OpenCV图像的全局阈值二值化函数（OTSU）
cv::threshold(GrayImg, Bw, 0, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);//灰度图像二值化
CV_THRESH_OTSU是提取图像最佳阈值算法。该方法在类间方差最大的情况下是最佳的，就图像的灰度值而言，OTSU给出最好的类间分离的阈值。
OpenCV阈值分割的几种方法（types_c.h中的定义):
/* Threshold types */
enum
{
CV_THRESH_BINARY =0, /* value = value > threshold ? max_value : 0 */
CV_THRESH_BINARY_INV =1, /* value = value > threshold ? 0 : max_value */
CV_THRESH_TRUNC =2, /* value = value > threshold ? threshold : value */
CV_THRESH_TOZERO =3, /* value = value > threshold ? value : 0 */
CV_THRESH_TOZERO_INV =4, /* value = value > threshold ? 0 : value */
CV_THRESH_MASK =7,
CV_THRESH_OTSU =8 /* use Otsu algorithm to choose the optimal threshold value;
combine the flag with one of the above CV_THRESH_* values */
}