一：基本原理
该方法是图像二值化处理常见方法之一，在Matlab与OpenCV中均有实现。
Otsu Threshing方法是一种基于寻找合适阈值实现二值化的方法，其最重
要的部分是寻找图像二值化阈值，然后根据阈值将图像分为前景（白色）
或者背景（黑色）。假设有6x6的灰度图像，其像素数据及其对应的直方
图如下图：
阈值寻找方法首先假设是为T=3，则背景像素的比重、均值、方差的计算
结果如下：
根据前景像素直方图，计算比重、均值、方差的过程如下：
, 0, width, height, inPixels );
int index = 0;
for(int row=0; row<height; row++) {
int ta = 0, tr = 0, tg = 0, tb = 0;
for(int col=0; col<width; col++) {
index = row * width + col;
ta = (inPixels[index] >> 24) & 0xff;
tr = (inPixels[index] >> 16) & 0xff;
tg = (inPixels[index] >> 8) & 0xff;
tb = inPixels[index] & 0xff;
int gray= (int)(0.299 *tr + 0.587*tg + 0.114*tb);
inPixels[index] = (ta << 24) | (gray << 16) | (gray << 8) | gray;
}
}
// 获取直方图
int[] histogram = new int[256];
for(int row=0; row<height; row++) {
int tr = 0;
for(int col=0; col<width; col++) {
index = row * width + col;
tr = (inPixels[index] >> 16) & 0xff;
histogram[tr]++;
}
}
// 图像二值化 - OTSU 阈值化方法
double total = width * height;
double[] variances = new double[256];
for(int i=0; i<variances.length; i++)
{
double bw = 0;
double bmeans = 0;
double bvariance = 0;
double count = 0;
for(int t=0; t<i; t++)
{
count += histogram[t];
bmeans += histogram[t] * t;
}
bw = count / total;
bmeans = (count == 0) ? 0 :(bmeans / count);
for(int t=0; t<i; t++)
{
bvariance += (Math.pow((t-bmeans),2) * histogram[t]);
}
bvariance = (count == 0) ? 0 : (bvariance / count);
double fw = 0;
double fmeans = 0;
double fvariance = 0;
count = 0;
for(int t=i; t<histogram.length; t++)
{
count += histogram[t];
fmeans += histogram[t] * t;
}
fw = count / total;
fmeans = (count == 0) ? 0 : (fmeans / count);
for(int t=i; t<histogram.length; t++)
{
fvariance += (Math.pow((t-fmeans),2) * histogram[t]);
}
fvariance = (count == 0) ? 0 : (fvariance / count);
variances[i] = bw * bvariance + fw * fvariance;
}
// find the minimum within class variance
double min = variances[0];
int threshold = 0;
for(int m=1; m<variances.length; m++)
{
if(min > variances[m]){
threshold = m;
min = variances[m];
}
}
// 二值化
System.out.println("final threshold value : " + threshold);
for(int row=0; row<height; row++) {
for(int col=0; col<width; col++) {
index = row * width + col;
int gray = (inPixels[index] >> 8) & 0xff;
if(gray > threshold)
{
gray = 255;
outPixels[index] = (0xff << 24) | (gray << 16) | (gray << 8) | gray;
}
else
{
gray = 0;
outPixels[index] = (0xff << 24) | (gray << 16) | (gray << 8) | gray;
}
}
}
setRGB(dest, 0, 0, width, height, outPixels );
return dest;
}
}