缺点：RGB对光强的变化很敏感。
优点：HSV对光强变化步敏感。
hue: which is represented as angular position around the cylinder as describing what color is in a pixel.
saturation: which is measured as radial distance from the center axes as being the intensity of that color.
value: value or brightness along the vertial axes.
RGB 转为 HSＶ, 使用openCV.
hsv_image = cv2.cvtColor(rgb_image,cv2.COLOR_RGB2HSV)
改变光强对比：
