From 26a61941cf0216202aee3378f17b05255170da17 Mon Sep 17 00:00:00 2001
From: Nick White <git@njw.name>
Date: Tue, 29 Jan 2019 13:33:50 +0000
Subject: Switch binarization to Sauvola algorithm

---
 binarize/sauvola.go | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 binarize/sauvola.go

(limited to 'binarize/sauvola.go')

diff --git a/binarize/sauvola.go b/binarize/sauvola.go
new file mode 100644
index 0000000..f1d0512
--- /dev/null
+++ b/binarize/sauvola.go
@@ -0,0 +1,91 @@
+package main
+
+import (
+	"image"
+	"image/color"
+	"math"
+)
+
+func mean(i []int) float64 {
+	sum := 0
+	for _, n := range i {
+		sum += n
+	}
+	return float64(sum) / float64(len(i))
+}
+
+// TODO: is there a prettier way of doing this than float64() all over the place?
+func stddev(i []int) float64 {
+	m := mean(i)
+
+	var sum float64
+	for _, n := range i {
+		sum += (float64(n) - m) * (float64(n) - m)
+	}
+	variance := float64(sum) / float64(len(i) - 1)
+	return math.Sqrt(variance)
+}
+
+func meanstddev(i []int) (float64, float64) {
+	m := mean(i)
+
+	var sum float64
+	for _, n := range i {
+		sum += (float64(n) - m) * (float64(n) - m)
+	}
+	variance := float64(sum) / float64(len(i) - 1)
+	return m, math.Sqrt(variance)
+}
+
+// gets the pixel values surrounding a point in the image
+func surrounding(img *image.Gray, x int, y int, size int) []int {
+	b := img.Bounds()
+
+	miny := y - size/2
+	if miny < b.Min.Y {
+		miny = b.Min.Y
+	}
+	minx := x - size/2
+	if minx < b.Min.X {
+		minx = b.Min.X
+	}
+	maxy := y + size/2
+	if maxy > b.Max.Y {
+		maxy = b.Max.Y
+	}
+	maxx := x + size/2
+	if maxx > b.Max.X {
+		maxx = b.Max.X
+	}
+
+	var s []int
+	for yi := miny; yi < maxy; yi++ {
+		for xi := minx; xi < maxx; xi++ {
+			s = append(s, int(img.GrayAt(xi, yi).Y))
+		}
+	}
+	return s
+}
+
+// TODO: parallelize
+// TODO: switch to using integral images to make faster; see paper
+//       "Efficient Implementation of Local Adaptive Thresholding Techniques Using Integral Images"
+func Sauvola(img *image.Gray, ksize float64, windowsize int) *image.Gray {
+	b := img.Bounds()
+	new := image.NewGray(b)
+
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			window := surrounding(img, x, y, windowsize)
+			m, dev := meanstddev(window)
+			threshold := m * (1 + ksize * ((dev / 128) - 1))
+			if img.GrayAt(x, y).Y < uint8(threshold) {
+				new.SetGray(x, y, color.Gray{0})
+			} else {
+				new.SetGray(x, y, color.Gray{255})
+			}
+		}
+	}
+
+	return new
+}
-- 
cgit v1.2.1-24-ge1ad