Look on my works, ye Mighty, and despair!

UPDATE: this is fake news, see my newer post for more info.

It seems that neither iOS devices, nor Safari on OS X, support mp4; so if you’re trying to stream video, you need to provide another format.

The recommendation is to use HLS, which fortunately is supported by ffmpeg, you merely need to adjust your incantation:

app.get('/hls/video', function(req, res) {
    res.contentType('application/vnd.apple.mpegurl');
    var proc = ffmpeg();
    proc.stdout.pipe(res);
});
 
function ffmpeg() {
    var cmd = "ffmpeg";
    var filter = "some complex filter expr";
    var args = ["-i", "video1.mp4"];
    ...
    args.push(
        "-vcodec", "libx264",
        "-f", "hls",
        "-hls_time", "9",
        "-hls_list_size", "0",
        "-profile:v", "baseline",
        "-level", "3.0",
        "pipe:1"
    );
    return spawn(cmd, args);
}

I could probably use conneg to decide which format to return, rather than the uri, but I’m not convinced that my caching infrastructure (varnish and cloudfront now!) would handle that correctly.