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Change existing cv::Mat dimensions while retaining the content that fits the new size – Photoshop

I have a collection of cv::Mat objects of different sizes. I want to them all to have the same number of columns as the widest matrix in the collection. Matrices that have less columns should be padded to the right with a fixed color. Essentially I want the same functionality as Photoshop’s “Canvas Size…” operation. How should I do that in C++?

cv::resize doesn’t cut it, because it stretches content, instead of padding it. cv::Mat::resize also doesn’t fit the bill because it can only add rows, but not columns.

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  1. Chosen as BEST ANSWER
    0

    The trick is to create a new matrix with the desired dimensions and then copy data from the original image into a ROI representing the retained data:

    // Creates a new matrix with size newSize with data copied from source.
// Source data outside the new size is discarded.
// If any of the dimensions of newSize is larger than that dimension in source,
// the extra area is filled with emptyColor.
cv::Mat resizeCanvas(const cv::Mat& source, cv::Size newSize, cv::Scalar emptyColor) {
    cv::Mat result(newSize, source.type(), emptyColor);

    int height = std::min(source.rows, newSize.height);
    int width = std::min(source.cols, newSize.width);
    cv::Rect roi(0, 0, width, height);

    auto sourceWindow = source(roi);
    auto targetWindow = result(roi);
    sourceWindow.copyTo(targetWindow);

    return result;
}

  2. 0

    You can also use copyMakeBorder:

    void resizeCanvas(const cv::Mat& src, cv::Mat& dst, const cv::Size& canvasSize, const cv::Scalar& emptyColor)
{
    if((canvasSize.height < src.rows) || canvasSize.width < src.cols) {
        // Canvas is smaller than source image
        return;
    }

    int bottom = canvasSize.height - src.rows;
    int right = canvasSize.width - src.cols;

    cv::copyMakeBorder(src, dst, 0 /*top*/, bottom, 0 /*left*/, right, cv::BORDER_CONSTANT, emptyColor);
}

    Usage:

    cv::Mat3b img = cv::imread("...");
cv::Mat3b resized;
resizeCanvas(img, resized, cv::Size(1000, 1000), cv::Scalar(0,0,0,0));
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