Fog of war is a lie

Most virtual tabletops implement fog as a mask over an image. Reveal a region, the players see it, done. It is simple and it is wrong in a way that shows up about twenty minutes into any dungeon.

Characters do not know things because a rectangle was uncovered. They know things because someone with a torch walked into a room, or because the rogue scouted ahead and came back, or because a player remembered a corridor from the last session even though their character was unconscious at the time.

We ended up modelling knowledge per character rather than visibility per region.