how to change the image from Irregular rectangle to a rectangle?

Question

the gold I want to do is change the Matrix from the left of the images1 to the right. From what I know is we can not change just with basic transformation method.

image1. change matrix from left to right

The real problem is I have a rectangle in the following image. I need to change the irregular rectangle to a regular rectangle.

image2.

Maybe this could help you: [link](http://stackoverflow.com/questions/7838487/executing-cvwarpperspective-for-a-fake-deskewing-on-a-set-of-cvpoint) — s1hofmann, Jul 26 '16 at 07:21
@s1h thank u so much. The [link](http://stackoverflow.com/questions/7838487/executing-cvwarpperspective-for-a-fake-deskewing-on-a-set-of-cvpoint) is right my answer. — Sanjay Chan, Aug 01 '16 at 02:33

score 2 · Accepted Answer · edited Jul 14 '18 at 12:21

So, first problem is corner order. They must be in the same order in both vectors. So, if in the first vector your order is:(top-left, bottom-left, bottom-right, top-right) , they MUST be in the same order in the other vector.

Second, to have the resulting image contain only the object of interest, you must set its width and height to be the same as resulting rectangle width and height. Do not worry, the src and dst images in warpPerspective can be different sizes.

Third, a performance concern. While your method is absolutely accurate, because you are doing only affine transforms (rotate, resize, deskew), mathematically, you can use the affine corespondent of your functions. They are much faster.

getAffineTransform()
warpAffine().

Important note: getAffine transform needs and expects ONLY 3 points, and the result matrix is 2-by-3, instead of 3-by-3.

How to make the result image have a different size than the input:

cv::warpPerspective(src, dst, dst.size(), ... );
use

cv::Mat rotated;
cv::Size size(box.boundingRect().width, box.boundingRect().height);
cv::warpPerspective(src, dst, size, ... );

So here you are, and your programming assignment is over.

void main()
{
    cv::Mat src = cv::imread("r8fmh.jpg", 1);


    // After some magical procedure, these are points detect that represent 
    // the corners of the paper in the picture: 
    // [408, 69] [72, 2186] [1584, 2426] [1912, 291]

    vector<Point> not_a_rect_shape;
    not_a_rect_shape.push_back(Point(408, 69));
    not_a_rect_shape.push_back(Point(72, 2186));
    not_a_rect_shape.push_back(Point(1584, 2426));
    not_a_rect_shape.push_back(Point(1912, 291));

    // For debugging purposes, draw green lines connecting those points 
    // and save it on disk
    const Point* point = &not_a_rect_shape[0];
    int n = (int)not_a_rect_shape.size();
    Mat draw = src.clone();
    polylines(draw, &point, &n, 1, true, Scalar(0, 255, 0), 3, CV_AA);
    imwrite("draw.jpg", draw);

    // Assemble a rotated rectangle out of that info
    RotatedRect box = minAreaRect(cv::Mat(not_a_rect_shape));
    std::cout << "Rotated box set to (" << box.boundingRect().x << "," << box.boundingRect().y << ") " << box.size.width << "x" << box.size.height << std::endl;

    Point2f pts[4];

    box.points(pts);

    // Does the order of the points matter? I assume they do NOT.
    // But if it does, is there an easy way to identify and order 
    // them as topLeft, topRight, bottomRight, bottomLeft?

    cv::Point2f src_vertices[3];
    src_vertices[0] = pts[0];
    src_vertices[1] = pts[1];
    src_vertices[2] = pts[3];
    //src_vertices[3] = not_a_rect_shape[3];

    Point2f dst_vertices[3];
    dst_vertices[0] = Point(0, 0);
    dst_vertices[1] = Point(box.boundingRect().width-1, 0); 
    dst_vertices[2] = Point(0, box.boundingRect().height-1);

    Mat warpAffineMatrix = getAffineTransform(src_vertices, dst_vertices);

    cv::Mat rotated;
    cv::Size size(box.boundingRect().width, box.boundingRect().height);
    warpAffine(src, rotated, warpAffineMatrix, size, INTER_LINEAR, BORDER_CONSTANT);

    imwrite("rotated.jpg", rotated);
}

how to change the image from Irregular rectangle to a rectangle?

1 Answers1