Summary

Allow interpolation outside of range [0,1], and accept cubic Bézier functions with control points outside of range [0,1] on the Y axis (output progress).

Problem

JavaFX unnecessarily restricts interpolation in the following ways:

• Interpolatable implementations often clamp intermediate values to the interpolation factor range [0,1].

• SplineInterpolator doesn't accept Y coordinates outside of [0,1] for its control points. While this was probably done so that the computed interpolation factor doesn't exceed [0,1], the restriction goes far beyond that. For example, the following function is not representable, even though its values are all within the [0,1] range: <cubic-bezier-1.png>. The following function is also not representable, but would be very useful for bouncy animations: <cubic-bezier-2.png>.

Solution

Fortunately, there is no technical reason why JavaFX can't support the full range of animations that can be represented with a cubic Beziér interpolation function.

1. The specification of Interpolatable is changed to require implementations to accept interpolation factors outside of [0,1].
2. All implementations of Interpolatable now correctly return intermediate values outside of [0,1].
3. SplineInterpolator accepts control points with any Y coordinate (it was never specified that it should reject Y coordinates outside of [0,1]).

Specification

--- a/modules/javafx.graphics/src/main/java/javafx/animation/Interpolatable.java
+++ b/modules/javafx.graphics/src/main/java/javafx/animation/Interpolatable.java
@@ -39,10 +39,10 @@
  *                 {@link #interpolate(Object, double)} method.</td>
  *         </tr>
  *         <tr><td style="vertical-align: top"><a id="linear" style="white-space: nowrap">linear</a></td>
- *             <td>Two components are combined by linear interpolation such that {@code t = 0} produces
- *                 the start value, {@code t = 1} produces the end value, and {@code 0 < t < 1} produces
- *                 {@code (1 - t) * start + t * end}. This interpolation type is usually applicable for
- *                 numeric components.</td>
+ *             <td>Two components are combined by linear interpolation such that the intermediate value is
+ *                 produced by computing {@code (1 - t) * start + t * end}. Note that this formula produces
+ *                 values less than {@code start} for {@code t < 0} and values greater than {@code end} for
+ *                 {@code t > 1}. This interpolation type is usually applicable for numeric components.</td>
  *         </tr>
  *         <tr><td style="vertical-align: top"><a id="discrete" style="white-space: nowrap">discrete</a></td>
  *             <td>If two components cannot be meaningfully combined, the intermediate component value
@@ -70,8 +70,8 @@

     /**
      * Returns an intermediate value between the value of this {@code Interpolatable} and the specified
-     * {@code endValue} using the linear interpolation factor {@code t}, ranging from 0 (inclusive)
-     * to 1 (inclusive).
+     * {@code endValue} using the linear interpolation factor {@code t}. The interpolation factor can
+     * be any finite value.
      * <p>
      * The returned value might not be a new instance; the implementation might also return one of the
      * two existing instances if the intermediate value would be equal to one of the existing values.
@@ -78,10 +78,8 @@
      * However, this is an optimization and applications should not assume any particular identity
      * of the returned value.
      *
-     * @implSpec An implementation is not required to reject interpolation factors less than 0 or larger
-     *           than 1, but this specification gives no meaning to values returned outside of this range.
-     *           For example, an implementation might clamp the interpolation factor to [0..1], or it might
-     *           continue the linear interpolation outside of this range.
+     * @implSpec An implementation must accept any interpolation factor, but it can return a clamped
+     *           intermediate value that falls within its value range.
      *
      * @param endValue the target value
      * @param t the interpolation factor