<timing-function>

The <single-transition-timing-function> CSS data type denotes a mathematical function that describes how fast one-dimensional values change during transitions or animations. This in essence lets you establish an acceleration curve, so that the speed of the animation can vary over its duration. These functions are often called easing functions.

It is a function linking the time with a ratio of the output value, expressed as a <number>, with 0.0 representing the initial state, 1.0 the final state.

The output ratio can be greater than 1.0 (or smaller than 0.0). This causes the animation to go farther than the final state, then come back, in a kind of bouncing effect.

Nevertheless, if the output value goes outside of its possible range, such as a component of a color going greater than 255 or smaller than 0, the value is clipped to its closest allowed value (in the case of a color-component 255 and 0 respectively. Some cubic-bezier() curves exhibit this property.

Values

Syntax

cubic-bezier(x1, y1, x2, y2)

where:

x₁, y₁, x₂, y₂

Are <number> values representing the abscissas and ordinates of the P₁ and P₂ points defining the cubic Bézier curve. x₁ and x₂ must be in the range [0, 1] or the value is invalid.

Examples

These cubic Bézier curves are valid for use in CSS :

/* The canonical Bézier curve with four <number> in the [0,1] range. */
cubic-bezier(0.1, 0.7, 1.0, 0.1)   

/* Using <integer> is valid as any <integer> is also a <number>. */
cubic-bezier(0, 0, 1, 1)           

/* Negative values for ordinates are valid, leading to bouncing effects.*/
cubic-bezier(0.1, -0.6, 0.2, 0)    

/* Values > 1.0 for ordinates are also valid. */
cubic-bezier(0, 1.1, 0.8, 4)

These cubic Bézier curves definitions are invalid :

/* Though the animated output type may be a color, 
   Bézier curves work w/ numerical ratios.*/
cubic-bezier(0.1, red, 1.0, green) 

/* Abscissas must be in the [0, 1] range or 
   the curve is not a function of time. */
cubic-bezier(2.45, 0.6, 4, 0.1)    

/* The two points must be defined, there is no default value. */
cubic-bezier(0.3, 2.1)   

/* Abscissas must be in the [0, 1] range or 
   the curve is not a function of time. */
cubic-bezier(-1.9, 0.3, -0.2, 2.1) 

The steps() class of timing-functions

		The steps() functional notation defines a step function dividing the domain of output values in equidistant steps. This subclass of step functions are sometimes also called staircase functions.
steps(2, start)	steps(4, end)

Syntax

steps(number_of_steps, direction)

where:

number_of_steps

Is a strictly positive <integer> representing the amount of equidistant treads composing the stepping function.

direction

Is a keyword indicating if it the function is left- or right-continuous:

• start denotes a left-continuous function, so that the first step happens when the animation begins;
• end denotes a right-continuous function, so that the last step happens when the animation ends.

end is the default.

Examples

These timing functions are valid :

/* There is 5 treads, the last one happens 
   right before the end of the animation. */
steps(5, end)

/* A two-step staircase, the first one happening 
   at the start of the animation. */
steps(2, start)        

/* The second parameter is optional. */
steps(2)               

These timing function are invalid :

/* The first parameter must be an <integer> and 
   cannot be a real value, even if it is equal to one. */
steps(2.0, end)

/* The amount of steps must be non-negative. */
steps(-3, start)

/* There must be at least one step.*/
steps(0, end)       

Keywords for common timing-functions

linear

This keyword represents the timing function cubic-bezier(0.0, 0.0, 1.0, 1.0). Using this timing function, the animation goes from its initial state to its final one regularly, with a constant velocity.

ease

This keyword represents the timing function cubic-bezier(0.25, 0.1, 0.25, 1.0). This function is similar to ease-in-out, though it accelerates more sharply at the beginning, starts to slow down before the time is halfway through, and gently comes to rest.

ease-in

This keyword represents the timing function cubic-bezier(0.42, 0.0, 1.0, 1.0). The animation begins slowly, then progressively accelerates until the final state is reached and the animation stops abruptly.

ease-in-out

This keyword represents the timing function cubic-bezier(0.42, 0.0, 0.58, 1.0). With this timing function, the animation starts slowly, accelerates faster, and then slows down when approaching its final state. At the beginning, it behaves similarly to the ease-in function; at the end, it is similar to the ease-out function.

ease-out

This keyword represents the timing function cubic-bezier(0.0, 0.0, 0.58, 1.0). The animation starts quickly and then slows down when approaching to its final state.

step-start

This keyword represents the timing function steps(1, start). Using this timing function, the animation jumps immediately to the end state and stays in that position until the end of the animation.

step-end

This keyword represents the timing function steps(1, end). Using this timing function, the animation stays in its initial state until the end, where it directly jumps to its final position.

Specifications

Specification	Status	Comment
CSS Animations The definition of '<single-transition-timing-function>' in that specification.	Working Draft	Defines <single-timing-function> as synonym for <single-transition-timing-function> of CSS Transitions Module.
CSS Transitions The definition of '<single-transition-timing-function>' in that specification.	Working Draft	Initial definition

Browser compatibility

See also

• CSS Reference