opencv image remapping cv::remap() (image up and down, left and right mirroring)

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concept

What is remapping?
This is the process of taking pixels from one location in an image and positioning them to another location in a new image.
To complete the mapping process, some interpolation of non-integer pixel positions may be required, since there is not always a one-to-one pixel correspondence between the source and destination images.
We can express the remapping ( x , y ) for each pixel location as:

where g() is the mapped image, f() is the source image, and h(x, y) is the mapping function applied to (x, y) .

opencv function supports cv::remap()

1. Function prototype:

CV_EXPORTS_W void remap( InputArray src, OutputArray dst,
                         InputArray map1, InputArray map2,
                         int interpolation, int borderMode = BORDER_CONSTANT,
                         const Scalar& borderValue = Scalar());

2. Parameter description

src: source image.
dst: target image. It has the same size as map1 and the same type as src.
map1: The coordinate value of x when map1 is stored, there are types CV_16SC2,
CV_32FC1, or CV_32FC2
CV_32FC1 or CV_32FC2. For more information on floating point conversions, see ConvertMaps
Representation to Fixed Point Speed.
map2: map2 stores the y coordinate value, which has the type CV_16SC2,
CV_32FC1, or CV_32FC2
interpolation: interpolation method (see #InterpolationFlags). Method #INTER_AREA is
not supported by this function.
borderMode: pixel extrapolation method (see #BorderTypes) when
borderMode: border handling mode
borderValue: value to use for borders. The default value is 0.

Test code:

   cv::Mat src;
    src = cv::imread("D:\\QtProject\\Opencv_Example\\remap\\remap.png", cv::IMREAD_GRAYSCALE);
    if (src.empty()) {
    
    
        cout << "matTemplate Cannot load image" << endl;
        return;
    }
    cv::imshow("src", src);

    cv::Mat dstUpDown(src.size(), src.type());
    cv::Mat map_x(src.size(), CV_32FC1);
    cv::Mat map_y(src.size(), CV_32FC1);

    //上下镜像
    for( int i = 0; i < map_x.rows; i++ )
     {
    
    
         for( int j = 0; j < map_x.cols; j++ )
         {
    
    
            map_x.at<float>(i, j) = (float)j;
            map_y.at<float>(i, j) = (float)(map_x.rows - i);
         }
     }

    cv::remap( src, dstUpDown, map_x, map_y, cv::INTER_LINEAR, cv::BORDER_CONSTANT, cv::Scalar(0));
    cv::imshow("dstUpDown", dstUpDown);

    //左右镜像
    for( int i = 0; i < map_x.rows; i++ )
     {
    
    
         for( int j = 0; j < map_x.cols; j++ )
         {
    
    
            map_x.at<float>(i, j) = (float)(map_x.cols - j);
            map_y.at<float>(i, j) = (float)i;
         }
     }
    cv::Mat dstLeftRight(src.size(), src.type());
    cv::remap( src, dstLeftRight, map_x, map_y, cv::INTER_LINEAR, cv::BORDER_CONSTANT, cv::Scalar(0));
    cv::imshow("dstLeftRight", dstLeftRight);

Program running effect:

Mirror up and down: mirror

left and right