>time drift</a> value described in
<a href="#seeking-and-pausing" class="sectionRef"></a>.</li>
</ul>
<p>
If the <a>timeline</a> with which is <a>player</a> is associated is
<a>not started</a> then the <a>current time</a> is <code>null</code>.
</p>
<p>
It is often useful to treat the <a>current time</a> as zero when it
would otherwise be <code>null</code>.
We define the <dfn>effective current time</dfn> as being equal
to <a>current time</a> unless the <a>current time</a> is
<code>null</code>,
in which case the <var>effective current time</var> is zero.</li>
</p>
<p>
The procedure for performing manual updates to the <a>current time</a>
is defined in <a href="#performing-a-seek" class="sectionRef"></a>.
</p>
</section>
<section>
<h3>Seeking and pausing</h3>
<p>
Seeking and pausing a player are closely related and are described
here together.
</p>
<section class="informative">
<h4>Introduction to seeking</h4>
<p>
Changing the current playback position of a player can be used to
rewind its <a>source content</a> to its start point, fast-forward
to a point in the future, or to provide ad-hoc synchronization
between timed items.
</p>
<p>
However, in Web Animations, the <a title="player start time">start
time</a> of a <a>player</a> has special significance in determining
the priority of animations (see <a href="#combining-animations"
class="sectionRef"></a>) and so we cannot simply adjust the <a
title="player start time">start time</a>.
Instead, an additional offset is introduced called the <a>time
drift</a> that further offsets a player's <a>current time</a> from
its timeline.
The effect of the <a>time drift</a> when seeking is illustrated
below.
</p>
<div class="figure">
<img src="img/seeking.svg" width="600"
alt="The effect of seeking a player.">
</div>
<p class="caption">
At time <var>t</var>, a seek is performed on the player changing its
<a>current time</a> from 1.5s to 2s.<br>
As a result, the <a>time drift</a> is set to -0.5s.<br>
Note that the <a title="player start time">start time</a> indicated
by a red star does not change.
</p>
<p>
It is possible to seek a player even if its <a>timeline</a> is
<a>not started</a>.
Once the timeline begins, the player will begin playback from the
seeked time.
</p>
</section>
<section class="informative">
<h4>Introduction to pausing</h4>
<p>
Pausing can be used to temporarily suspend a <a>player</a>.
Like seeking, pausing effectively causes the <a>current time</a> of
a player to be offset from its <a>timeline</a> by means of
setting the <a>time drift</a>.
</p>
<p>
The effect of pausing on a <a>player</a>'s <a>time drift</a> is
illustrated below.
</p>
<div class="figure">
<img src="img/pausing.svg" width="600"
alt="The effect of pausing a player.">
</div>
<p class="caption">
The effect of pausing a <a>player</a>.<br>
Whether pausing before or after a <a>player</a>'s <a
title="player start time">start time</a>
the duration of the interval during which the <a>player</a> was
paused is added to the <a>player</a>'s <a>time drift</a> whilst the
<a title="player start time">start time</a> remains unaffected.
</p>
</section>
<section>
<h4>Seeking and pausing properties</h4>
<p>
<a>Players</a> track three properties related to seeking and
pausing.
</p>
<dl>
<dt><dfn>time drift</dfn></dt>
<dd>
The offset from a player's scheduled current time as defined by
its <a title="player start time">start time</a>, and its actual
current time after accounting for the effects of seeking and
pausing.
The <a>time drift</a> is initially zero and is updated as per the
definition in <a href="#calculating-the-time-drift"
class="sectionRef"></a>.
</dd>
<dt><dfn>paused state</dfn></dt>
<dd>
A boolean value that is true if the player is currently paused.
The <a>paused state</a> is initially false.
</dd>
<dt><dfn>pause start time</dfn></dt>
<dd>
The player's <a>effective current time</a> at the time when the
<a>paused state</a> was last newly true.
The <a>pause start time</a> is initially zero.
</dd>
</dl>
<p>
A number of calculations for performing seeking and pausing are
defined to operate even when the associated <a>timeline</a> is
<a>not started</a>.
For such situations we define the <dfn>effective timeline time</dfn>
as the current <a>time value</a> of the <a>timeline</a> associated
with a <a>player</a>;
if the timeline is <a>not started</a>, then the <var>effective
timeline time</var> is zero.
</p>
</section>
<section>
<h4>Calculating the time drift</h4>
<p>
The <a>time drift</a> value is both a stored and a calculated value.
When a player is paused, the value is calculated from the <a>pause
start time</a>.
When a player is not paused, the stored value is used.
The stored value is initially zero, and is updated when the player
is unpaused or seeked.
</p>
<p>
The value of <a>time drift</a> at a given moment is calculated as
follows:
</p>
<dl class="switch">
<dt>If the <a>paused state</a> is true,</dt>
<dd>
Return the result of
<code>(<a>effective timeline time</a> - <a
title="player start time">start time</a>) * <a
title="player playback rate">playback rate</a> -
<a>pause start time</a></code>.
</dd>
<dt>Otherwise,</dt>
<dd>
Return the stored value for this property.
</dd>
</dl>
<p class="todo">
These formulae need to be verified since removing the special
behavior regarding pausing before the start time.
</p>
</section>
<section>
<h4>Updating the <a>paused state</a></h4>
<p>
The procedure for updating the <a>paused state</a> is as
follows:
</p>
<ol>
<li>
Let <var>new value</var> be the new paused state to set.
</li>
<li>
If <var>new value</var> equals the current <a>paused state</a>,
return.
</li>
<li>
The next step depends on the current <a>paused state</a> as
follows,
<dl class="switch">
<dt>If <a>paused state</a> is true,</dt>
<dd>
Set the <em>stored</em> value of <a>time drift</a> to the
<em>current calculated</em> value of <a>time drift</a> as
defined in <a href="#calculating-the-time-drift"
class="sectionRef"></a>.
</dd>
<dt>Otherwise,</dt>
<dd>
Record the current value of the <a>effective current time</a>
as <a>pause start time</a>.
</dd>
</dl>
</li>
<li>
Update <a>paused state</a> to <var>new value</var>.
</li>
</ol>
</section>
<section>
<h4>Performing a seek</h4>
<p>
<dfn>Seeking</dfn> is the process of updating a player's <a>current
time</a> to a desired value.
It is achieved using the following procedure:
</p>
<ol>
<li>Let <var>seek time</var> be the desired <a>time value</a> for
the player's <a>current time</a>.</li>
<li>The steps for adjusting the <a>current time</a> depend on the
<a>paused state</a> as follows:
<dl class="switch">
<dt>If the <a>paused state</a> is true,</dt>
<dd>
Set <a>pause start time</a> to <var>seek time</var>.
</dd>
<dt>If the <a>paused state</a> is false,</dt>
<dd>
Set the stored value for the <a>time drift</a> to
the result of evaluating
<code>(<a>effective timeline time</a> - <a
title="player start time">start time</a>)
* <a title="player playback rate">playback rate</a> -
<var>seek time</var></code>.
</dd>
</dl>
</li>
</ol>
<p>
The <a>timing events</a> queued when a seek is performed are
described in <a href="#event-dispatch-and-seeking-a-player"
class="sectionRef"></a>.
</p>
</section>
</section>
<section>
<h3>Speed control</h3>
<div class="informative">
<p>
The rate of play of a player can be controlled by setting its
<em>playback rate</em>.
For example, setting a playback rate of 2 will cause the player's
<a>current time</a> to increase at twice the rate of its
<a>timeline</a>.
Similarly, a playback rate of -1 will cause the player's <a>current
time</a> to decrease at the same rate as the <a>time values</a> from
its <a>timeline</a> increase.
</p>
<p>
Note that <a>timed items</a> also have a <a
title="timed item playback rate">playback rate</a> associated with
them that behaves differently to that defined here.
</p>
</div>
<p>
<a>Players</a> have a <dfn title="player playback
rate">playback rate</dfn> that provides a scaling factor from the rate
of change of the associated <a>timeline</a>'s <a>time values</a> to
the player's <a>current time</a>.
The <a title="player playback rate">playback rate</a> is initially 1.
</p>
<p>
Setting a player's <a title="player playback rate">playback rate</a>
to zero effectively pauses the player but without affecting the
player's <a>paused state</a>.
</p>
<section>
<h4>Updating the playback rate</h4>
<p>
Changes to the <a title="player playback rate">playback rate</a>
also trigger a compensatory seek so that that the player's
<a>current time</a> is unaffected by the change to the playback
rate.
</p>
<p>
The procedure is as follows:
</p>
<ol>
<li>Let <var>previous time</var> be the value of the <a>effective
current time</a> before updating the <a
title="player playback rate">playback rate</a>.</li>
<li>Update the <a title="player playback rate">playback rate</a> to
the new value.</li>
<li>Seek to <var>previous time</var> using the procedure defined in
<a href="#performing-a-seek" class="sectionRef"></a>.</li>
</ol>
</section>
</section>
</section>
<section>
<h2>Timed items</h2>
<p>
The <a>source content</a> of a <a>player</a>, if set, is a type of
<dfn>timed item</dfn>.
There are two types of <a>timed item</a>:
</p>
<ul>
<li><a>animations</a>, and</li>
<li><a>timing groups</a>.</li>
</ul>
<p>
At a given moment, a <a>timed item</a> can be associated with at most
one <a>player</a>.
</p>
<p>
<a>Timed items</a> can be chained together into a hierarchy using
<a>timings groups</a>.
Only the <a>root</a> <a>timed item</a> of such a hierarchy can be <dfn
title="directly associated with a player">directly associated</dfn> with
a <a>player</a>.
Attempting to associate a <a>timed item</a> that has a <a>parent timing
group</a> with a <a>player</a> results in the <a>timed item</a> being
removed from the <a>parent timing group</a> first.
A <a>timed item</a> with an <a>ancestor</a> <a>timing group</a> that is
<a>directly associated with a player</a> is <dfn
title="indirectly associated with a player">indirectly associated</dfn>
with that <a>player</a>.
</p>
<p>
A <a>timed item</a>, <var>item</var>, is <dfn>associated with
a timeline</dfn>, <var>timeline</var>, if <var>item</var> is
<a title="directly associated with a player">directly</a> or
<a title="indirectly associated with a player">indirectly</a> associated
with a <a>player</a> which, in turn, is associated with
<var>timeline</var>.
</p>
<p>
All types of <a>timed item</a> define a number of common properties
which are described in the following sections.
</p>
<section>
<h3>The active interval</h3>
<div class="informative">
<p>
The period that a <a>timed item</a> is scheduled to run is called
its <a>active interval</a>.
Each <a>timed item</a> has only one such interval.
</p>
<p>
The lower bound of the <a>active interval</a> is determined by the
<a title="timed item start time">start time</a> of the <a>timed
item</a> but may be shifted by a <a>start delay</a> on the <a>timed
item</a>.
</p>
<p>
The upper bound of the interval is determined by the <a>active
duration</a>.
</p>
<p>
The relationship between the <a title="timed item start time">start
time</a>, <a>start delay</a>, and <a>active duration</a> is
illustrated below.
</p>
<div class="figure">
<img src="img/active-interval-examples.svg" width="600"
alt="Examples of the effect of the start delay on the endpoints
of the active interval">
</div>
<p class="caption">
Examples of the effect of the <a>start delay</a> on the endpoints of
the <a>active interval</a>.<br>
(a) A timed item with no delay; the <a
title="timed item start time">start time</a> and beginning of the
<a>active interval</a> are coincident.<br>
(b) A timed item with a positive delay; the beginning of the
<a>active interval</a> is deferred by the delay.<br>
(c) A timed item with a negative delay; the beginning of the
<a>active interval</a> is brought forward by the delay.
</p>
</div>
<p>
<a>Timed items</a> define an <dfn>active interval</dfn> which is the
period of time during which the item is scheduled to produce its
effect with the exception of <a>fill modes</a> which apply outside
the <a>active interval</a>.
</p>
<p>
The lower bound of the <a>active interval</a> is defined by the
combination of the timed item's <a title="timed item start time">start
time</a> and <a>start delay</a>
</p>
<p>
A <a>timed item</a>'s <dfn title="timed item start time">start
time</dfn> is the moment at which the <a>parent timing group</a>, if
any, has scheduled the <a>timed item</a> to begin.
It is expressed in <a>inherited time</a>.
In most cases, including the case when the <a>timed item</a> has no
<a>parent timing group</a>, the <a title="timed item start time">start
time</a> is zero.
The singular exception is <a>sequence timing groups</a> which set the
<a title="timed item start time">start times</a> of their children as
described in <a
href="#the-start-time-of-children-of-a-sequence-timing-group"
class="sectionRef"></a>.
</p>
<p>
In addition to the <a title="timed item start time">start time</a>,
a <a>timed item</a> also has a <dfn>start delay</dfn> which is an
offset from the <a title="timed item start time">start time</a>.
Unlike the <a title="timed item start time">start time</a> which is
determined by the <a>parent timing group</a>, the <a>start delay</a>