burns.easing#

Timing functions (easing) for Ken Burns motion.

Easing maps normalized clock time t in [0, 1] to normalized progress [0, 1]. It is composed over the geometry: a path’s viewport at clock time t is path.evaluate_geometry(easing(t)) — the After-Effects split of what shape the motion traces (the path) from how fast it moves along it (the timing). Keeping them orthogonal is the single most useful design lesson from the NLE prior art.

The public currency is the CSS timing-function string — "linear", "ease", "ease-in", "ease-out", "ease-in-out", or an explicit "cubic-bezier(x1, y1, x2, y2)" — because that vocabulary is understood verbatim by CSS, Remotion’s Easing.bezier, and GSAP’s CustomEase alike. The cinematic default is "ease-in-out" (Final Cut Pro’s Ken Burns default), not linear.

parse_easing() also accepts a 4-tuple of bezier control values or any callable float -> float, so power users can inject an arbitrary curve.

Functions

cubic_bezier(x1, y1, x2, y2)

A CSS-style cubic-bezier easing f: [0, 1] -> [0, 1].

parse_easing([spec])

Resolve an easing spec to a callable f: [0, 1] -> [0, 1].
burns.easing.cubic_bezier(x1, y1, x2, y2)[source]#

A CSS-style cubic-bezier easing f: [0, 1] -> [0, 1].

The curve runs from (0, 0) to (1, 1) with control points (x1, y1) and (x2, y2). Evaluation inverts x(t) for the curve parameter (bisection — robust for any monotone-x curve) then returns y(t).

Return type:

Callable[[float], float]

Examples

>>> linear = cubic_bezier(0.0, 0.0, 1.0, 1.0)
>>> round(linear(0.5), 6)
0.5
>>> round(cubic_bezier(0.42, 0.0, 0.58, 1.0)(0.5), 6)  # ease-in-out
0.5
burns.easing.parse_easing(spec='ease-in-out')[source]#

Resolve an easing spec to a callable f: [0, 1] -> [0, 1].

Accepts:

  • a CSS name ("linear", "ease", "ease-in", "ease-out", "ease-in-out");

  • a CSS "cubic-bezier(x1, y1, x2, y2)" string;

  • a 4-element sequence (x1, y1, x2, y2);

  • any callable, returned unchanged.

Examples

>>> parse_easing("linear")(0.3)
0.3
>>> round(parse_easing("cubic-bezier(0,0,1,1)")(0.7), 6)
0.7
>>> parse_easing(lambda t: t * t)(0.5)
0.25
Return type:

Callable[[float], float]