顏色空間，常見的有三種模式：

灰色圖
BGR
HSV

備註：在opencv裡面，BGR，這個順序要注意的，常見的是RGB的稱呼

經常用到的 兩種：BGR↔Gray 和 BGR↔HSV

函式是：cv2.cvtColor(input_image ，flag)，
其中 flag是轉換型別

對於 BGR↔Gray 的轉換，使用的 flag 就是 cv2.COLOR_BGR2GRAY
對於 BGR↔HSV 的轉換，我們用的 flag 就是 cv2.COLOR_BGR2HSV

注意：在 OpenCV 的 HSV 格式中，
H（色彩/色度）的取值範圍是 [0，179]，
S（飽和度）的取值範圍 [0，255]，
V（亮度）的取值範圍 [0，255]
但是不同的軟體使用的值可能不同 ，需要歸一化

列印所有的flag

import cv2

for i in dir(cv2) :
    if i.startswith('COLOR_'):
        print (i)

結果：
COLOR_BAYER_BG2BGR
COLOR_BAYER_BG2BGRA
COLOR_BAYER_BG2BGR_EA
COLOR_BAYER_BG2BGR_VNG
COLOR_BAYER_BG2GRAY
COLOR_BAYER_BG2RGB
COLOR_BAYER_BG2RGBA
COLOR_BAYER_BG2RGB_EA
COLOR_BAYER_BG2RGB_VNG
COLOR_BAYER_GB2BGR
COLOR_BAYER_GB2BGRA
COLOR_BAYER_GB2BGR_EA
COLOR_BAYER_GB2BGR_VNG
COLOR_BAYER_GB2GRAY
COLOR_BAYER_GB2RGB
COLOR_BAYER_GB2RGBA
COLOR_BAYER_GB2RGB_EA
COLOR_BAYER_GB2RGB_VNG
COLOR_BAYER_GR2BGR
COLOR_BAYER_GR2BGRA
COLOR_BAYER_GR2BGR_EA
COLOR_BAYER_GR2BGR_VNG
COLOR_BAYER_GR2GRAY
COLOR_BAYER_GR2RGB
COLOR_BAYER_GR2RGBA
COLOR_BAYER_GR2RGB_EA
COLOR_BAYER_GR2RGB_VNG
COLOR_BAYER_RG2BGR
COLOR_BAYER_RG2BGRA
COLOR_BAYER_RG2BGR_EA
COLOR_BAYER_RG2BGR_VNG
COLOR_BAYER_RG2GRAY
COLOR_BAYER_RG2RGB
COLOR_BAYER_RG2RGBA
COLOR_BAYER_RG2RGB_EA
COLOR_BAYER_RG2RGB_VNG
COLOR_BGR2BGR555
COLOR_BGR2BGR565
COLOR_BGR2BGRA
COLOR_BGR2GRAY
COLOR_BGR2HLS
COLOR_BGR2HLS_FULL
COLOR_BGR2HSV
COLOR_BGR2HSV_FULL
COLOR_BGR2LAB
COLOR_BGR2LUV
COLOR_BGR2Lab
COLOR_BGR2Luv
COLOR_BGR2RGB
COLOR_BGR2RGBA
COLOR_BGR2XYZ
COLOR_BGR2YCR_CB
COLOR_BGR2YCrCb
COLOR_BGR2YUV
COLOR_BGR2YUV_I420
COLOR_BGR2YUV_IYUV
COLOR_BGR2YUV_YV12
COLOR_BGR5552BGR
COLOR_BGR5552BGRA
COLOR_BGR5552GRAY
COLOR_BGR5552RGB
COLOR_BGR5552RGBA
COLOR_BGR5652BGR
COLOR_BGR5652BGRA
COLOR_BGR5652GRAY
COLOR_BGR5652RGB
COLOR_BGR5652RGBA
COLOR_BGRA2BGR
COLOR_BGRA2BGR555
COLOR_BGRA2BGR565
COLOR_BGRA2GRAY
COLOR_BGRA2RGB
COLOR_BGRA2RGBA
COLOR_BGRA2YUV_I420
COLOR_BGRA2YUV_IYUV
COLOR_BGRA2YUV_YV12
COLOR_BayerBG2BGR
COLOR_BayerBG2BGRA
COLOR_BayerBG2BGR_EA
COLOR_BayerBG2BGR_VNG
COLOR_BayerBG2GRAY
COLOR_BayerBG2RGB
COLOR_BayerBG2RGBA
COLOR_BayerBG2RGB_EA
COLOR_BayerBG2RGB_VNG
COLOR_BayerGB2BGR
COLOR_BayerGB2BGRA
COLOR_BayerGB2BGR_EA
COLOR_BayerGB2BGR_VNG
COLOR_BayerGB2GRAY
COLOR_BayerGB2RGB
COLOR_BayerGB2RGBA
COLOR_BayerGB2RGB_EA
COLOR_BayerGB2RGB_VNG
COLOR_BayerGR2BGR
COLOR_BayerGR2BGRA
COLOR_BayerGR2BGR_EA
COLOR_BayerGR2BGR_VNG
COLOR_BayerGR2GRAY
COLOR_BayerGR2RGB
COLOR_BayerGR2RGBA
COLOR_BayerGR2RGB_EA
COLOR_BayerGR2RGB_VNG
COLOR_BayerRG2BGR
COLOR_BayerRG2BGRA
COLOR_BayerRG2BGR_EA
COLOR_BayerRG2BGR_VNG
COLOR_BayerRG2GRAY
COLOR_BayerRG2RGB
COLOR_BayerRG2RGBA
COLOR_BayerRG2RGB_EA
COLOR_BayerRG2RGB_VNG
COLOR_COLORCVT_MAX
COLOR_GRAY2BGR
COLOR_GRAY2BGR555
COLOR_GRAY2BGR565
COLOR_GRAY2BGRA
COLOR_GRAY2RGB
COLOR_GRAY2RGBA
COLOR_HLS2BGR
COLOR_HLS2BGR_FULL
COLOR_HLS2RGB
COLOR_HLS2RGB_FULL
COLOR_HSV2BGR
COLOR_HSV2BGR_FULL
COLOR_HSV2RGB
COLOR_HSV2RGB_FULL
COLOR_LAB2BGR
COLOR_LAB2LBGR
COLOR_LAB2LRGB
COLOR_LAB2RGB
COLOR_LBGR2LAB
COLOR_LBGR2LUV
COLOR_LBGR2Lab
COLOR_LBGR2Luv
COLOR_LRGB2LAB
COLOR_LRGB2LUV
COLOR_LRGB2Lab
COLOR_LRGB2Luv
COLOR_LUV2BGR
COLOR_LUV2LBGR
COLOR_LUV2LRGB
COLOR_LUV2RGB
COLOR_Lab2BGR
COLOR_Lab2LBGR
COLOR_Lab2LRGB
COLOR_Lab2RGB
COLOR_Luv2BGR
COLOR_Luv2LBGR
COLOR_Luv2LRGB
COLOR_Luv2RGB
COLOR_M_RGBA2RGBA
COLOR_RGB2BGR
COLOR_RGB2BGR555
COLOR_RGB2BGR565
COLOR_RGB2BGRA
COLOR_RGB2GRAY
COLOR_RGB2HLS
COLOR_RGB2HLS_FULL
COLOR_RGB2HSV
COLOR_RGB2HSV_FULL
COLOR_RGB2LAB
COLOR_RGB2LUV
COLOR_RGB2Lab
COLOR_RGB2Luv
COLOR_RGB2RGBA
COLOR_RGB2XYZ
COLOR_RGB2YCR_CB
COLOR_RGB2YCrCb
COLOR_RGB2YUV
COLOR_RGB2YUV_I420
COLOR_RGB2YUV_IYUV
COLOR_RGB2YUV_YV12
COLOR_RGBA2BGR
COLOR_RGBA2BGR555
COLOR_RGBA2BGR565
COLOR_RGBA2BGRA
COLOR_RGBA2GRAY
COLOR_RGBA2M_RGBA
COLOR_RGBA2RGB
COLOR_RGBA2YUV_I420
COLOR_RGBA2YUV_IYUV
COLOR_RGBA2YUV_YV12
COLOR_RGBA2mRGBA
COLOR_XYZ2BGR
COLOR_XYZ2RGB
COLOR_YCR_CB2BGR
COLOR_YCR_CB2RGB
COLOR_YCrCb2BGR
COLOR_YCrCb2RGB
COLOR_YUV2BGR
COLOR_YUV2BGRA_I420
COLOR_YUV2BGRA_IYUV
COLOR_YUV2BGRA_NV12
COLOR_YUV2BGRA_NV21
COLOR_YUV2BGRA_UYNV
COLOR_YUV2BGRA_UYVY
COLOR_YUV2BGRA_Y422
COLOR_YUV2BGRA_YUNV
COLOR_YUV2BGRA_YUY2
COLOR_YUV2BGRA_YUYV
COLOR_YUV2BGRA_YV12
COLOR_YUV2BGRA_YVYU
COLOR_YUV2BGR_I420
COLOR_YUV2BGR_IYUV
COLOR_YUV2BGR_NV12
COLOR_YUV2BGR_NV21
COLOR_YUV2BGR_UYNV
COLOR_YUV2BGR_UYVY
COLOR_YUV2BGR_Y422
COLOR_YUV2BGR_YUNV
COLOR_YUV2BGR_YUY2
COLOR_YUV2BGR_YUYV
COLOR_YUV2BGR_YV12
COLOR_YUV2BGR_YVYU
COLOR_YUV2GRAY_420
COLOR_YUV2GRAY_I420
COLOR_YUV2GRAY_IYUV
COLOR_YUV2GRAY_NV12
COLOR_YUV2GRAY_NV21
COLOR_YUV2GRAY_UYNV
COLOR_YUV2GRAY_UYVY
COLOR_YUV2GRAY_Y422
COLOR_YUV2GRAY_YUNV
COLOR_YUV2GRAY_YUY2
COLOR_YUV2GRAY_YUYV
COLOR_YUV2GRAY_YV12
COLOR_YUV2GRAY_YVYU
COLOR_YUV2RGB
COLOR_YUV2RGBA_I420
COLOR_YUV2RGBA_IYUV
COLOR_YUV2RGBA_NV12
COLOR_YUV2RGBA_NV21
COLOR_YUV2RGBA_UYNV
COLOR_YUV2RGBA_UYVY
COLOR_YUV2RGBA_Y422
COLOR_YUV2RGBA_YUNV
COLOR_YUV2RGBA_YUY2
COLOR_YUV2RGBA_YUYV
COLOR_YUV2RGBA_YV12
COLOR_YUV2RGBA_YVYU
COLOR_YUV2RGB_I420
COLOR_YUV2RGB_IYUV
COLOR_YUV2RGB_NV12
COLOR_YUV2RGB_NV21
COLOR_YUV2RGB_UYNV
COLOR_YUV2RGB_UYVY
COLOR_YUV2RGB_Y422
COLOR_YUV2RGB_YUNV
COLOR_YUV2RGB_YUY2
COLOR_YUV2RGB_YUYV
COLOR_YUV2RGB_YV12
COLOR_YUV2RGB_YVYU
COLOR_YUV420P2BGR
COLOR_YUV420P2BGRA
COLOR_YUV420P2GRAY
COLOR_YUV420P2RGB
COLOR_YUV420P2RGBA
COLOR_YUV420SP2BGR
COLOR_YUV420SP2BGRA
COLOR_YUV420SP2GRAY
COLOR_YUV420SP2RGB
COLOR_YUV420SP2RGBA
COLOR_YUV420p2BGR
COLOR_YUV420p2BGRA
COLOR_YUV420p2GRAY
COLOR_YUV420p2RGB
COLOR_YUV420p2RGBA
COLOR_YUV420sp2BGR
COLOR_YUV420sp2BGRA
COLOR_YUV420sp2GRAY
COLOR_YUV420sp2RGB
COLOR_YUV420sp2RGBA
COLOR_mRGBA2RGBA