1 files changed, 52 insertions, 0 deletions
diff --git a/Plugins/MonoGame.Extended/source/MonoGame.Extended.Particles/Modifiers/Containers/CircleContainerModifier.cs b/Plugins/MonoGame.Extended/source/MonoGame.Extended.Particles/Modifiers/Containers/CircleContainerModifier.cs
new file mode 100644
index 0000000..30d2802
--- /dev/null
+++ b/Plugins/MonoGame.Extended/source/MonoGame.Extended.Particles/Modifiers/Containers/CircleContainerModifier.cs
@@ -0,0 +1,52 @@
+using System;
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Particles.Modifiers.Containers
+{
+    public class CircleContainerModifier : Modifier
+    {
+        public float Radius { get; set; }
+        public bool Inside { get; set; } = true;
+        public float RestitutionCoefficient { get; set; } = 1;
+
+        public override unsafe void Update(float elapsedSeconds, ParticleBuffer.ParticleIterator iterator)
+        {
+            var radiusSq = Radius*Radius;
+            while (iterator.HasNext)
+            {
+                var particle = iterator.Next();
+                var localPos = particle->Position - particle->TriggerPos;
+
+                var distSq = localPos.LengthSquared();
+                var normal = localPos;
+                normal.Normalize();
+
+                if (Inside)
+                {
+                    if (distSq < radiusSq) continue;
+
+                    SetReflected(distSq, particle, normal);
+                }
+                else
+                {
+                    if (distSq > radiusSq) continue;
+
+                    SetReflected(distSq, particle, -normal);
+                }
+            }
+        }
+
+        private unsafe void SetReflected(float distSq, Particle* particle, Vector2 normal)
+        {
+            var dist = (float) Math.Sqrt(distSq);
+            var d = dist - Radius; // how far outside the circle is the particle
+
+            var twoRestDot = 2*RestitutionCoefficient*
+                             Vector2.Dot(particle->Velocity, normal);
+            particle->Velocity -= twoRestDot*normal;
+
+            // exact computation requires sqrt or goniometrics
+            particle->Position -= normal*d;
+        }
+    }
+}
+\ No newline at end of file