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// Copyright 2019 Nick White.
// Use of this source code is governed by the GPLv3
// license that can be found in the LICENSE file.
package preproc
import (
"image"
"image/color"
"image/draw"
"math"
"rescribe.xyz/integral"
)
// Implements Sauvola's algorithm for text binarization, see paper
// "Adaptive document image binarization" (2000)
func Sauvola(img image.Image, ksize float64, windowsize int) *image.Gray {
b := img.Bounds()
gray := image.NewGray(b)
draw.Draw(gray, b, img, b.Min, draw.Src)
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(gray, x, y, windowsize)
m, dev := meanstddev(window)
threshold := m * (1 + ksize*((dev/128)-1))
if gray.GrayAt(x, y).Y < uint8(math.Round(threshold)) {
new.SetGray(x, y, color.Gray{0})
} else {
new.SetGray(x, y, color.Gray{255})
}
}
}
return new
}
// Implements Sauvola's algorithm using Integral Images, see paper
// "Efficient Implementation of Local Adaptive Thresholding Techniques Using Integral Images"
// and
// https://stackoverflow.com/questions/13110733/computing-image-integral
func IntegralSauvola(img image.Image, ksize float64, windowsize int) *image.Gray {
b := img.Bounds()
intImg := integral.NewImage(b)
draw.Draw(intImg, b, img, b.Min, draw.Src)
intSqImg := integral.NewSqImage(b)
draw.Draw(intSqImg, b, img, b.Min, draw.Src)
return PreCalcedSauvola(*intImg, *intSqImg, img, ksize, windowsize)
}
// PreCalcedSauvola Implements Sauvola's algorithm using precalculated Integral Images
func PreCalcedSauvola(intImg integral.Image, intSqImg integral.SqImage, img image.Image, ksize float64, windowsize int) *image.Gray {
b := img.Bounds()
gray := image.NewGray(b)
draw.Draw(gray, b, img, b.Min, draw.Src)
new := image.NewGray(b)
for y := b.Min.Y; y < b.Max.Y; y++ {
for x := b.Min.X; x < b.Max.X; x++ {
r := centeredRectangle(x, y, windowsize)
m, dev := integral.MeanStdDev(intImg, intSqImg, r)
// Divide by 255 to adjust from Gray16 used by integral.Image to 8 bit Gray
m8 := m / 255
dev8 := dev / 255
threshold := m8 * (1 + ksize*((dev8/128)-1))
if gray.GrayAt(x, y).Y < uint8(math.Round(threshold)) {
new.SetGray(x, y, color.Gray{0})
} else {
new.SetGray(x, y, color.Gray{255})
}
}
}
return new
}
func centeredRectangle(x, y, size int) image.Rectangle {
step := size / 2
return image.Rect(x - step - 1, y - step - 1, x + step, y + step)
}
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