官方解析

cvtColor

把图像从一个颜色空间转换到另外一个。通道的顺序说明了颜色的顺序，比如RGB或BGR。在OpenCV默认是RGB但，通常传入BGR，因为字节序是相反的。在标准的图像文件（24bit）中前8bit是蓝色，中间8bit是绿，最后的8bit是红。

通常情况下R、G、B颜色通道的取值范围：
0-255是CV_8U图像
0-65535是CV_16U图像
0-1是CV_32F图像

注意：单通道的图像（灰度图像）转成32位3通道的图像是会有丢失的，灰色图像不能转化为彩色，但彩色可以转化为灰度。
如果要增加alpha通道则要增加这个通道对应值的范围：
255是CV_8U
65535是CV_16U
1是CV_32F

CV_EXPORTS_W void cvtColor( InputArray src, OutputArray dst, int code, int dstCn = 0 );

这里只说一下最后两个参数：
code是值转化为的颜色空间，具体可查cv::ColorConversionCodes
dstCn，指的是dst的通道数量，0表示由src与code自动生成对应的通道数

关于第三个参数的枚举值：

enum ColorConversionCodes {COLOR_BGR2BGRA     = 0, //!< add alpha channel to RGB or BGR imageCOLOR_RGB2RGBA     = COLOR_BGR2BGRA,COLOR_BGRA2BGR     = 1, //!< remove alpha channel from RGB or BGR imageCOLOR_RGBA2RGB     = COLOR_BGRA2BGR,COLOR_BGR2RGBA     = 2, //!< convert between RGB and BGR color spaces (with or without alpha channel)COLOR_RGB2BGRA     = COLOR_BGR2RGBA,COLOR_RGBA2BGR     = 3,COLOR_BGRA2RGB     = COLOR_RGBA2BGR,COLOR_BGR2RGB      = 4,COLOR_RGB2BGR      = COLOR_BGR2RGB,COLOR_BGRA2RGBA    = 5,COLOR_RGBA2BGRA    = COLOR_BGRA2RGBA,COLOR_BGR2GRAY     = 6, //!< convert between RGB/BGR and grayscale, @ref color_convert_rgb_gray "color conversions"COLOR_RGB2GRAY     = 7,COLOR_GRAY2BGR     = 8,COLOR_GRAY2RGB     = COLOR_GRAY2BGR,COLOR_GRAY2BGRA    = 9,COLOR_GRAY2RGBA    = COLOR_GRAY2BGRA,COLOR_BGRA2GRAY    = 10,COLOR_RGBA2GRAY    = 11,COLOR_BGR2BGR565   = 12, //!< convert between RGB/BGR and BGR565 (16-bit images)COLOR_RGB2BGR565   = 13,COLOR_BGR5652BGR   = 14,COLOR_BGR5652RGB   = 15,COLOR_BGRA2BGR565  = 16,COLOR_RGBA2BGR565  = 17,COLOR_BGR5652BGRA  = 18,COLOR_BGR5652RGBA  = 19,COLOR_GRAY2BGR565  = 20, //!< convert between grayscale to BGR565 (16-bit images)COLOR_BGR5652GRAY  = 21,COLOR_BGR2BGR555   = 22,  //!< convert between RGB/BGR and BGR555 (16-bit images)COLOR_RGB2BGR555   = 23,COLOR_BGR5552BGR   = 24,COLOR_BGR5552RGB   = 25,COLOR_BGRA2BGR555  = 26,COLOR_RGBA2BGR555  = 27,COLOR_BGR5552BGRA  = 28,COLOR_BGR5552RGBA  = 29,COLOR_GRAY2BGR555  = 30, //!< convert between grayscale and BGR555 (16-bit images)COLOR_BGR5552GRAY  = 31,COLOR_BGR2XYZ      = 32, //!< convert RGB/BGR to CIE XYZ, @ref color_convert_rgb_xyz "color conversions"COLOR_RGB2XYZ      = 33,COLOR_XYZ2BGR      = 34,COLOR_XYZ2RGB      = 35,COLOR_BGR2YCrCb    = 36, //!< convert RGB/BGR to luma-chroma (aka YCC), @ref color_convert_rgb_ycrcb "color conversions"COLOR_RGB2YCrCb    = 37,COLOR_YCrCb2BGR    = 38,COLOR_YCrCb2RGB    = 39,COLOR_BGR2HSV      = 40, //!< convert RGB/BGR to HSV (hue saturation value), @ref color_convert_rgb_hsv "color conversions"COLOR_RGB2HSV      = 41,COLOR_BGR2Lab      = 44, //!< convert RGB/BGR to CIE Lab, @ref color_convert_rgb_lab "color conversions"COLOR_RGB2Lab      = 45,COLOR_BGR2Luv      = 50, //!< convert RGB/BGR to CIE Luv, @ref color_convert_rgb_luv "color conversions"COLOR_RGB2Luv      = 51,COLOR_BGR2HLS      = 52, //!< convert RGB/BGR to HLS (hue lightness saturation), @ref color_convert_rgb_hls "color conversions"COLOR_RGB2HLS      = 53,COLOR_HSV2BGR      = 54, //!< backward conversions to RGB/BGRCOLOR_HSV2RGB      = 55,COLOR_Lab2BGR      = 56,COLOR_Lab2RGB      = 57,COLOR_Luv2BGR      = 58,COLOR_Luv2RGB      = 59,COLOR_HLS2BGR      = 60,COLOR_HLS2RGB      = 61,COLOR_BGR2HSV_FULL = 66, //!<COLOR_RGB2HSV_FULL = 67,COLOR_BGR2HLS_FULL = 68,COLOR_RGB2HLS_FULL = 69,COLOR_HSV2BGR_FULL = 70,COLOR_HSV2RGB_FULL = 71,COLOR_HLS2BGR_FULL = 72,COLOR_HLS2RGB_FULL = 73,COLOR_LBGR2Lab     = 74,COLOR_LRGB2Lab     = 75,COLOR_LBGR2Luv     = 76,COLOR_LRGB2Luv     = 77,COLOR_Lab2LBGR     = 78,COLOR_Lab2LRGB     = 79,COLOR_Luv2LBGR     = 80,COLOR_Luv2LRGB     = 81,COLOR_BGR2YUV      = 82, //!< convert between RGB/BGR and YUVCOLOR_RGB2YUV      = 83,COLOR_YUV2BGR      = 84,COLOR_YUV2RGB      = 85,//! YUV 4:2:0 family to RGBCOLOR_YUV2RGB_NV12  = 90,COLOR_YUV2BGR_NV12  = 91,COLOR_YUV2RGB_NV21  = 92,COLOR_YUV2BGR_NV21  = 93,COLOR_YUV420sp2RGB  = COLOR_YUV2RGB_NV21,COLOR_YUV420sp2BGR  = COLOR_YUV2BGR_NV21,COLOR_YUV2RGBA_NV12 = 94,COLOR_YUV2BGRA_NV12 = 95,COLOR_YUV2RGBA_NV21 = 96,COLOR_YUV2BGRA_NV21 = 97,COLOR_YUV420sp2RGBA = COLOR_YUV2RGBA_NV21,COLOR_YUV420sp2BGRA = COLOR_YUV2BGRA_NV21,COLOR_YUV2RGB_YV12  = 98,COLOR_YUV2BGR_YV12  = 99,COLOR_YUV2RGB_IYUV  = 100,COLOR_YUV2BGR_IYUV  = 101,COLOR_YUV2RGB_I420  = COLOR_YUV2RGB_IYUV,COLOR_YUV2BGR_I420  = COLOR_YUV2BGR_IYUV,COLOR_YUV420p2RGB   = COLOR_YUV2RGB_YV12,COLOR_YUV420p2BGR   = COLOR_YUV2BGR_YV12,COLOR_YUV2RGBA_YV12 = 102,COLOR_YUV2BGRA_YV12 = 103,COLOR_YUV2RGBA_IYUV = 104,COLOR_YUV2BGRA_IYUV = 105,COLOR_YUV2RGBA_I420 = COLOR_YUV2RGBA_IYUV,COLOR_YUV2BGRA_I420 = COLOR_YUV2BGRA_IYUV,COLOR_YUV420p2RGBA  = COLOR_YUV2RGBA_YV12,COLOR_YUV420p2BGRA  = COLOR_YUV2BGRA_YV12,COLOR_YUV2GRAY_420  = 106,COLOR_YUV2GRAY_NV21 = COLOR_YUV2GRAY_420,COLOR_YUV2GRAY_NV12 = COLOR_YUV2GRAY_420,COLOR_YUV2GRAY_YV12 = COLOR_YUV2GRAY_420,COLOR_YUV2GRAY_IYUV = COLOR_YUV2GRAY_420,COLOR_YUV2GRAY_I420 = COLOR_YUV2GRAY_420,COLOR_YUV420sp2GRAY = COLOR_YUV2GRAY_420,COLOR_YUV420p2GRAY  = COLOR_YUV2GRAY_420,//! YUV 4:2:2 family to RGBCOLOR_YUV2RGB_UYVY = 107,COLOR_YUV2BGR_UYVY = 108,//COLOR_YUV2RGB_VYUY = 109,//COLOR_YUV2BGR_VYUY = 110,COLOR_YUV2RGB_Y422 = COLOR_YUV2RGB_UYVY,COLOR_YUV2BGR_Y422 = COLOR_YUV2BGR_UYVY,COLOR_YUV2RGB_UYNV = COLOR_YUV2RGB_UYVY,COLOR_YUV2BGR_UYNV = COLOR_YUV2BGR_UYVY,COLOR_YUV2RGBA_UYVY = 111,COLOR_YUV2BGRA_UYVY = 112,//COLOR_YUV2RGBA_VYUY = 113,//COLOR_YUV2BGRA_VYUY = 114,COLOR_YUV2RGBA_Y422 = COLOR_YUV2RGBA_UYVY,COLOR_YUV2BGRA_Y422 = COLOR_YUV2BGRA_UYVY,COLOR_YUV2RGBA_UYNV = COLOR_YUV2RGBA_UYVY,COLOR_YUV2BGRA_UYNV = COLOR_YUV2BGRA_UYVY,COLOR_YUV2RGB_YUY2 = 115,COLOR_YUV2BGR_YUY2 = 116,COLOR_YUV2RGB_YVYU = 117,COLOR_YUV2BGR_YVYU = 118,COLOR_YUV2RGB_YUYV = COLOR_YUV2RGB_YUY2,COLOR_YUV2BGR_YUYV = COLOR_YUV2BGR_YUY2,COLOR_YUV2RGB_YUNV = COLOR_YUV2RGB_YUY2,COLOR_YUV2BGR_YUNV = COLOR_YUV2BGR_YUY2,COLOR_YUV2RGBA_YUY2 = 119,COLOR_YUV2BGRA_YUY2 = 120,COLOR_YUV2RGBA_YVYU = 121,COLOR_YUV2BGRA_YVYU = 122,COLOR_YUV2RGBA_YUYV = COLOR_YUV2RGBA_YUY2,COLOR_YUV2BGRA_YUYV = COLOR_YUV2BGRA_YUY2,COLOR_YUV2RGBA_YUNV = COLOR_YUV2RGBA_YUY2,COLOR_YUV2BGRA_YUNV = COLOR_YUV2BGRA_YUY2,COLOR_YUV2GRAY_UYVY = 123,COLOR_YUV2GRAY_YUY2 = 124,//CV_YUV2GRAY_VYUY    = CV_YUV2GRAY_UYVY,COLOR_YUV2GRAY_Y422 = COLOR_YUV2GRAY_UYVY,COLOR_YUV2GRAY_UYNV = COLOR_YUV2GRAY_UYVY,COLOR_YUV2GRAY_YVYU = COLOR_YUV2GRAY_YUY2,COLOR_YUV2GRAY_YUYV = COLOR_YUV2GRAY_YUY2,COLOR_YUV2GRAY_YUNV = COLOR_YUV2GRAY_YUY2,//! alpha premultiplicationCOLOR_RGBA2mRGBA    = 125,COLOR_mRGBA2RGBA    = 126,//! RGB to YUV 4:2:0 familyCOLOR_RGB2YUV_I420  = 127,COLOR_BGR2YUV_I420  = 128,COLOR_RGB2YUV_IYUV  = COLOR_RGB2YUV_I420,COLOR_BGR2YUV_IYUV  = COLOR_BGR2YUV_I420,COLOR_RGBA2YUV_I420 = 129,COLOR_BGRA2YUV_I420 = 130,COLOR_RGBA2YUV_IYUV = COLOR_RGBA2YUV_I420,COLOR_BGRA2YUV_IYUV = COLOR_BGRA2YUV_I420,COLOR_RGB2YUV_YV12  = 131,COLOR_BGR2YUV_YV12  = 132,COLOR_RGBA2YUV_YV12 = 133,COLOR_BGRA2YUV_YV12 = 134,//! DemosaicingCOLOR_BayerBG2BGR = 46,COLOR_BayerGB2BGR = 47,COLOR_BayerRG2BGR = 48,COLOR_BayerGR2BGR = 49,COLOR_BayerBG2RGB = COLOR_BayerRG2BGR,COLOR_BayerGB2RGB = COLOR_BayerGR2BGR,COLOR_BayerRG2RGB = COLOR_BayerBG2BGR,COLOR_BayerGR2RGB = COLOR_BayerGB2BGR,COLOR_BayerBG2GRAY = 86,COLOR_BayerGB2GRAY = 87,COLOR_BayerRG2GRAY = 88,COLOR_BayerGR2GRAY = 89,//! Demosaicing using Variable Number of GradientsCOLOR_BayerBG2BGR_VNG = 62,COLOR_BayerGB2BGR_VNG = 63,COLOR_BayerRG2BGR_VNG = 64,COLOR_BayerGR2BGR_VNG = 65,COLOR_BayerBG2RGB_VNG = COLOR_BayerRG2BGR_VNG,COLOR_BayerGB2RGB_VNG = COLOR_BayerGR2BGR_VNG,COLOR_BayerRG2RGB_VNG = COLOR_BayerBG2BGR_VNG,COLOR_BayerGR2RGB_VNG = COLOR_BayerGB2BGR_VNG,//! Edge-Aware DemosaicingCOLOR_BayerBG2BGR_EA  = 135,COLOR_BayerGB2BGR_EA  = 136,COLOR_BayerRG2BGR_EA  = 137,COLOR_BayerGR2BGR_EA  = 138,COLOR_BayerBG2RGB_EA  = COLOR_BayerRG2BGR_EA,COLOR_BayerGB2RGB_EA  = COLOR_BayerGR2BGR_EA,COLOR_BayerRG2RGB_EA  = COLOR_BayerBG2BGR_EA,COLOR_BayerGR2RGB_EA  = COLOR_BayerGB2BGR_EA,//! Demosaicing with alpha channelCOLOR_BayerBG2BGRA = 139,COLOR_BayerGB2BGRA = 140,COLOR_BayerRG2BGRA = 141,COLOR_BayerGR2BGRA = 142,COLOR_BayerBG2RGBA = COLOR_BayerRG2BGRA,COLOR_BayerGB2RGBA = COLOR_BayerGR2BGRA,COLOR_BayerRG2RGBA = COLOR_BayerBG2BGRA,COLOR_BayerGR2RGBA = COLOR_BayerGB2BGRA,COLOR_COLORCVT_MAX  = 143
};

注意这个2是to的意思和p2p一样，国外的人真会完！

博主例子

来个普通图，转灰度图的例子！

运行截图如下：

源码如下：

#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/imgproc/imgproc_c.h>using namespace std;
using namespace cv;int main(){Mat matSrc=imread("5.png",IMREAD_COLOR);Mat matDts;imshow("matStr",matSrc);cvtColor(matSrc,matDts,COLOR_RGB2GRAY);imshow("matDts",matDts);waitKey();getchar();return 0;
}

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官方解析

博主例子

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