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Here is a function to convert Mat to QPixmap or QImage. It's much faster than the direct pixel access using the Mat::at(y,x) function:

QPixmap MatToPixmap(cv::Mat src)
{
    QImage::Format format=QImage::Format_Grayscale8;
    int bpp=src.channels();
    if(bpp==3)format=QImage::Format_RGB888;
    QImage img(src.cols,src.rows,format);
    uchar *sptr,*dptr;
    int linesize=src.cols*bpp;
    for(int y=0;y<src.rows;y++){
        sptr=src.ptr(y);
        dptr=img.scanLine(y);
        memcpy(dptr,sptr,linesize);
    }
    if(bpp==3)return QPixmap::fromImage(img.rgbSwapped());
    return QPixmap::fromImage(img);
}

To convert a QImage to Mat, invert the parameters of the memcpy function.