code: func blend(d, s uint8, alpha float64) uint8 {
  	return uint8(float64(s)*alpha + float64(d)*(1-alpha))
  }
docs: // blend calculates the value of a color given some alpha value in [0, 1]
  // and a source and destination color. Note that this assumes that the
  // destination is fully opaque (has an alpha value of 1).