Show and tell: using Virtual-Display-Driver to turn an Android phone into a second monitor

[vensilver]

Hi! 👋 Just wanted to say thank you and show what we built on top of Virtual-Display-Driver.

I make Remote Mouse & Keyboard | AX, an Android app that turns a phone into a
mouse, keyboard, webcam and mic for a Windows PC over Wi-Fi. The latest update adds
a Second display feature: the phone becomes an extra monitor that Windows can
extend the desktop onto, with the pointer working on it. VDD is what makes that
possible.

Why VDD: Windows has no user-mode way for an ordinary app to add a monitor, so
it needs an IddCx driver. Writing and signing our own would have meant attestation
and maintaining a driver forever. VDD being MIT licensed and properly signed
(no test-signing, no root certificates) made it the clear choice over the older
IddSampleDriver forks.

How we use it:

• We don't bundle it. Our PC receiver detects the driver (Root\MttVDD), and if it's
  missing, it walks the user through installing it from your Releases page.
• When the phone asks for a second display, the receiver writes a phone-shaped
  resolution to vdd_settings.xml and uses \\.\pipe\MTTVirtualDisplayPipe
  (PING / SETDISPLAYCOUNT) to attach the monitor, then detaches it when the
  session ends. Nice bonus: this all works without elevation.
• The new display is captured, streamed to the phone as H.264, and touch on the
  phone becomes pointer input on that display.

A few things we ran into, in case they help anyone else integrating with the pipe:

• When the driver closes the connection after replying, Windows reports it as
  ERROR_BROKEN_PIPE (109) or ERROR_PIPE_NOT_CONNECTED (233), not as a clean
  end-of-file, so a working pipe can look like a missing one.
• The pipe is single-instance: while the driver recreates it, opens fail with
  ERROR_PIPE_BUSY (231), so clients need to wait and retry.
• Uninstalling can leave stale device nodes behind, which can confuse a later
  reinstall.

Links:

• App: https://play.google.com/store/apps/details?id=com.ax.remote
• PC receiver: https://ax.huntbits.com/download/

Thanks again for maintaining this. It saved us weeks and a lot of driver-signing
pain. Happy to share more details or test anything on our side. 🙏