float x,y,r;
float vx,vy;
float t,dt;

boolean set_pos,set_vel;

void setup() {
  size(700,500);
  background(0);
  frameRate(200);
  noStroke();
  x = mouseX;
  y = mouseY;
  set_pos = false;
  set_vel = false;
  dt = 0.25;
}

void draw() {
  fill(34,34,68,20);
  rect(0,0,width,height);
  if (set_vel) {
    if (x<=r || x>width-r)  vx = -vx;
    if (y<=r || y>height-r) vy = -vy;   
    x += vx*dt;
    y += vy*dt;
    float u=x,v=y;
    float vu=vx,vv=vy;
    stroke(15,150,255,100);
    strokeWeight(4);
    alpha(1);
    for (int i=0; i<500; i++) {
      if (u<=r || u>width-r)  vu = -vu;
      if (v<=r || v>height-r) vv = -vv;
      stroke(15,150,255,255*(1-i/500.));   
      line(u,v,u+vu*dt,v+vv*dt);
      u += vu*dt;
      v += vv*dt;
    }
    noStroke();
  } else if (set_pos) {
    strokeWeight(2);
    line(x,y,mouseX,mouseY);
    noStroke();
  } else {
    ellipse(mouseX,mouseY,20,20);
  }
  fill(50,100,255,120);
  ellipse(x,y,20,20);
  if (mousePressed && set_pos) {
    vx = mouseX-x;
    vy = mouseY-y;
    set_vel = true;
  } else if (mousePressed) {
    x = (int)(mouseX);
    y = (int)(mouseY);
    set_pos = true;
  }
}