# Sets the background gray value based on the distance of the mouse from the
# center of the screen
draw <- function() {
    noStroke()
    d = dist(width/2, height/2, mouseX, mouseY)
    maxDist = dist(0, 0, width/2, height/2)
    gray = map(d, 0, maxDist, 0, 255)
    fill(gray)
    rect(0, 0, width, height)
}