您正在为蒙版设置颜色(0,255,0)，但是该蒙版是单通道的，因此您可以使用颜色0绘制轮廓.

you are setting color (0,255,0) to the mask, but the mask is single channel so you draw the contour in color 0.

尝试

 cv.drawContours(mask, contours, -1, (255),1)

或

 cv.drawContours(mask, contours, -1, (255,255,255),1)