2.opencv由浅入深--图像色彩空间转换

2.opencv由浅入深–图像色彩空间转换

1.色彩空间

颜色空间也称彩色模型(又称彩色空间或彩色系统）它的用途是在某些标准下用通常可接受的方式对彩色加以说明。详情去百度百科查看：
百度百科

1.1 数字图像处理常用颜色空间

1.1.1 RGB颜色空间

RGB颜色空间以R(Red:红)、G(Green:绿)、B(Blue:蓝)三种基本色为基础，进行不同程度的叠加，产生丰富而广泛的颜色，所以俗称三原色模式。
RGB（红绿蓝）是依据人眼识别的颜色定义出的空间，可表示大部分颜色。但在科学研究一般不采用RGB颜色空间，因为它的细节难以进行数字化的调整。它将色调，亮度，饱和度三个量放在一起表示，很难分开。它是最通用的面向硬件的彩色模型。该模型用于彩色监视器和一大类彩色视频摄像。

1.1.2 CMY/CMYK颜色空间

CMY是工业印刷采用的颜色空间。它与RGB对应。简单的类比RGB来源于是物体发光，而CMY是依据反射光得到的。具体应用如打印机：一般采用四色墨盒，即CMY加黑色墨盒。

1.1.3 HSV/HSB颜色空间

由色调（Hue）、饱和度（Saturation）、亮度（Value）三个分量构成，HSV更接近于人眼的主观感受

1.1.4 HSI/HSL颜色空间

与HSV类似，只不过把V：Value替换为了L：Lightness。这两种表示在用目的上类似，但在方法上有区别。二者在数学上都是圆柱，但HSV（色相，饱和度，色调）在概念上可以被认为是颜色的倒圆锥体（黑点在下顶点，白色在上底面圆心），HSL在概念上表示了一个双圆锥体和圆球体（白色在上顶点，黑色在下顶点，最大横切面的圆心是半程灰色）。注意尽管在HSL和HSV中“色相”指称相同的性质，它们的“饱和度”的定义是明显不同的。对于一些人，HSL更好的反映了“饱和度”和“亮度”作为两个独立参数的直觉观念，但是对于另一些人，它的饱和度定义是错误的，因为非常柔和的几乎白色的颜色在HSL可以被定义为是完全饱和的。对于HSV还是HSL更适合于人类用户界面是有争议的。

1.1.5 Lab颜色空间

同RGB颜色空间相比，Lab(Commission International EclairageLab)是一种不常用的色彩空间。它是在1931年国际照明委员会（Commission International Eclairage, CIE）制定的颜色度量国际标准的基础上建立起来的。1976年，经修改后被正式命名为CIELab。它是一种设备无关的颜色系统，也是一种基于生理特征的颜色系统。这也就意味着，它是用数字化的方法来描述人的视觉感应。Lab颜色空间中的L分量用于表示像素的亮度，取值范围是[0,100],表示从纯黑到纯白；a表示从红色到绿色的范围，取值范围是[127,-128]；b表示从黄色到蓝色的范围，取值范围是[127,-128]。

1.1.6 YUV/YCbCr颜色空间

YUV，是一种颜色编码方法。常使用在各个视频处理组件中。 YUV在对照片或视频编码时，考虑到人类的感知能力，允许降低色度的带宽。YUV是编译true-color颜色空间（color space）的种类，Y’UV, YUV, YCbCr，YPbPr等专有名词都可以称为YUV，彼此有重叠。“Y”表示明亮度（Luminance、Luma），“U”和“V”则是色度、浓度（Chrominance、Chroma）。

1.2 opencv中常用颜色空间

Opencv 中常用颜色空间有RGB、HSV、YUV、Lab等，具体详情请参考cvtColor函数。

3.opencv图像颜色空间转换

在进行图像处理时，需要用到灰度图、二值图、HSV、HSI等颜色制式，opencv提供了cvtColor函数来实现这些功能。

在modules\imgproc\src\color.cpp中
void cvtColor( InputArray _src, OutputArray _dst, int code, int dcn )
{
    CV_INSTRUMENT_REGION();

    CV_Assert(!_src.empty());

    if(dcn <= 0)
            dcn = dstChannels(code);

    CV_OCL_RUN( _src.dims() <= 2 && _dst.isUMat() &&
                !(CV_MAT_DEPTH(_src.type()) == CV_8U && (code == COLOR_Luv2BGR || code == COLOR_Luv2RGB)),
                ocl_cvtColor(_src, _dst, code, dcn) )

    switch( code )
    {
        case COLOR_BGR2BGRA: case COLOR_RGB2BGRA: case COLOR_BGRA2BGR:
        case COLOR_RGBA2BGR: case COLOR_RGB2BGR:  case COLOR_BGRA2RGBA:
            if(_src.channels() == 1)
                cvtColorGray2BGR(_src, _dst, dcn);
            else
                cvtColorBGR2BGR(_src, _dst, dcn, swapBlue(code));
            break;

        case COLOR_BGR2BGR565:  case COLOR_BGR2BGR555: case COLOR_BGRA2BGR565: case COLOR_BGRA2BGR555:
        case COLOR_RGB2BGR565:  case COLOR_RGB2BGR555: case COLOR_RGBA2BGR565: case COLOR_RGBA2BGR555:
            cvtColorBGR25x5(_src, _dst, swapBlue(code), greenBits(code));
            break;

        case COLOR_BGR5652BGR:  case COLOR_BGR5552BGR: case COLOR_BGR5652BGRA: case COLOR_BGR5552BGRA:
        case COLOR_BGR5652RGB:  case COLOR_BGR5552RGB: case COLOR_BGR5652RGBA: case COLOR_BGR5552RGBA:
            cvtColor5x52BGR(_src, _dst, dcn, swapBlue(code), greenBits(code));
            break;

        case COLOR_BGR2GRAY: case COLOR_BGRA2GRAY:
        case COLOR_RGB2GRAY: case COLOR_RGBA2GRAY:
            cvtColorBGR2Gray(_src, _dst, swapBlue(code));
            break;

        case COLOR_BGR5652GRAY:
        case COLOR_BGR5552GRAY:
            cvtColor5x52Gray(_src, _dst, greenBits(code));
            break;

        case COLOR_GRAY2BGR:
        case COLOR_GRAY2BGRA:
            cvtColorGray2BGR(_src, _dst, dcn);
            break;

        case COLOR_GRAY2BGR565:
        case COLOR_GRAY2BGR555:
            cvtColorGray25x5(_src, _dst, greenBits(code));
            break;

        case COLOR_BGR2YCrCb: case COLOR_RGB2YCrCb:
        case COLOR_BGR2YUV:   case COLOR_RGB2YUV:
            cvtColorBGR2YUV(_src, _dst, swapBlue(code), code == COLOR_BGR2YCrCb || code == COLOR_RGB2YCrCb);
            break;

        case COLOR_YCrCb2BGR: case COLOR_YCrCb2RGB:
        case COLOR_YUV2BGR:   case COLOR_YUV2RGB:
            cvtColorYUV2BGR(_src, _dst, dcn, swapBlue(code), code == COLOR_YCrCb2BGR || code == COLOR_YCrCb2RGB);
            break;

        case COLOR_BGR2XYZ:
        case COLOR_RGB2XYZ:
            cvtColorBGR2XYZ(_src, _dst, swapBlue(code));
            break;

        case COLOR_XYZ2BGR:
        case COLOR_XYZ2RGB:
            cvtColorXYZ2BGR(_src, _dst, dcn, swapBlue(code));
            break;

        case COLOR_BGR2HSV: case COLOR_BGR2HSV_FULL:
        case COLOR_RGB2HSV: case COLOR_RGB2HSV_FULL:
            cvtColorBGR2HSV(_src, _dst, swapBlue(code), isFullRangeHSV(code));
            break;

        case COLOR_BGR2HLS: case COLOR_BGR2HLS_FULL:
        case COLOR_RGB2HLS: case COLOR_RGB2HLS_FULL:
            cvtColorBGR2HLS(_src, _dst, swapBlue(code), isFullRangeHSV(code));
            break;

        case COLOR_HSV2BGR: case COLOR_HSV2BGR_FULL:
        case COLOR_HSV2RGB: case COLOR_HSV2RGB_FULL:
            cvtColorHSV2BGR(_src, _dst, dcn, swapBlue(code), isFullRangeHSV(code));
            break;

        case COLOR_HLS2BGR: case COLOR_HLS2BGR_FULL:
        case COLOR_HLS2RGB: case COLOR_HLS2RGB_FULL:
            cvtColorHLS2BGR(_src, _dst, dcn, swapBlue(code), isFullRangeHSV(code));
            break;

        case COLOR_BGR2Lab: case COLOR_LBGR2Lab:
        case COLOR_RGB2Lab: case COLOR_LRGB2Lab:
            cvtColorBGR2Lab(_src, _dst, swapBlue(code), is_sRGB(code));
            break;

        case COLOR_BGR2Luv: case COLOR_LBGR2Luv:
        case COLOR_RGB2Luv: case COLOR_LRGB2Luv:
            cvtColorBGR2Luv(_src, _dst, swapBlue(code), is_sRGB(code));
            break;

        case COLOR_Lab2BGR: case COLOR_Lab2LBGR:
        case COLOR_Lab2RGB: case COLOR_Lab2LRGB:
            cvtColorLab2BGR(_src, _dst, dcn, swapBlue(code), is_sRGB(code));
            break;

        case COLOR_Luv2BGR: case COLOR_Luv2LBGR:
        case COLOR_Luv2RGB: case COLOR_Luv2LRGB:
            cvtColorLuv2BGR(_src, _dst, dcn, swapBlue(code), is_sRGB(code));
            break;

        case COLOR_BayerBG2GRAY: case COLOR_BayerGB2GRAY: case COLOR_BayerRG2GRAY: case COLOR_BayerGR2GRAY:
        case COLOR_BayerBG2BGR: case COLOR_BayerGB2BGR: case COLOR_BayerRG2BGR: case COLOR_BayerGR2BGR:
        case COLOR_BayerBG2BGR_VNG: case COLOR_BayerGB2BGR_VNG: case COLOR_BayerRG2BGR_VNG: case COLOR_BayerGR2BGR_VNG:
        case COLOR_BayerBG2BGR_EA: case COLOR_BayerGB2BGR_EA: case COLOR_BayerRG2BGR_EA: case COLOR_BayerGR2BGR_EA:
        case COLOR_BayerBG2BGRA: case COLOR_BayerGB2BGRA: case COLOR_BayerRG2BGRA: case COLOR_BayerGR2BGRA:
            {
                Mat src;
                if (_src.getObj() == _dst.getObj()) // inplace processing (#6653)
                    _src.copyTo(src);
                else
                    src = _src.getMat();
                demosaicing(src, _dst, code, dcn);
                break;
            }

        case COLOR_YUV2BGR_NV21:  case COLOR_YUV2RGB_NV21:  case COLOR_YUV2BGR_NV12:  case COLOR_YUV2RGB_NV12:
        case COLOR_YUV2BGRA_NV21: case COLOR_YUV2RGBA_NV21: case COLOR_YUV2BGRA_NV12: case COLOR_YUV2RGBA_NV12:
            // http://www.fourcc.org/yuv.php#NV21 == yuv420sp -> a plane of 8 bit Y samples followed by an interleaved V/U plane containing 8 bit 2x2 subsampled chroma samples
            // http://www.fourcc.org/yuv.php#NV12 -> a plane of 8 bit Y samples followed by an interleaved U/V plane containing 8 bit 2x2 subsampled colour difference samples
            cvtColorTwoPlaneYUV2BGR(_src, _dst, dcn, swapBlue(code), uIndex(code));
            break;

        case COLOR_YUV2BGR_YV12: case COLOR_YUV2RGB_YV12: case COLOR_YUV2BGRA_YV12: case COLOR_YUV2RGBA_YV12:
        case COLOR_YUV2BGR_IYUV: case COLOR_YUV2RGB_IYUV: case COLOR_YUV2BGRA_IYUV: case COLOR_YUV2RGBA_IYUV:
            //http://www.fourcc.org/yuv.php#YV12 == yuv420p -> It comprises an NxM Y plane followed by (N/2)x(M/2) V and U planes.
            //http://www.fourcc.org/yuv.php#IYUV == I420 -> It comprises an NxN Y plane followed by (N/2)x(N/2) U and V planes
            cvtColorThreePlaneYUV2BGR(_src, _dst, dcn, swapBlue(code), uIndex(code));
            break;

        case COLOR_YUV2GRAY_420:
            cvtColorYUV2Gray_420(_src, _dst);
            break;

        case COLOR_RGB2YUV_YV12: case COLOR_BGR2YUV_YV12: case COLOR_RGBA2YUV_YV12: case COLOR_BGRA2YUV_YV12:
        case COLOR_RGB2YUV_IYUV: case COLOR_BGR2YUV_IYUV: case COLOR_RGBA2YUV_IYUV: case COLOR_BGRA2YUV_IYUV:
            cvtColorBGR2ThreePlaneYUV(_src, _dst, swapBlue(code), uIndex(code));
            break;

        case COLOR_YUV2RGB_UYVY: case COLOR_YUV2BGR_UYVY: case COLOR_YUV2RGBA_UYVY: case COLOR_YUV2BGRA_UYVY:
        case COLOR_YUV2RGB_YUY2: case COLOR_YUV2BGR_YUY2: case COLOR_YUV2RGB_YVYU: case COLOR_YUV2BGR_YVYU:
        case COLOR_YUV2RGBA_YUY2: case COLOR_YUV2BGRA_YUY2: case COLOR_YUV2RGBA_YVYU: case COLOR_YUV2BGRA_YVYU:
            //http://www.fourcc.org/yuv.php#UYVY
            //http://www.fourcc.org/yuv.php#YUY2
            //http://www.fourcc.org/yuv.php#YVYU
            {
                int ycn  = (code==COLOR_YUV2RGB_UYVY || code==COLOR_YUV2BGR_UYVY ||
                            code==COLOR_YUV2RGBA_UYVY || code==COLOR_YUV2BGRA_UYVY) ? 1 : 0;
                cvtColorOnePlaneYUV2BGR(_src, _dst, dcn, swapBlue(code), uIndex(code), ycn);
                break;
            }

        case COLOR_YUV2GRAY_UYVY:
        case COLOR_YUV2GRAY_YUY2:
            cvtColorYUV2Gray_ch(_src, _dst, code == COLOR_YUV2GRAY_UYVY ? 1 : 0);
            break;

        case COLOR_RGBA2mRGBA:
            cvtColorRGBA2mRGBA(_src, _dst);
            break;

        case COLOR_mRGBA2RGBA:
            cvtColormRGBA2RGBA(_src, _dst);
            break;
        default:
            CV_Error( CV_StsBadFlag, "Unknown/unsupported color conversion code" );
    }
}

4.opencv图像颜色空间转换实例

#include 

#include 

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    Mat src, dst;
    // 读取图像，格式为BGR
    src = imread("G:/opencv/vs_opencv/images/lena.png"); 
    if (src.empty())
    {
        cout << "src is empty." << endl;
        return -1;
    }

    imshow("src", src);

    // 转换为灰度图像 BGR TO GRAY
    cvtColor(src, dst, COLOR_BGR2GRAY);
    imshow("gray", dst);

    // 转换为HSV BGR TO HSV
    cvtColor(src, dst, COLOR_BGR2HSV);
    imshow("hsv",dst);

    // 转换为LAB  BGR TO LAB
    cvtColor(src, dst, COLOR_BGR2Lab);
    imshow("lab", dst);

    // 转换为LAB  BGR TO LAB
    cvtColor(src, dst, COLOR_BGR2YCrCb);
    imshow("Ycrcb", dst);

    // 转换为LAB  BGR TO XYZ
    cvtColor(src, dst, COLOR_BGR2XYZ);
    imshow("xyz", dst);

    waitKey(0);

    destroyAllWindows();

    return 0;
}