float ang = 0;
void setup(){
  size(600, 400, P3D);  
  framerate(30);
}
void draw(){
  background(0); 
  ambientLight(140, 140, 140);
  translate(width/2, height/2, 0);

  //rotate 90 degs and then find 
  //theta(angle of rotation in radians) 
  //based on mouse position
  rotateZ(PI/2+atan2(mouseY-height/2, mouseX-width/2));
  
  //this second rotation just
  //keeps pyramid spinnning
  rotateY(radians(ang-=PI/2.0));
  scale(50, 50, 50);

  beginShape(TRIANGLES);
  //face 1
  fill(45, 200, 20);
  vertex(-1, 1, -1); 
  vertex(0, -1, 0);  
  vertex(1, 1 , -1); 

  //face 2
  fill(255, 30, 30);
  vertex(-1, 1, 1); 
  vertex(0, -1, 0);  
  vertex(1, 1 , 1);

  //face 3
  fill(140, 45, 255);
  vertex(-1, 1, -1); 
  vertex(-1, 1, 1); 
  vertex(0, -1, 0); 

  //face 4
  fill(255, 255, 30);
  vertex(1, 1, -1); 
  vertex(1, 1, 1); 
  vertex(0, -1, 0); 
  endShape(); 
}