code: func Histogram(img image.Image) [256]float64 {
  	var mu sync.Mutex
  	var histogram [256]float64
  	var total float64
  
  	src := newScanner(img)
  	if src.w == 0 || src.h == 0 {
  		return histogram
  	}
  
  	parallel(0, src.h, func(ys <-chan int) {
  		var tmpHistogram [256]float64
  		var tmpTotal float64
  		scanLine := make([]uint8, src.w*4)
  		for y := range ys {
  			src.scan(0, y, src.w, y+1, scanLine)
  			i := 0
  			for x := 0; x < src.w; x++ {
  				s := scanLine[i : i+3 : i+3]
  				r := s[0]
  				g := s[1]
  				b := s[2]
  				y := 0.299*float32(r) + 0.587*float32(g) + 0.114*float32(b)
  				tmpHistogram[int(y+0.5)]++
  				tmpTotal++
  				i += 4
  			}
  		}
  		mu.Lock()
  		for i := 0; i < 256; i++ {
  			histogram[i] += tmpHistogram[i]
  		}
  		total += tmpTotal
  		mu.Unlock()
  	})
  
  	for i := 0; i < 256; i++ {
  		histogram[i] = histogram[i] / total
  	}
  	return histogram
  }
docs: // Histogram returns a normalized histogram of an image.
  //
  // Resulting histogram is represented as an array of 256 floats, where
  // histogram[i] is a probability of a pixel being of a particular luminance i.