Every in-person DM who uses a TV at the table has the same problem: how do you get the map onto the big screen without running a full VTT, casting your entire desktop, or dealing with HDMI cables and mirrored displays? The existing options are either too heavy (Foundry VTT running on a second monitor), too fragile (screen sharing over Discord), or too expensive (dedicated hardware like a Game Master screen).
The peer-to-peer approach
WebRTC was built for video calls, but it works just as well for pushing a single image and a fog overlay between two browsers. The DM's laptop sends the current map and fog state directly to the TV's browser. There is a small public signalling broker (PeerJS at peerjs.com) that handles the initial handshake, but the actual image data never touches a server. Latency on a local network is measured in milliseconds.
What the DM sees versus what the players see
The DM view shows the full map with a fog overlay and painting controls. The player view shows the map with the same fog overlay applied: everything covered is hidden behind a dark wash, everything revealed is fully visible. The player view has no painting controls, no replace button, no chrome. Just the map.
Why not just use screen mirroring?
Screen mirroring shows your entire desktop, browser tabs, notes, stat blocks, and the secret boss room you have not revealed yet. You can crop the window, but then your map fills only part of the TV. Dedicated casting tools like Chromecast introduce latency and compression artifacts. A purpose-built tool sends exactly one thing to the TV: the player view, full screen, crisp, with no chrome and no accidental reveals.
Hardware requirements
Any laptop with a modern browser (Chrome, Firefox, Edge, Safari) works as the DM device. Any screen with a browser works as the player display, smart TVs, Fire Sticks, Chromecasts with a browser, tablets, even a phone. The simplest setup is a laptop with HDMI to a TV, two browser tabs side by side. For separate devices the join code links them across your local network.
Fog of war on the shared screen
The display earns its place when players only see what their characters can see. Paint fog over the unexplored parts of the map on the DM view and the same dark wash appears instantly on the TV. As the party moves, wipe the fog room by room and the reveal happens live in front of them. Because the fog is painted on the DM device and only the finished image is pushed to the player screen, there is no way for a curious player to peek at the layers underneath the way they could if you shared a file or handed round a VTT scene.
Where it fits in your prep
Most DMs build their maps before the session: cropping a battlemap, adding a grid, or stitching several tiles into one scene. The display tool sits at the end of that pipeline. Export the finished map from whatever you used to make it, drop it into the DM view, paint your starting fog, and open the player view on the TV. Nothing is uploaded, nothing is stored, and there is no account to create, so a one-shot at a friend's house works exactly like your home table. When the session ends you close the tab and the connection is gone.