OpenCv C++, Assigning a passed-in Mat to Mat within function fails

Question

I have a function, resizeToShortSide, which takes a Mat and resizes its short side to the specified value. To resize, I setup a destination mat, resizedMat, perform the resize, and then assign resizedMat to the input Mat, mat. This occurs successfully.

However, when the function ends, the Mat which was passed to the function is again its original size, as if the assignment to resizedMat had not occured! OpenCv Mat are always passed by reference, so I'm not sure why it is acting like a copy of the Mat is being passed. Here is the function in question...

void resizeToShortSide(Mat mat, int shortSide, int resamplingMethod)
{
    //determine which side is the short side
    bool shortSideIsRows;
    if (mat.rows <= mat.cols) {
        shortSideIsRows = true;
    } else {
        shortSideIsRows = false;
    }
    //caluculate the size of the long side
    Size outputSize;
    if (shortSideIsRows) {
        int cols = (shortSide / (float) mat.rows) * mat.cols;
        int rows = shortSide;
        outputSize = Size(cols, rows);
    } else {
        int rows = (shortSide / (float) mat.cols) * mat.rows;
        int cols = shortSide;
        outputSize = Size(cols, rows);
    }

    //setup a destination mat
    Mat resizedMat(outputSize, CV_8UC4);

    //resize
    if (resamplingMethod == BashSettings::ResamplingMethod::NearestNeighbor)
        resize(mat, resizedMat, outputSize, 0, 0, INTER_NEAREST);
    else
        resize(mat, resizedMat, outputSize);    //defaults to INTER_LINEAR

    //assign mat to resized mat
    mat = resizedMat;
    qDebug() << "resize to short side " << shortSide;
    qDebug() << "resized mat width, height " << resizedMat.cols << ", " << resizedMat.rows;
    qDebug() << "input mat width, height " << mat.cols << ", " << mat.rows;
    qDebug() << " ";
}

Answer 1

The Mat class itself contains some "header information" as well as a pointer to a UMatData object. The UMatData handles the reference counting. Unfortunately, the MatSize size definition is in the Mat object. Since your function resizeToShortSide takes a Mat by value, there's no way for the size to update upon return by your function. You'll still need to pass your Mat by reference. Here is a relevant portion of the Mat class definition:

class CV_EXPORTS Mat
{

     ...

    //! interaction with UMat
    UMatData* u;

    MatSize size;
    MatStep step;

    ...

}

Notice that the cv::resize function is defined with InputArray and OutputArray parameters.

void cv::resize(InputArray  src, OutputArray dst, Size  dsize, double fx = 0, double fy = 0, int interpolation = INTER_LINEAR) 

These InputArray and OutputArray classes implement constructors that wrap a Mat reference and so changes can persist to dst

_InputArray(const Mat& m);
_OutputArray(Mat& m);

Answer 2

您按值将mat传递给函数，按引用重试或返回resizeMat 。