void setup() { 
  size(100, 100, P3D); 
} 
 
void draw() { 
  background(204); 
  
  float x = mouseX; 
  float y = mouseY; 
  float z = -100; 
  
  // Draw "X" at z = -100 
  stroke(255); 
  line(x-10, y-10, z, x+10, y+10, z); 
  line(x+10, y-10, z, x-10, y+10, z); 
  
  // Draw line in 2D at same y value 
  // Notice the parallax 
  stroke(102); 
  line(0, y, 0, width, y, 0); 
  
  // Draw 2D line to match the y value 
  // element drawn at z = -100 
  stroke(0); 
  float theY = screenY(x, y, z); 
  line(0, theY, 0, width, theY, 0);    
}