double threshold( InputArray src, OutputArray dst, double thresh, double maxval, int type );

参数： src：原图像。 dst：结果图像。 thresh：当前阈值。 maxVal：最大阈值，一般为255。 thresholdType：阈值类型。

返回值： retval： 与参数thresh一致 dst： 结果图像

阈值类型： THRESH_BINARY = 0, THRESH_BINARY_INV = 1, THRESH_TRUNC = 2, THRESH_TOZERO = 3, THRESH_TOZERO_INV = 4, THRESH_MASK = 7, THRESH_OTSU = 8, THRESH_TRIANGLE = 16

注意： THRESH_OTSU和THRESH_TRIANGLE是作为优化算法配合THRESH_BINARY、THRESH_BINARY_INV、THRESH_TRUNC、THRESH_TOZERO以及THRESH_TOZERO_INV来使用的。

当使用了THRESH_OTSU和THRESH_TRIANGLE两个标志时，输入图像必须为单通道。

固定的方法获取图像阈值 THRESH_OTSU THRESH_TRIANGLE 忽略设置的 thresh 值，会自动生成阈值 threshold(src, dst, 0, 255, THRESH_TRIANGLE | type);