for (int y = 0; y < res; y++) {
    int ry = (y % (block_size * 2) < block_size)? 0: 1;
    for (int x = 0; x < res; x++) {
       int rx = (x % (block_size * 2) >= block_size)? 0: 1;
       vec[y][x] = 255 * ry * rx;
    }
}