RFR: 8372203: Piecewise linear easing function

[Andy Goryachev]

On Wed, 19 Nov 2025 21:36:36 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> Implementation of the [linear](https://www.w3.org/TR/css-easing-2/#the-linear-easing-function) easing function, which is now widely supported by all browsers, but still missing in JavaFX.
> 
> It allows developers to approximate arbitrary easing functions with linear segments:
> 
> 
> linear(
>     /* Start to 1st bounce */
>     0, 0.063, 0.25, 0.563, 1 36.4%,
>     /* 1st to 2nd bounce */
>     0.812, 0.75, 0.813, 1 72.7%,
>     /* 2nd to 3rd bounce */
>     0.953, 0.938, 0.953, 1 90.9%,
>     /* 3rd bounce to end */
>     0.984, 1 100% 100%
>   )
> 
> 
> <img src="https://github.com/user-attachments/assets/73aac0d4-0185-47c5-8791-bdb247eb69c8" width="400"/>

Could you provide an example (in the JBS maybe) of a CSS that illustrates the usage?  Or maybe add an example to the javadoc?

modules/javafx.graphics/src/main/java/com/sun/scenario/animation/LinearInterpolator.java line 76:

> 74:         // Ensure that the input progress value of each control point is greater than or equal to the
> 75:         // input progress values of all preceding control points (monotonically non-decreasing).
> 76:         double largestX = controlPoints[0];

Question: what happens when a esquence is specified which is somehow invalid?  Will it throw an exception, write to stderr, or silently ignore?

(I can't think of an invalid sequence, maybe `0, 0 0% 0%, 0 -10%, NaN` ?)

modules/javafx.graphics/src/main/java/com/sun/scenario/animation/LinearInterpolator.java line 198:

> 196: 
> 197:         // Linearly interpolate (or extrapolate) along the segment (ax, ay) -> (bx, by).
> 198:         if (ax == bx) {

FP alert: is it possible for `ax != bx` yet the result of division on L203 to produce an infinity?

modules/javafx.graphics/src/test/java/test/com/sun/scenario/animation/LinearInterpolatorTest.java line 41:

> 39:         try {
> 40:             Field f = LinearInterpolator.class.getDeclaredField("controlPoints");
> 41:             f.setAccessible(true);

should an Accessor be used instead?

-------------

PR Review: https://git.openjdk.org/jfx/pull/1977#pullrequestreview-3494451006
PR Review Comment: https://git.openjdk.org/jfx/pull/1977#discussion_r2551033935
PR Review Comment: https://git.openjdk.org/jfx/pull/1977#discussion_r2551042657
PR Review Comment: https://git.openjdk.org/jfx/pull/1977#discussion_r2551048049