Cast your iPad's Screen to Linux

Overview

This post is going to explain how to record the screen of your iPad and stream it to your Linux system with latencies of about two seconds. Unfortunately Apple makes things quite a bit more complicated than they have to be to keep you within their walled garden, so lets see how to break free.

There are three components to the solution I found:

An app to stream the screen of your iPad to a Real-Time Messaging Protocol (RTMP) server.
An RTMP server that receives the stream and distributes it to clients that request the stream.
A program to connect to the RTMP server and playback the stream.

RTMP Server

The most complicated part in this setup is the RTMP server. The RTMP server we will use here is nginx using the RTMP module. On Arch Linux you can install the packages nginx-mainline nginx-mainline-mod-rtmp. On Ubuntu everything can be installed via: apt install nginx libnginx-mod-rtmp. On other distributions you may need to build the RTMP module from source, to do so refer to the official docs at https://github.com/arut/nginx-rtmp-module.

After the installation nginx needs to be configured for livestreaming. To do so the file /etc/nginx/nginx.conf needs to be modified. If it does not yet exist, create it. The following configuration should get you up and running, apply customizations if necessary.

# this tells nginx to load the RTMP module
# the path to it may vary depending on your distribution
# make sure it actually points to the RTMP module you installed
# also make sure it is placed somwhere at the top of this file
load_module "/usr/lib/nginx/modules/ngx_rtmp_module.so";

rtmp {
    server {

        listen 1935;

        chunk_size 4000;

        application live {
            # enable livestreaming
            live on;

            # no need to record the livestreams
            record off;

            # everyone is allowed to livestream to this server,
            # you may want to limit this to the IP of your iPad
            allow publish all;

            # only localhost can access and play any livestreams
            allow play 127.0.0.1;
            deny play all;
        }
    }
}

For documentation on all RTMP options refer to https://github.com/arut/nginx-rtmp-module/wiki/Directives . After configuring RTMP nginx needs to be restarted or at least reloaded to read the new configuration. On Arch Linux and Ubuntu this is done with systemctl restart nginx, on Arch Linux nginx needs to be enabled first with systemctl enable nginx.

The App

On your iPad an app is required to stream your screen via RTMP. After extensive research the only app I found to be acceptable is Larix Screencaster. Unfortunately as of version 1.0.5 it still contains a bug that cuts off a part of your screen if you try to enable anything else than streaming in vertical/portrait mode. So make sure to set all your settings to vertical!

The settings in the main menu should look similar to this: Larix Screencaster settings 1 To tell the app where to direct the video stream to, you need to add a new connection. Give it a name and point it to the RTMP server on your computer by pasting a URL like rtmp://192.168.42.42:1935/live/ipad into the URL field. Make sure to replace the IP address with the actual IP address of your computer! After configuring the connection it should look similar to this: Larix Screencaster settings 2 To start a stream from your iPad follow the instructions show in https://www.youtube.com/watch?v=yvkkcRP8-2Y .

Playing the Stream

To playback the stream captured on your iPad you need a program capable of playing RTMP streams. If you are a friend of running everything in your terminal mpv is the right choice for you. To play the stream the command mpv rtmp://127.0.0.1:1935/live/ipad --profile=low-latency -demuxer-lavf-analyzeduration=5 can be used. To workaround Larix Screencasters problems with non vertical streams mpv can be used to rotate the video with the additional options --video-rotate=90. Sometimes it appears mpv does not recognize the video part of the stream, consider increasing -demuxer-lavf-analyzeduration in that case. Alternatively VLC media player is equally capable of playing RTMP streams. To play the stream go to the menu Media and choose Open Network Stream. This will open a prompt for the Network Stream to be opened, paste rtmp://127.0.0.1:1935/live/ipad into it. The stream should start playing then. VLC can also rotate the video, the option can be found under Tools > Effects and Filters > Video Effects > Geometry > Transform. I recommend mpv though as I could not find an option for VLC to minimize playback latency. Fin.