1 files changed, 38 insertions, 0 deletions
diff --git a/gsk/vulkan/resources/ellipse.glsl b/gsk/vulkan/resources/ellipse.glsl
new file mode 100644
index 0000000000..08986de731
--- /dev/null
+++ b/gsk/vulkan/resources/ellipse.glsl
@@ -0,0 +1,38 @@
+#ifndef _ELLIPSE_
+#define _ELLIPSE_
+
+struct Ellipse
+{
+  vec2 center;
+  vec2 radius;
+};
+
+float
+ellipse_distance (Ellipse r, vec2 p)
+{
+  vec2 e = r.radius;
+  p = p - r.center;
+
+  if (e.x == e.y)
+    return length (p) - e.x;
+
+  /* from https://www.shadertoy.com/view/tt3yz7 */
+  vec2 pAbs = abs(p);
+  vec2 ei = 1.0 / e;
+  vec2 e2 = e*e;
+  vec2 ve = ei * vec2(e2.x - e2.y, e2.y - e2.x);
+  
+  vec2 t = vec2(0.70710678118654752, 0.70710678118654752);
+  for (int i = 0; i < 3; i++) {
+      vec2 v = ve*t*t*t;
+      vec2 u = normalize(pAbs - v) * length(t * e - v);
+      vec2 w = ei * (v + u);
+      t = normalize(clamp(w, 0.0, 1.0));
+  }
+  
+  vec2 nearestAbs = t * e;
+  float dist = length(pAbs - nearestAbs);
+  return dot(pAbs, pAbs) < dot(nearestAbs, nearestAbs) ? -dist : dist;
+}
+
+#endif