for (int frame = 1; frame <= 20; ++frame) {
        // Advance animations manually. By default animations would advance
        // based on whatever time the rendering, readback and PNG generation
        // throttle this thread to. That is not ideal so maintain our own
        // "global time".
        viewer.presentation()->setGlobalAnimationTime(t);
        t += 16; // 60 fps

        // Render the next frame.
        ...

        // Write it to a file.
       ...
    }