How to set up dynamic lighting for online D&D
Dynamic lighting is the feature that makes a dungeon feel like a dungeon. It's also the one GMs put off, because the first time you try it the fighter can see through a wall and everyone laughs. This is how to set it up so that doesn't happen, on any VTT that has it.
The four parts
Every dynamic lighting system is the same four things. Walls: lines that block light and sight. Doors: walls that open. Light sources: things that emit light, with a bright radius and a dim one. Vision: which tokens can see, and how far, in the dark.
If one of the four is wrong, the map looks wrong. Usually it's the walls.
Step 1: walls (10 minutes for a normal map)
Trace the walls of the map with the wall tool. Click at each corner. You don't need pixel accuracy; you need the lines to meet at the corners so light can't leak through a gap. Do the outer walls first, then the internal ones.
Two tips. Don't wall around furniture; a table doesn't block vision at eye level. And don't wall the whole map at once if it's huge: wall the part the party will see this session and do the rest next week.
If you built the map in a VTT's own map builder (RollQuest's, for instance), skip this step. The walls came with the map.
Step 2: doors (2 minutes)
Every gap in a wall that should open is a door. Place them where the artwork shows a door. Set them closed. During play you open them, and light and vision go through.
Secret doors are doors the players can't see the control for. Windows are walls that block movement but not light, if your VTT has that type.
Step 3: lights (5 minutes)
Place the light sources the map implies: torches in sconces, a fireplace, the glowing rune. Set a bright radius (the rules say 20 feet for a torch) and a dim one (another 20). Give them a colour. Warm orange for fire, cold white for a lantern, whatever green means in your campaign.
Then attach a torch to a player token. That's the one that moves with the party and is the light they'll mostly see by.
Step 4: vision (1 minute)
Give player tokens vision. Set darkvision for the ones who have it (60 feet, dim). Tokens without it see only where there's light. Enemies usually don't need vision set at all; you can see everything as the GM.
Step 5: test it as a player
Open the map in a private window logged in as a player, or use your VTT's player perspective view. Walk a token down a corridor. Open a door. If you can see something you shouldn't, the wall has a gap at a corner. Fix it and try again.
Mistakes that make it look wrong
Gaps at corners are the big one. The second is lighting every room at once: if the whole dungeon is lit, there's no dark for the torch to matter in. Set the scene's ambient darkness and only light what's lit. The third is forgetting the dim radius, which makes lights end in a hard circle instead of fading.
Frequently asked questions
How long does it take to set up lighting for a map?
About 15 to 20 minutes for a typical dungeon level if you have to draw the walls. Under 5 if the map came with them.
Do I need dynamic lighting to play?
No. Fog of war on its own is fine for most games. Lighting is for when the dark is part of the story.
Which VTTs have free dynamic lighting?
RollQuest has it on the free plan. Foundry includes it in the one-off licence. Roll20 requires a Plus subscription. Owlbear Rodeo needs an extension.