1package imaging
 2
 3import (
 4	"image"
 5	"sync"
 6)
 7
 8// Histogram returns a normalized histogram of an image.
 9//
10// Resulting histogram is represented as an array of 256 floats, where
11// histogram[i] is a probability of a pixel being of a particular luminance i.
12func Histogram(img image.Image) [256]float64 {
13	var mu sync.Mutex
14	var histogram [256]float64
15	var total float64
16
17	src := newScanner(img)
18	if src.w == 0 || src.h == 0 {
19		return histogram
20	}
21
22	parallel(0, src.h, func(ys <-chan int) {
23		var tmpHistogram [256]float64
24		var tmpTotal float64
25		scanLine := make([]uint8, src.w*4)
26		for y := range ys {
27			src.scan(0, y, src.w, y+1, scanLine)
28			i := 0
29			for x := 0; x < src.w; x++ {
30				s := scanLine[i : i+3 : i+3]
31				r := s[0]
32				g := s[1]
33				b := s[2]
34				y := 0.299*float32(r) + 0.587*float32(g) + 0.114*float32(b)
35				tmpHistogram[int(y+0.5)]++
36				tmpTotal++
37				i += 4
38			}
39		}
40		mu.Lock()
41		for i := 0; i < 256; i++ {
42			histogram[i] += tmpHistogram[i]
43		}
44		total += tmpTotal
45		mu.Unlock()
46	})
47
48	for i := 0; i < 256; i++ {
49		histogram[i] = histogram[i] / total
50	}
51	return histogram
52}