Re-adding Topics to SVN

java/examples/Topics/Simulate/Chain/Chain.pde

@@ -0,0 +1,68 @@
+/**
+ * Chain. 
+ * 
+ * One mass is attached to the mouse position and the other 
+ * is attached the position of the other mass. The gravity
+ * in the environment pulls down on both. 
+ */
+
+
+Spring2D s1, s2;
+
+float gravity = 6.0;
+float mass = 2.0;
+
+void setup() 
+{
+  size(200, 200);
+  smooth();
+  fill(0);
+  // Inputs: x, y, mass, gravity
+  s1 = new Spring2D(0.0, width/2, mass, gravity);
+  s2 = new Spring2D(0.0, width/2, mass, gravity);
+}
+
+void draw() 
+{
+  background(204);
+  s1.update(mouseX, mouseY);
+  s1.display(mouseX, mouseY);
+  s2.update(s1.x, s1.y);
+  s2.display(s1.x, s1.y);
+}
+
+class Spring2D {
+  float vx, vy; // The x- and y-axis velocities
+  float x, y; // The x- and y-coordinates
+  float gravity;
+  float mass;
+  float radius = 20;
+  float stiffness = 0.2;
+  float damping = 0.7;
+  
+  Spring2D(float xpos, float ypos, float m, float g) {
+    x = xpos;
+    y = ypos;
+    mass = m;
+    gravity = g;
+  }
+  
+  void update(float targetX, float targetY) {
+    float forceX = (targetX - x) * stiffness;
+    float ax = forceX / mass;
+    vx = damping * (vx + ax);
+    x += vx;
+    float forceY = (targetY - y) * stiffness;
+    forceY += gravity;
+    float ay = forceY / mass;
+    vy = damping * (vy + ay);
+    y += vy;
+  }
+  
+  void display(float nx, float ny) {
+    noStroke();
+    ellipse(x, y, radius*2, radius*2);
+    stroke(255);
+    line(x, y, nx, ny);
+  }
+}