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Tweening & timelines for animation

local tw = require'tweening'

A library for the management of gradual value changes for the purposes of animation, inspired by GSAP.


  • timing parameters: duration, speed, loop, backwards, yoyo, ease, way, offset.
  • independent timing parameters for each target object and/or attribute.
  • timelines that can be nested, overlapping, and tweened.
  • stagger tweens: alternate end-values over a list of targets.
  • extensible attribute types, value converters and interpolation functions.
  • relative values including directional rotation.
  • pause/resume/restart/reverse individual tweens even when in a timeline.
  • no allocations while tweening.


tw:tween(t) -> tween

Create a new tween. A tween is an object which repeatedly updates a value on a target object in time based on an interpolation function and timing parameters.

NOTE: t itself is turned into a tween (no new table is created). This allows any method to be overriden by specifying it directly as a field of t.

Timing model

field default description
start current clock start clock (relative when in a timeline)
duration 1 duration of one iteration (can’t be negative)
ease 'quad' ease function f(p) -> d or name from easing module
way 'in' easing way: 'in', 'out', 'inout' or 'outin'
ease_args none optional args to pass to the ease function
backwards false start backwards
yoyo true alternate between forwards and backwards on each iteration
loop 1 number of iterations (can be fractional; use 1/0 for infinite)
speed 1 speed factor (must be > 1)
offset 0 progress at start
running true running or paused
clock start current clock when running, set by update()
resume_clock clock current clock when paused, set by update()

NOTE: Timing model fields can be changed anytime (there’s no internal state that must be kept in sync).

method description
total_duration() -> dt total duration incl. repeats (can be +/-inf)
end_clock() -> t end clock (can be +/-inf)
is_infinite() -> bool true if loop and/or duration are inf
is_backwards(i) -> bool true if iteration i goes backwards
progress([t]) -> P linear progress in 0..1 incl. repeats (can be inf)
clock_at(P) -> t clock at total linear progress (which is in 0..1)
status([t]) -> status status: ‘before_start’, ‘running’, ‘paused’, ‘finished’
loop_progress([t]) -> p linear progress in 0..loop
loop_clock_at(p) -> t clock at loop progress
distance([t]) -> d eased progress in current iteration (in 0..1)
update([t]) update internal clock and target value

NOTE: progress(), clock_at() and seek() map time to 0..1 while loop_progress(), loop_clock_at() and loop_seek() map time to 0..loop. The first set is more convenient but doesn’t work when loop is inf.

Changing the state of the tween

method description
pause() pause (changes running, prevents updating clock)
resume() resume (changes running and start)
seek(P) update target value based on progress (changes start).
loop_seek(p) update target value based on loop progress (changes start).
stop() pause and remove from timeline
restart() restart (changes start)
reverse() reverse (changes start, offset, backwards)

NOTE: The methods above change the timing model of the tween, which is normally supposed to be immutable in order to make the tween stateless. This is done to allow a tween that is part of a timeline to be paused and resumed independently of other tweens and of the timeline. Use clone() to preseve the initial definition of the tween.

Animation model

field default description
target (required) target object
attr (required) attribute in the target object to tween
from target[attr] start value (defaults to target’s value)
to target[attr] end value (defaults to target’s value)
type per attr force attribute type
interpolate default for type f(t, x1, x2[, xout]) -> x
get_value() -> v target[attr] -> v value getter
set_value(v) target[attr] = v value setter

See below for how to add attribute type matching rules and interpolators.

NOTE: Animation model fields are read/only. Changing them requires a call to reset().

Relative values

from and to can be given in the format '<number>', '<number><unit>, '<operator>=<number>' or '<operator>=<number><unit>', where operator can be +, - or * and unit can be:

unit description
% percent, converted to 0..1
ms milliseconds, converted to seconds
deg degrees, converted to radians
cw rotation: clockwise
ccw rotation: counter-clockwise
short rotation: shortest direction
deg_cw rotation: clockwise in degrees, converted to radians
deg_ccw rotation: counter-clockwise in degrees, converted to radians
deg_short rotation: shortest direction in degrees, converted to radians

Examples: '+=10deg', '25%'.

NOTE: '25%' means 25% the initial value, while '+=25%' means 125% the initial value.

NOTE: cw and ccw assume that increasing angles rotate the target clockwise (like cairo and other systems where the y-coord grows from top to bottom).

NOTE: '+=90deg' means “rotate the target another 90 degrees clockwise”, while '90deg_cw' means “rotate the target to the 90 degrees mark by moving clockwise”. Don’t combine relative rotations with cw and ccw.


method description
tween:clone() -> tween clone a tween
tween:totarget() -> obj convert to tweenable object

tween:clone() -> tween

Clone a tween in its current state.

tween:totarget() -> obj

Create a proxy object for the tween with the additional tweenable fields progress and loop_progress. Other tweenable properties like speed remain accessible.


tw:timeline(t) -> tl

Create a new timeline. A timeline is a list of tweens which are updated in parallel (or one after another, depending on their start time). The list can also contain other timelines, resulting in a hierarchy of timelines.

NOTE: t itself is turned into a timeline (no new table is created).

Timelines are tweens

A timeline is itself a tween, containing all the fields and methods of the timing model of a tween (but none of the fields and methods of the animation model since it’s not animating a target object, it’s updating other tweens). This means that a timeline can be used to tween the progress of its child tweens instead of just updating them on the same clock, since it has a distance resulting from its timing model. It also means that the timeline can be itself tweened on its progress (which only works if the timeline is not infinite).

Tweening other tweens

Setting tween_progress = true on a timeline switches the timeline into tweening its child tweens on their progress (so tweening a temporal value) instead of just updating them on the same clock. In this mode, the child’s start and duration are ignored: instead, the timeline’s distance is interpolated over the child’s progress.

Timeline-specific fields and methods

field default description
tweens {} the list of tweens in the timeline
ease 'linear' (a better default for the tween_progress mode)
duration 0 auto-adjusted when adding tweens
auto_duration true auto-increase duration to include all tweens
auto_remove true remove tweens automatically when finished
tween_progress false progress-tweening mode
method description
add(tween|opt[, start]) add a tween or create multiple tweens
each(func, ...) iterate tweens recursively
remove(tween|attr|target) remove matching tweens recursively
clear() remove all tweens (non-recursively)
status([t]) adds 'empty' status

tl:add(tween|opt[, start]) -> tl

Add a new tween to the timeline, set its start field and its timeline field, and, if auto_duration is true, increase timeline’s duration to include the entire tween. When part of a timeline, a tween’s start is relative to the timeline’s start time. If start is not given, the tween is added to the end of the timeline (when the timeline’s duration is infinite then the tween’s start is set to 0 instead).

If an options table is given instead, multiple tweens are created and added to the timeline as follows: targets specifies a list of targets, otherwise target specifies a single target, from and to specifies a table of from/to attribute -> value pairs. cycle_from, cycle_to, cycle specifies a table of from/to attribute -> list-of-values pairs such that values will be distributed to each target in a round-robin fashion. The values can also be functions. More on cylcling here.

NOTE: start can be a relative value relative to the timeline’s current total duration, eg. '+=500ms' means half a second after the last tween, while '500ms' means half a second from the start of the timeline.

tl:each(func, ...)

Run func(tween) for each tween of a timeline, recursively. Returning false from func breaks the iteration. Returning 'remove' removes the tween. Returning another tween replaces the tween.


Remove a tween or all tweens with a specific attribute, target object or id recursively.


Remove all tweens from the timeline (non-recursively).

The tweening module

tw() -> tw

Create a new tweening module. Useful for extending the tweening module with new attribute types, new interpolators or different ways of getting the wall clock without affecting the original module table.

Attribute types and interpolators

tw.type.<name|pattern|match_func> = attr_type

Tell tweening about the type of an attribute, eg. tw.type['_color$'] = 'list'

tw.interpolate.<attr_type> = function(d, x1, x2[, xout]) -> x

Add a new interpolation function for an attribute type. Interpolators are called as x = f(d, x1, x2, x) and can update x in-place and return it thus avoiding an allocation on every frame if x is a non-scalar type.

Value parsers

tw:parse_value(v, relative_to, tween) -> v

Parse a value.

The wall clock

tw:clock([t|false]) -> t

Get/freeze/unfreeze the wall clock. With t it freezes the wall clock such that subsequent tw:clock() calls will return t. With false it unfreezes it such that subsequent tw:clock() calls will return tw:current_clock().

Freezing is useful for creating multiple tweens which start at the exact same time without having to specify the time.

tw:current_clock() -> t

Current monotonic performance counter, in seconds. Implemented in terms of time.clock(). Override this in order to remove the dependency on the time module.


tw:ease(ease, way, p, i) -> d

Implemented in terms of easing.ease(). Override this in order to remove the dependency on the easing module.

Last updated: 2 years ago | Edit on GitHub

Pkg type:Lua
Version: d6e7693
Last commit:
Author: Cosmin Apreutesei
License: Public Domain

Requires: none

Required by: none