This pages tests screenY().
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 x value // Notice the parallax stroke(102); line(x, 0, 0, x, height, 0); // Draw 2D line to match the x value // element drawn at z = -100 stroke(0); float theX = screenX(x, y, z); line(theX, 0, 0, theX, height, 0); }