What is a watch party with your own video files?

A watch party lets people in different places watch the same video at the same moment, with play, pause and seeking shared by everyone. Streaming sites have one built in for their own videos, but with a video you have as a file, such as a home movie, a recorded talk or a film you bought, people usually count down, press play at the same time and drift apart after the first pause.

Tool description

This tool keeps everyone's copy of a video file in sync. Open it, open your video file and send the link: everyone who opens the link opens their own copy of the same file, and from then on play, pause and seeking happen for everyone at once.

The video never leaves anyone's device. Each browser plays its own file, and the sync server only gets a fingerprint of it, which tells whether everyone opened the same file. The panel next to the player lists who is in the room and whether they have the same video, and has a chat.

Features

  • Your own files - any video your browser can play, such as MP4 or WebM, straight from your device, with nothing uploaded
  • Same-file check - the fingerprint of each person's file shows next to their name as "Same video" or "Different video"
  • Real-time sync - play, pause and seeking from anyone reach the whole room at once, and a player that falls behind catches up by itself
  • Live drift monitoring - your connection latency and the largest gap between your video and the others'

How it works

When you open a file, your browser makes its fingerprint: a SHA-256 hash of the file's size and of pieces spread from its start to its end, so it's ready at once, even for a long film. Only this fingerprint goes to the sync server. It tells the server whether two people have the same file, but nothing about the video, not even its name.

The first file opened in a room becomes the room's video. Someone with a different file still follows along, with a warning, and can make their file the room's video, which starts it over for everyone. The server keeps whether the video plays and where, and every player follows it. When someone opens their file while the others watch, everyone pauses where they are, so the newcomer starts with them.

Limitations

  • Everyone needs their own copy of the video: the tool doesn't send the file to the others.
  • What plays depends on the browser: MKV, AVI or HEVC files may not play everywhere, and a file can play without sound when the browser can't decode its audio.
  • Anyone in the room can play, pause, seek or change the video.
  • Nothing is saved. Once everyone has left, the room is gone.

Tips

  • Send everyone the same file, for example through a shared folder, so the fingerprints match. Copies from different places often differ, even of the same film.
  • If your browser won't start the video by itself, press play once. Until you do, the room waits for you.
  • After reloading the page, open the file again: the browser doesn't keep it.