Mat gray;
cvtColor(src, gray, COLOR_BGR2GRAY);
Mat thresh;
threshold(gray, thresh, 0, 255, THRESH_BINARY_INV | THRESH_OTSU);
//使用形态学开操作去除一些小轮廓
Mat kernel = getStructuringElement(MORPH_RECT, Size(3, 3));
Mat open;
morphologyEx(thresh, open, MORPH_OPEN, kernel);
以上每句代码是什么意思