Implement simple easing curves

kitty/animation.c

@@ -0,0 +1,37 @@
+/*
+ * animation.c
+ * Copyright (C) 2024 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#include "animation.h"
+#include <stdbool.h>
+#include "data-types.h"
+
+static double
+unit_value(double x) { return MAX(0., MIN(x, 1.)); }
+
+double
+linear_easing_curve(easing_curve_parameters p, double val) {
+    for (size_t i = p.count - 1; i-- > 0;) if (p.positions[i] <= val) return p.params[i];
+    return p.params[0];
+}
+
+double
+cubic_bezier_easing_curve(easing_curve_parameters p, double t) {
+    const double u = 1. - t, uu = u * u, uuu = uu * u, tt = t * t, ttt = tt * t;
+    // p0 is start, p3 is end. p1, p2 are control points
+    return uuu * p.params[0] + 3 * uu * t * p.params[1] + 3 * u * tt * p.params[2] + ttt * p.params[3];
+}
+
+double
+apply_easing_curve(const Animation *a, double val) {
+    if (a->first_half.curve) {
+        if (a->second_half.curve) {
+            if (val <= 0.5) return unit_value(a->first_half.curve(a->first_half.params, 2 * val));
+            return unit_value(a->second_half.curve(a->second_half.params, 2 * (val - 0.5)));
+        } else return unit_value(a->first_half.curve(a->first_half.params, val));
+    } else return (val <= 0.5) ? 1. : 0.;
+}
+