图像的打开、修改、显示和保存示例（>OpenCV 2.0）

代码如下：

#include <opencv2/opencv.hpp>
using namespace cv;
int main( int argc, char** argv )
{
    char* imageName = argv[1];
    char* outName = argv[2];

    Mat image;
    image = imread( imageName, 1 );//打开
    if( argc != 2 || !image.data )
    {
        printf( " No image data \n " );
        return -1;
    }

    Mat gray_image;
    cvtColor( image, gray_image, COLOR_BGR2GRAY );//修改

    imwrite( outName, gray_image );//保存

    namedWindow( imageName, WINDOW_AUTOSIZE );
    namedWindow( "Gray image", WINDOW_AUTOSIZE );
    imshow( imageName, image );//显示
    imshow( "Gray image", gray_image );//显示

    waitKey(0);

    return 0;
}

１　 cv::imread　读取

　Mat cv::imread (const String &  filename, int  flags = IMREAD_COLOR )

• IMREAD_UNCHANGED (<0) 按照图像原有的模式打开 (包含ａｌｐｈａ通道)
• IMREAD_GRAYSCALE ( 0)　以灰度模式打开图像
• IMREAD_COLOR (>0) 打开图像为 RGB通道模式

The function imread loads an image from the specified file and returns it. If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format), the function returns an empty matrix (Mat::data==NULL ). Currently, the following file formats are supported:

• Windows bitmaps - *.bmp, *.dib (always supported)
• JPEG files - *.jpeg, *.jpg, *.jpe (see the Notes section)
• JPEG 2000 files - *.jp2 (see the Notes section)
• Portable Network Graphics - *.png (see the Notes section)
• WebP - *.webp (see the Notes section)
• Portable image format - *.pbm, *.pgm, *.ppm (always supported)
• Sun rasters - *.sr, *.ras (always supported)
• TIFF files - *.tiff, *.tif (see the Notes section)

２　 cvtColor( image, gray_image, COLOR_BGR2GRAY );

• a source image (image)
• a destination image (gray_image), in which we will save the converted image.
• an additional parameter that indicates what kind of transformation will be performed. In this case we useCOLOR_BGR2GRAY (because of cv::imread has BGR default channel order in case of color images).

void cv::cvtColor ( InputArray  src, OutputArray  dst, int  code, int  dstCn = 0 )

Converts an image from one color space to another.

The function converts an input image from one color space to another. In case of a transformation to-from RGB color space, the order of the channels should be specified explicitly (RGB or BGR). Note that the default color format in OpenCV is often referred to as RGB but it is actually BGR (the bytes are reversed). So the first byte in a standard (24-bit) color image will be an 8-bit Blue component, the second byte will be Green, and the third byte will be Red. The fourth, fifth, and sixth bytes would then be the second pixel (Blue, then Green, then Red), and so on.

The conventional ranges for R, G, and B channel values are:

• 0 to 255 for CV_8U images
• 0 to 65535 for CV_16U images
• 0 to 1 for CV_32F images

In case of linear transformations, the range does not matter. But in case of a non-linear transformation, an input RGB image should be normalized to the proper value range to get the correct results, for example, for RGB → L*u*v* transformation. For example, if you have a 32-bit floating-point image directly converted from an 8-bit image without any scaling, then it will have the 0..255 value range instead of 0..1 assumed by the function. So, before calling cvtColor , you need first to scale the image down:

img *= 1./255;

cvtColor(img, img, COLOR_BGR2Luv);

If you use cvtColor with 8-bit images, the conversion will have some information lost. For many applications, this will not be noticeable but it is recommended to use 32-bit images in applications that need the full range of colors or that convert an image before an operation and then convert back.

If conversion adds the alpha channel, its value will set to the maximum of corresponding channel range: 255 for CV_8U, 65535 for CV_16U, 1 for CV_32F.

３　cv::namedWindow

void cv::namedWindow ( const String &winname,int  flags = WINDOW_AUTOSIZE )

• WINDOW_NORMAL If this is set, the user can resize the window (no constraint).
• WINDOW_AUTOSIZE If this is set, the window size is automatically adjusted to fit the displayed image (see imshow ), and you cannot change the window size manually.
• WINDOW_OPENGL If this is set, the window will be created with OpenGL support.

Qt backend supports additional flags:

• CV_WINDOW_NORMAL or CV_WINDOW_AUTOSIZE: CV_WINDOW_NORMAL enables you to resize the window, whereas CV_WINDOW_AUTOSIZE adjusts automatically the window size to fit the displayed image (see imshow ), and you cannot change the window size manually.
• CV_WINDOW_FREERATIO or CV_WINDOW_KEEPRATIO: CV_WINDOW_FREERATIO adjusts the image with no respect to its ratio, whereas CV_WINDOW_KEEPRATIO keeps the image ratio.
• CV_GUI_NORMAL or CV_GUI_EXPANDED: CV_GUI_NORMAL is the old way to draw the window without statusbar and toolbar, whereas CV_GUI_EXPANDED is a new enhanced GUI. By default, flags == CV_WINDOW_AUTOSIZE | CV_WINDOW_KEEPRATIO | CV_GUI_EXPANDED

４　cv::imshow

void cv::imshow ( const String &winname, InputArray  mat )

Displays an image in the specified window.

Parameters

winname	Name of the window.
mat	Image to be shown.

The function imshow displays an image in the specified window. If the window was created with the CV_WINDOW_AUTOSIZE flag, the image is shown with its original size, however it is still limited by the screen resolution. Otherwise, the image is scaled to fit the window. The function may scale the image, depending on its depth:

• If the image is 8-bit unsigned, it is displayed as is.
• If the image is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].
• If the image is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].

If window was created with OpenGL support, imshow also support ogl::Buffer , ogl::Texture2D and cuda::GpuMat as input.

If the window was not created before this function, it is assumed creating a window with CV_WINDOW_AUTOSIZE.

If you need to show an image that is bigger than the screen resolution, you will need to call namedWindow("", WINDOW_NORMAL) before the imshow.

Note

This function should be followed by waitKey function which displays the image for specified milliseconds. Otherwise, it won't display the image. For example, waitKey(0) will display the window infinitely until any keypress (it is suitable for image display). waitKey(25) will display a frame for 25 ms, after which display will be automatically closed. (If you put it in a loop to read videos, it will display the video frame-by-frame)

５　waitKey(0)

int cv::waitKey ( int  delay = 0)

Waits for a pressed key.

Parameters

delay

Delay in milliseconds. 0 is the special value that means "forever".

The function waitKey waits for a key event infinitely (when