Resize an image to a square but keep aspect ratio c++ opencv

Question

Is there a way of resizing images of any shape or size to say [500x500] but have the image's aspect ratio be maintained, levaing the empty space be filled with white/black filler?

So say the image is [2000x1000], after getting resized to [500x500] making the actual image itself would be [500x250], with 125 either side being white/black filler.

Something like this:

Input

Output

EDIT

I don't wish to simply display the image in a square window, rather have the image changed to that state and then saved to file creating a collection of same size images with as little image distortion as possible.

The only thing I came across asking a similar question was this post, but its in php.

Answer 1

A general approach:

cv::Mat utilites::resizeKeepAspectRatio(const cv::Mat &input, const cv::Size &dstSize, const cv::Scalar &bgcolor)
{
    cv::Mat output;

    double h1 = dstSize.width * (input.rows/(double)input.cols);
    double w2 = dstSize.height * (input.cols/(double)input.rows);
    if( h1 <= dstSize.height) {
        cv::resize( input, output, cv::Size(dstSize.width, h1));
    } else {
        cv::resize( input, output, cv::Size(w2, dstSize.height));
    }

    int top = (dstSize.height-output.rows) / 2;
    int down = (dstSize.height-output.rows+1) / 2;
    int left = (dstSize.width - output.cols) / 2;
    int right = (dstSize.width - output.cols+1) / 2;

    cv::copyMakeBorder(output, output, top, down, left, right, cv::BORDER_CONSTANT, bgcolor );

    return output;
}