simulcast-mpv

I was curious how easy it would be to sync two mpv players across the internet, even though it's' overengineering a solution to a not-very problem. If one person pauses, then pause for the other person. Add in some ping calculation between clients. That's basically what simulcast-mpv is.

Usage

TL;DR:

• Download simulcast-mpv
• Run simulcast-mpv. It will install itself.
• Start mpv. It should just work™.
• (optional) Hit a once to show some info. Hit a a few times really fast to open up an input window for custom room codes. (Maybe you and your friend are watching the same thing, but your file names are different.)

The simulcast-mpv executable has 4 "modes":

• simulcast-mpv
  • This "installs" simulcast-mpv to your mpv scripts directory (%APPDATA%\mpv\scripts or ~/.config/mpv/scripts). It also writes a helper lua script (simulcast-mpv.lua) to the directory.
• simulcast-mpv client
  • This is ran when mpv starts. It acts as a middle-man for sending pause/resume/seek messages between mpv and the relay server.
• simulcast-mpv relay
  • A websocket server
• simulcast-mpv input-reader
  • A popup command prompt window for inputting custom room codes.

TODO:

• setup github actions to compile binaries x86_64 Windows, x86_64 Linux, 64-bit ARM Linux.
  • cargo +1.75 build --release (1.75 for Windows 7 support)
  • cargo zigbuild --release --target x86_64-unknown-linux-musl
  • cargo-dist? I don't particularly want to package .msi installers though...
• better logic to sync the position when someone joins a party (other than having an existing user just seek backwards from their position by 5s (which will sync the new user)...)
• Things don't always pause correctly when a party starts. Seems related to mpv automatically resuming/playing media when mpv opens.
• Fix some logic bug that gets you trapped in a pause/unpause loop.
• Link to simulcast-mpv source-code (repository) in Hello message from the server?
  • log to stdout and log file...
• Drop mpvipc dependency?

similar projects (for mpv)

• Syncplay: website / github
  • More feature-complete than simulcast-mpv.
• https://github.com/po5/groupwatch_sync
  • Manual groupwatch project.
• others? dunno...

similar projects (for not-mpv)

• Jellyfin has some "SyncPlay" thing.
• Plex (ew) has a "Watch Together" thing.

Environment variables / .env

simulcast-mpv allows environment variables and files to configure some of the settings.

client

• SIMULCAST_RELAY_URL / --relay-url (default: reads the server from here)
• SIMULCAST_RELAY_ROOM / --relay-room (default abcd1234)
• SIMULCAST_CLIENT_SOCK / --client-sock (passed by mpv to the simulcast-mpv executable)
• SIMULCAST_BIND_ADDRESS / --bind-address (default 127.0.0.1)
• SIMULCAST_BIND_PORT / --bind-port (default 30777)

Configuration files can be placed at

• %APPDATA%\mpv\scripts\simulcast-mpv.env (Windows)
• ~/.config/mpv/scripts/simulcast-mpv.env (Unix)
• $PWD/simulcast-mpv.env (current directory AKA where mpv is started from) (Windows + Unix)

Relay server privacy

Relay server "rooms" are public to anyone who joins using the same "room ID".

"Room IDs" are calculated client-side as blake3_hash(filename + relay_room) where relay_room is configurable with SIMULCAST_RELAY_ROOM/--relay-room.

This means the server cannot know which file you are playing unless the server already knows what the filename + relay_room combination is.