Store the intermediate of a ppm file

backup_ppm.cpp

// Store a backup ppm after rendering every 100 rows
if (j % 100 == 0) {
    ofs.close();

    temp_filename = "./out_" + std::to_string(ny-j) + ".ppm";

    std::ifstream  src("out.ppm", std::ios::binary);
    std::ofstream  dst(temp_filename,   std::ios::binary);

    dst << src.rdbuf();

    std::ofstream temp_ofs;
    temp_ofs.open(temp_filename, std::ios_base::app);

    for (int z = j+1; z>=0; z--) {
        for (int i = 0; i < nx; i++) {
            temp_ofs << "0 0 0\n";
        }
    }
    temp_ofs.close();
    ofs.open("./out.ppm", std::ios_base::app);
}