Good fog of war hides information, not just darkness. On Roll20, use the free Hide / Reveal Mask when you want direct manual control, or Dynamic Lighting with barriers, token vision, Explorer Mode, doors, and windows when you want visibility to react automatically to player movement. In Foundry VTT, enable Token Vision and Fog Exploration, then use walls, doors, secret doors, light sources, and token vision to determine what each player can actually see. The biggest mistake is testing everything from the GM view. Always preview the map as a player or player-controlled token before the session, because one tiny gap in a wall can reveal the room you spent all week trying to hide.
The rogue walks toward an ordinary stone wall. Before anyone rolls Investigation, the player stops their token and says, "There's definitely a room behind this."
They are right. A tiny gap in your vision barrier revealed three squares of furnished dungeon on the other side. The secret chamber is no longer secret. The hidden cultist token is visible. Everyone knows there is something behind the wall, and the encounter has changed before the characters actually discovered anything.
Fog of war is supposed to prevent exactly this problem, but hiding a map well requires more than covering unexplored rooms with black space. You need to think about line of sight, remembered areas, player movement, doors, windows, lighting, token ownership, and what happens when someone drags a token farther than you expected.
Roll20 and Foundry VTT approach these systems differently, but the goal is the same: players should know exactly as much about the map as their characters reasonably know, and no more.
What should fog of war actually hide from players?
Do not think of fog of war as a black blanket covering the map. Think of it as information control.
Some information should be completely unknown. A secret chamber behind solid stone belongs in that category. Other information can remain remembered after the party has passed through it. The players know the hallway exists, even if their characters cannot currently see what is standing there.
Then there is information that players should be able to see but not necessarily enter. A locked glass door, iron bars, a cliff edge, a window, and a magical force field all require different combinations of sight and movement restrictions.
| Map Information | Player Should See? | Best Treatment |
|---|---|---|
| Unexplored dungeon room | No | Fog or token vision |
| Previously explored hallway | Usually remembered | Exploration memory |
| Creature outside current sight | No | Current token vision |
| Secret chamber | No | Solid vision wall / secret door |
| Window into another room | Yes | Vision permitted, movement blocked |
The best setup therefore does not simply ask, "Is this square hidden?" It asks three separate questions: can the character see it now, have they seen it before, and can their token move into it?
How does fog of war work on Roll20 now?
Roll20 currently gives GMs two main ways to control map visibility: the Hide / Reveal Mask and Dynamic Lighting.
The Hide / Reveal Mask is the simpler option and is available to all GMs. It is the modern version of Roll20's older basic Fog of War workflow. You decide which parts of the map are hidden and manually reveal them as the party progresses.
Dynamic Lighting is available through Roll20's Plus and Pro tiers. Instead of requiring the GM to reveal every room manually, it uses barriers, light sources, token vision, doors, windows, and other page settings to calculate what players can see as they move.
Hide / Reveal Mask gives the GM direct control. Dynamic Lighting lets token position, vision, barriers, and light determine visibility automatically.
Neither method is inherently better. Manual revealing is fast and predictable. Dynamic vision takes more preparation but produces a much stronger first-person exploration experience once everything is configured correctly.
When should you use Roll20's Hide / Reveal Mask?
Use the Hide / Reveal Mask when you want the GM, rather than the software, deciding exactly when information appears.
Open the Hide / Reveal Mask tool from the left-hand menu. You can hide the full page, hide selected areas, or reveal portions of the mask using rectangular or polygonal shapes.
That workflow is ideal for simple dungeons, regional maps, investigation boards, town maps, puzzle rooms, and encounters where the characters' exact line of sight is less important than controlling the timing of a reveal.
Start hidden and reveal forward
For a traditional dungeon crawl, hide the full page before the session. Reveal individual rooms and corridors as the party discovers them. This is safer than starting with everything visible and trying to remember every area that contains a spoiler.
Use polygons for irregular rooms
Rectangular reveals are fast, but dungeon walls are rarely perfect rectangles. Polygonal reveals give you more control around diagonal walls, caves, curved tunnels, ruined structures, and irregular chambers.
Keep the GM darkness visible
Roll20 lets the GM adjust the opacity of hidden areas from the GM perspective. Keeping the mask partially transparent lets you prepare and navigate the hidden parts of the encounter while players continue to see those areas as fully obscured.
Manual fog is not less advanced if it gives you better control over the encounter you actually want to run.
How do you hide secrets with Roll20 Dynamic Lighting?
Dynamic Lighting works best when you think of the map as a collection of sight boundaries rather than a collection of rooms.
Enable Dynamic Lighting, move to the Dynamic Lighting layer, and draw barriers along walls and other objects that should stop vision. Then configure player tokens with vision and give the scene the appropriate lighting conditions.
Use walls to protect hidden rooms
Standard barriers block vision. Trace the actual visual boundary of a room rather than drawing a rough box around the entire building. A badly positioned barrier can either reveal part of a hidden room or prevent players from seeing terrain they should clearly be able to see.
Use doors instead of leaving gaps
Do not leave a permanent opening in your wall line just because there is a door on the artwork. Use Roll20's door tools. Closed doors preserve the vision barrier, while opening the door changes what the players can see and move through.
Use windows when characters should see but not walk through
Windows are useful because visibility and movement are not always the same thing. A player might see a monster through a window without being able to move their token directly into the room.
Turn on Restrict Movement
Restrict Movement helps prevent player-controlled tokens from crossing Dynamic Lighting barriers. That matters because a player accidentally dragging a token through a wall can reveal map information before anyone intended to enter the room.
If your game involves careful dungeon exploration, this is one of the most useful safeguards you can enable.
Should you use Explorer Mode on Roll20?
Explorer Mode is useful when you want players to remember the geography of places they have already visited without continuing to see everything currently happening there.
With Explorer Mode enabled, previously explored areas remain visible in a subdued treatment after they leave the character's current line of sight. That creates the familiar video-game effect where the player remembers the dungeon layout while current vision still determines which creatures and details they can actively perceive.
Without exploration memory, areas outside the token's current vision return to darkness. That can work well in horror games, magical darkness, shifting labyrinths, or encounters where spatial uncertainty is part of the challenge.
| Encounter Style | Explorer Mode | Why |
|---|---|---|
| Traditional dungeon crawl | Usually On | Players remember explored rooms |
| Horror dungeon | Consider Off | Darkness feels more oppressive |
| Large fortress | Usually On | Reduces navigation frustration |
| Shifting magical maze | Often Off | Uncertainty is part of the encounter |
Explorer Mode can also increase the work Roll20 has to perform on complex pages, so extremely large maps with many barriers, vision-enabled tokens, and lighting effects deserve additional testing.
How does fog of war work in Foundry VTT?
Foundry VTT approaches fog of war as part of a larger vision system. The Scene determines whether token vision and exploration are enabled. Tokens determine how characters see. Walls determine what blocks sight. Lights determine what is illuminated. Fog Exploration records what players have previously discovered.
For a dungeon-style scene, the basic setup is:
Token Vision: Enabled.
Fog Exploration: Enabled if players should remember explored territory.
Global Illumination: Usually disabled for dark interiors.
Walls: Drawn around significant vision boundaries.
Player Tokens: Vision enabled and configured appropriately.
Lights: Placed wherever the environment actually provides illumination.
Foundry stores fog exploration on a per-user basis. That becomes important when party members split up. One player can discover a room without automatically giving every other user the exact same exploration history.
Which Foundry wall type should you use for each secret?
Foundry's wall system is powerful because vision and movement are not forced to behave identically. Using the right wall type is the difference between a natural-feeling scene and one where every object behaves like a concrete bunker.
| Foundry Wall | Behavior | Good Use |
|---|---|---|
| Normal Wall | Blocks normal vision and movement | Stone walls, solid structures |
| Terrain Wall | Allows limited visibility beyond one obstruction | Boulders, statues, large terrain |
| Invisible Wall | Blocks movement without blocking vision | Transparent barriers |
| Ethereal Wall | Blocks vision but allows movement | Dark curtains, visual veils, unusual magic |
| Door | Changes restrictions when opened | Normal doors and gates |
| Secret Door | Acts like a door but hides its interaction marker from players | Hidden passages and concealed chambers |
| Window Wall | Visibility depends on proximity while movement remains restricted | Windows and similar openings |
Secret Doors are especially important when hiding map secrets. To most players, a Foundry Secret Door looks like an ordinary wall until the GM reveals or changes it. You avoid the classic VTT problem where the software itself tells players there is a door their characters have not discovered.
How should lighting interact with Foundry fog of war?
A wall answers whether a character has line of sight. Lighting answers whether there is enough illumination to perceive the area through that line of sight.
For an outdoor daytime scene, Global Illumination can make sense because characters should generally see as far as physical obstacles permit. For a dungeon, cave, crypt, or interior building, individual light sources usually produce a more believable result.
Foundry light sources interact with walls. Place a torch in a corridor and the light should stop at the surrounding walls rather than illuminating hidden chambers. You can also configure darkness sources for effects that mechanically obstruct vision rather than merely painting a dark graphic over the map.
Do not light every room simply because you want the map art to look nice. If an unexplored chamber is supposed to be dark, the vision system should reflect that.
A character can have an unobstructed line toward a room and still be unable to see its contents if the scene's lighting and their vision do not allow it.
How do you hide secret doors without giving them away?
The worst secret door is one that looks secret from the player side of the VTT.
On Roll20, make sure your lighting barrier continues across the hidden passage until the characters actually discover it. Do not leave a suspicious gap in the barrier at the exact point where the secret door exists.
In Foundry, use the Secret Door wall type instead of a normal player-visible door. Foundry hides the interaction control from normal player roles, allowing the segment to behave like an ordinary wall until the GM chooses to reveal or open it.
The artwork matters too. If your supposedly hidden door is represented by a giant glowing doorway on the base map, no fog configuration can make the reveal feel natural once the adjacent room becomes visible. Maps intended for secrets work best when the visual design supports the concealment.
This is one reason a good battle map pack should give you tactically useful layouts rather than simply attractive scenery.
Why do most fog-of-war setups accidentally spoil the map?
The first problem is tiny wall gaps. Vision systems are geometric. A barrier that looks almost connected at normal zoom can still create a narrow sightline into the next chamber.
The second problem is excessive wall detail. GMs sometimes trace every table, barrel, chair, pillar, tree, crate, and broken piece of masonry. That takes much longer to prepare, can hurt performance on complicated scenes, and often makes movement feel worse rather than more realistic.
Foundry's own content guidance recommends being selective about what actually needs to block sight or movement and favors efficient wall segments over unnecessary complexity. Roll20 likewise warns that very large maps, complicated lighting barriers, many sight-enabled tokens, and Explorer Mode can increase Dynamic Lighting demands.
The third problem is forgetting token ownership. Both platforms calculate player vision around what a user is actually allowed to control. A perfect wall setup will not help if the wrong token has vision or a player has access to a token they were never supposed to see through.
The fourth problem is never testing the player perspective.
A map that looks correct to the GM has not been tested yet.
What should you test before your players join?
Run through the map with a player-level perspective before you build the rest of the encounter around it. Roll20 provides a Preview as Token option for Dynamic Lighting, while Foundry's token vision lets you inspect the scene from the perspective of a controlled vision-enabled token.
| Test | What to Check |
|---|---|
| Starting Position | Players cannot see rooms they have not entered |
| Every Door | Closed doors block the intended information |
| Every Corner | No vision leaks through wall connections |
| Secret Areas | No door icon, token, light, or map detail exposes the secret |
| Token Movement | Players cannot accidentally drag through important barriers |
| Exploration Memory | Previously visited areas behave the way you want |
| Darkness | Characters without appropriate vision cannot see through unlit areas |
Move the test token along the actual paths players are likely to take. Hug walls. Stand directly beside secret chambers. Open every door. Move to the corner of rooms. Try to drag through barriers. Test darkvision or equivalent vision modes.
The goal is not to confirm that the obvious route works. The goal is to discover what happens when a player does the exact thing you did not expect.
Should the whole party share the same explored map?
This is partly a technical question and partly a table-style decision.
If your party normally stays together, shared information is often easier to run. You probably do not need to create tension around whether the fighter remembers the hallway that the rogue saw thirty seconds ago.
When the party splits, individual vision becomes much more interesting. The rogue can enter a side chamber while the cleric remains in the hallway. One group might discover the monster before the other group knows it exists.
Foundry can combine vision from multiple tokens a user controls under certain selection conditions, while its Fog Exploration is recorded per user. Roll20's Dynamic Lighting likewise centers visibility around controlled tokens and their vision settings.
Decide before the session whether you want separated information to be an actual part of play. If not, do not make everyone fight the VTT just to reproduce information the characters would immediately share anyway.
How much fog-of-war preparation is actually worth doing?
Wall the tactical information that matters. Skip the geometry that does not.
A stone dungeon probably needs accurate room walls, corridor boundaries, doors, secret doors, and major pillars. It probably does not need individual vision barriers around every chair, barrel, candle holder, skeleton, and five-inch section of broken masonry.
The more complicated the scene becomes, the more chances you create for leaks, awkward movement, and performance issues.
If an object changes what the character can reasonably see or where they can tactically move, consider walling it. If it is mostly decoration, the VTT usually does not need another barrier.
Start with the structural shell of the encounter. Test it. Then add only the extra barriers that materially improve the scene.
And if the base grid itself is giving you trouble before you even reach the fog setup, our grid alignment guide for Roll20 and Owlbear Rodeo covers the map setup side first.
Frequently Asked Questions
Is fog of war free on Roll20?
Roll20's Hide / Reveal Mask is available to all GMs and can manually hide or reveal parts of a page. Dynamic Lighting, which automatically calculates player vision using barriers, token sight, and lighting, is available to Plus and Pro games.
What is Explorer Mode in Roll20?
Explorer Mode lets players retain a subdued view of areas they have already explored even after those locations leave their current line of sight. It is useful for traditional dungeon exploration because players can remember the map layout without automatically seeing current creatures or events in areas their characters cannot presently observe.
How do I stop players from seeing through walls in Roll20?
With Dynamic Lighting enabled, draw continuous barriers along the walls of your map and make sure player tokens have vision configured correctly. Zoom in on corners and connections to check for small gaps. Use Preview as Token to verify the setup from the player's actual perspective.
How does Fog Exploration work in Foundry VTT?
When Fog Exploration is enabled for a scene, Foundry records explored fog on a per-user basis as the scene is explored. Areas currently visible to the player's token appear normally, while previously explored areas can remain represented as remembered territory depending on the scene's fog settings.
How do I create a secret door in Foundry VTT?
Use Foundry's Secret Door wall type where the hidden doorway exists. It behaves like a door but hides its interaction marker from normal player roles. When the characters discover the passage, the GM can open it or convert it to a normal door.
Should I wall every object on a Foundry map?
Usually not. Focus on walls and terrain that meaningfully affect sight or movement. Excessive wall segments can complicate setup, create more opportunities for errors, and negatively affect scene performance without adding much tactical value.
Should players remember rooms after leaving them?
For most traditional dungeon crawls, allowing players to remember explored geography keeps navigation from becoming unnecessarily frustrating. For horror, magical mazes, supernatural darkness, or changing environments, removing or resetting exploration memory can be part of the encounter design.
Explore battle map packs built for dungeon crawls, hidden rooms, dangerous corridors, tactical encounters, and the moment someone finally finds the door you have been hiding all night.
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| Roll20 | Hide / Reveal Mask |
| Roll20 | How to Set Up Dynamic Lighting |
| Roll20 | Placing Doors & Windows |
| Foundry VTT | Scenes |
| Foundry VTT | Walls |
| Foundry VTT | Lighting |
