view.opacity = 0.5

var origin = Point()
let size = Size(canvas.width/10, canvas.height)
let dx = Vector(x: size.width, y: 0)
repeat {
    let r = Rectangle(frame: Rect(origin, size))
    r.opacity = origin.x/canvas.width
    canvas.add(r)
    origin += dx
} while origin.x < canvas.width