How to resize down an image using every nth pixel?

edit

@bheek, you can use the nearest neighbor resize

int scale = 2;  // downscaling factor
int w = 10;     // input width
int h = 8;      // input height

// Fill the input
cv::Mat origin(h, w, CV_32F);
for (int y = 0; y < origin.rows; ++y)
{
    for (int x = 0; x < origin.cols; ++x)
    {
        origin.at<float>(y, x) = y * origin.cols + x;
    }
}
std::cout << origin << std::endl;

cv::Mat res;
cv::resize(origin, res, cv::Size(w/scale, h/scale), 0, 0, cv::INTER_NEAREST);
std::cout << res << std::endl;

output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9;
 10, 11, 12, 13, 14, 15, 16, 17, 18, 19;
 20, 21, 22, 23, 24, 25, 26, 27, 28, 29;
 30, 31, 32, 33, 34, 35, 36, 37, 38, 39;
 40, 41, 42, 43, 44, 45, 46, 47, 48, 49;
 50, 51, 52, 53, 54, 55, 56, 57, 58, 59;
 60, 61, 62, 63, 64, 65, 66, 67, 68, 69;
 70, 71, 72, 73, 74, 75, 76, 77, 78, 79]
[0, 2, 4, 6, 8;
 20, 22, 24, 26, 28;
 40, 42, 44, 46, 48;
 60, 62, 64, 66, 68]