Open technological standards for abstracting true display interfaces over arbitrary connections (I.E. Miracast) seem to be still in their infancy, and rife with (in)compatibility issues. On the other hand, writing your own virtual display drivers and clients will almost certainly lead to all sorts of headaches with different display protocols, hardware vendors, etc.
However, the only important part of a display interface is the video data, and there are all sorts of standards for capturing and streaming that.
So the easiest way to achieve the effect of a virtualized second screen may be to just configure your OS to render it (without a real display device connected), and then capture and transmit it using conventional off-the-shelf screen capturing and recording software, such as OBS, or even Skype.
If you're okay with including an inexpensive hardware component in the final solution, then the easiest way to make your local machine render that virtualized second screen might be to use a physical dongle like the following product:
CompuLab Display Emulator (fit-Headless)
(So you could plug that in, set it up like any monitor, and then connect your two computers using an OBS stream, Skype call, remote desktop session, or whichever method of video streaming works best for you.)
This also carries the benefit of requiring customization only in user-land, and even then only with widely supported systems.
This is an amazing project. Thank you for sharing this. This looks quite close to what windows seems to be working on with its miracast implementation. I am looking for something that I can use on a linux machine. – tensai – 2018-04-13T05:46:13.940
Miracast will allow you to connect only one display, which must have its own client (Miracast sink, not available for iOS and problems for Android). Spacedesk can connect up to 4 displays simultaneously and there is a client for Android / iOS / Windows. And what do you plan to use as source (host) / target (client)? For Linux, there are many developments in this area, you can also try to work out VirtualBox Guest addition driver. – IGRIG – 2018-04-17T00:05:32.753
Yes, this was the idea. Connect just one screen, to use for powerpoint presentations etc. The sink would either be a raspberry pi or a desktop machine connected to the screen. And yes, it seems like spacedesk has solved the problem for windows. I was just trying to see how difficult it would be to get started with a client for Linux. – tensai – 2018-04-17T11:14:22.773