Set up fog of war on a battle map

To set up fog of war on a map in Eldritch OS, open the scene as the GM, pick the Fog tool, and drag a fog shape over the map — everything under it is solid black on your players’ screens. As the party moves, right-click a shape and choose Reveal to Players; the reveal reaches every player at the table as it happens.

This guide covers putting a map on a scene, covering it, revealing it room by room, and the difference between what you see and what your players see.

Before you start

• You need to be the campaign’s GM. The Fog tool only appears in the tool rail for the GM; players never see it.
• Fog belongs to the scene. Every fog shape you draw is saved with the scene it is on, so it is there again when you switch back to that scene, or open it next week.
• You need an Eldritch OS account. During the alpha, access is by invitation — see the end of this page.

Step 1 — Put a map on a scene

If the scene already has a map, skip to Step 2.

1. In the campaign workspace, open Maps and click Add Map, then Upload Image. Give the map a Name, choose a Map Image (PNG, JPEG, WebP or GIF), and click Add Map.
2. Open Scenes and click New Scene. Name it, pick the map from the Map list, and click Create & Prep. You land on the scene ready to set it up.

If you drew the map in the Eldritch OS map editor rather than uploading an image, the New Scene dialog also offers Cover unexplored areas with fog. Leave it ticked and the scene is created with one fog shape over each room and corridor, and you can skip Step 2.

Step 2 — Cover the map

1. Press F, or click Fog in the tool rail on the left of the map.
2. In the bar that appears, pick a shape — Rectangle, Ellipse or Polygon — and draw it over the map. Rectangles and ellipses are a drag; for a polygon, click each corner, then double-click or press Enter to close it. Backspace removes the last corner and Escape cancels the shape.
3. New fog starts hidden. Draw as many shapes as you need.

Cover the map one room at a time, not all at once. A fog shape is revealed or hidden as a whole, so one shape over the entire map can only be lifted in one go. One shape per room or corridor is what lets you reveal the dungeon as the party walks it.

With the Fog tool active you can drag a shape to move it and pull its handles to resize it, and with grid snapping on, shapes snap to the grid as you do. The slider next to the shape buttons changes how dark the fog looks to you; players always see solid black.

Step 3 — Reveal areas as the party moves

1. With the Fog tool active, right-click a fog shape and choose Reveal to Players. The shape lifts on every player’s screen at once.
2. To take it back — the party retreats, or you revealed the wrong room — right-click the shape again and choose Hide from Players.
3. To do several at once, shift-click the shapes to select them together; the toolbar that appears offers Reveal All to Players and Hide All from Players. Merge fog shapes joins touching shapes into one, which is handy after you have tiled a large hall out of rectangles.
4. Delete in the same right-click menu removes a shape for good. Once a room is explored and will stay explored, deleting its fog keeps the scene tidy.

What players see, and what you see

Players see hidden fog as solid black and revealed fog not at all. Fog covers whatever is under it on their screen, tokens included — but it is paint over the picture, not a secret. A token under fog is still sent to the player’s browser. If a monster should stay unknown until it springs, hide the token as well: right-click it and untick Visible. A hidden token is not sent to players at all.

You see hidden fog as a translucent black with a dashed white border, at whatever darkness you set on the slider, so you can read the map underneath. Revealed shapes show as a dashed outline while the Fog tool is active and disappear when you switch to any other tool.

What fog does not do yet

We keep a page of exactly what ships and what does not. For fog specifically:

• No dynamic lighting and no line of sight. Fog is shapes you draw and reveal by hand. Nothing is computed from where a token stands, from walls, or from light sources. This is not built, and it is not on the roadmap page either, so do not plan a session around it.
• No brush or eraser. Reveal is per shape. To reveal half a room, draw the room as two shapes.
• Uploaded maps are not covered automatically. The automatic cover in Step 1 only works for maps drawn in the Eldritch OS map editor, because it reads the rooms from the drawing. For an image you upload, the cover is Step 2.
• Players cannot reveal fog for themselves. The Fog tool is GM-only.

Join the alpha playtest

Eldritch OS is in a closed alpha with a small group of GMs running real sessions. If you want to run yours here, join the alpha playtest and say a little about your table — we are inviting tables in waves.