macOS Multi Display Sync: VLC

This brief script came about from numerous requests to play multiple videos in sync with each other from a single machine. The videos were to have different content but would be of the same duration. Achieving this on macOS has proved to be a little tricky to say the least. It is possible to run multiple instance of VLC so that videos can be played concurrently from the same machine.

Be aware that this method does not enforce any syncing, so it is possible for the videos to fall out of sync, especially if the are not exactly the same frame count and encoding.

VLC CLI

You can play multiple videos just by using the VLC CLI tool, which should live at

/Applications/VLC.app/Contents/MacOS/VLC

Play multiple videos

You can run multiple instances of VLC concurrently

/Applications/VLC.app/Contents/MacOS/VLC $FILE1 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE2

This will run two instances but you can easily daisy chain this by adding more lines with the & operator

i.e.

/Applications/VLC.app/Contents/MacOS/VLC $FILE1 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE2 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE3 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE4 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE5 & \
/Applications/VLC.app/Contents/MacOS/VLC $FILE6

Repeat

you can repeat the video with the -R flag e.g.

/Applications/VLC.app/Contents/MacOS/VLC -R $FILE1 & \
/Applications/VLC.app/Contents/MacOS/VLC -R $FILE2

This requires that you manually set each instance to full screen in a seperate display, which becomes tiresome after a while. So what about playing in fullscreen from the get go?

Play on different diplays in fullscreen

To play on different displays with VLC CLI macOS you have to the flag

--macosx-vdev=<screenID>

Where screen ID is some integer identifying the screen. Unhelpfully, this is not a sequential 1, 2, 3, ... but rather an 8 digit decimal number whose value is elusive to find.

There is the odd forum post on the matter, but if you're not sure what you're looking for they are of little help.

Cutting to the chase, the screen ID can be found in the vlcrc file located at

/Users/USERNMAME/Library/Preferences/org.videolan.vlc/vlcrc

Find / search for --macosx-vdev, switch the display in VLC preferences and check this file again.

This is a little tedious, especially if you intend on using different displays. So I opted to automate it a little with Swift

Swift

Below is a CLI tool which you can compile in Xcode.

Create an Xcode Command Line Tool Project
Choose language Swift
Paste the below code
build
copy app in Product folder to the folder with your videos

Assuming the project was named vlc-multi-screen the CLI tool takes the form

./vlc-multi-screen $file1 $file2 ... $filexx

Where the number of files is equal to the number of displays

//
//  Play multiple instances of VLC player, fullscreen across multiple screens
//
//------------------------------------------------------------------------------
import Foundation
import Cocoa
//------------------------------------------------------------------------------
if (CommandLine.argc - 1) != NSScreen.screens.count
{
    print("Number of files does not match number screens\n")
    exit(0);
}
//------------------------------------------------------------------------------
var screen_id = [String]()

for screen in NSScreen.screens
{
    screen_id.append(String(screen.deviceDescription[NSDeviceDescriptionKey("NSScreenNumber")]! as!  Int)) ;
}

print(screen_id)

//------------------------------------------------------------------------------
// get files
var files = [String]()

for i in 1...Int(CommandLine.argc - 1)
{
    files.append(CommandLine.arguments[i])
}
//------------------------------------------------------------------------------
func shell(_ args: String...) -> Void
{
    let task = Process()
    task.launchPath = "/usr/bin/env"
    task.arguments = args
    task.launch()
}

//------------------------------------------------------------------------------
// Run VLC
let vlc_path = "/Applications/VLC.app/Contents/MacOS/VLC"

for file in files
{
    let display = "--macosx-vdev=" + screen_id.popLast()!
    shell(vlc_path, "-f", display, file);
}

mhamilt/vlc-multi-macos.md