stereoRectify( M1, D1, M2, D2, img_size, R, T, R1, R2, P1, P2, Q, CALIB_ZERO_DISPARITY, -1, img_size, &roi1, &roi2 );
    Mat map11, map12, map21, map22;
    initUndistortRectifyMap(M1, D1, R1, P1, img_size, CV_16SC2, map11, map12);
    initUndistortRectifyMap(M2, D2, R2, P2, img_size, CV_16SC2, map21, map22);

    Mat img1r, img2r;
    remap(img1, img1r, map11, map12, INTER_LINEAR);
    remap(img2, img2r, map21, map22, INTER_LINEAR);