Move the mouse the change the direction of the light. Directional light comes from one direction and is stronger when hitting a surface squarely and weaker if it hits at a a gentle angle. After hitting a surface, a directional lights scatters in all directions. 

def setup():
    size(640, 360, P3D)
    noStroke()
    fill(204)


def draw():
    noStroke()
    background(0)
    dirY = (mouseY / float(height) - 0.5) * 2
    dirX = (mouseX / float(width) - 0.5) * 2
    directionalLight(204, 204, 204, -dirX, -dirY, -1)
    translate(width / 2 - 100, height / 2, 0)
    sphere(80)
    translate(200, 0, 0)
    sphere(80)