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Fog of War System (fowSystem.ts)

screenhot

Purpose

This file implements a lightweight fog-of-war system for the isometric map in cell auto isometric.

It provides:

  • FogExplorerComponent — a marker component added to actors that should clear fog.
  • FOW — an Excalibur System that scans fog tiles and clears them when an explorer overlaps.

Key exports

FogExplorerComponent

  • Extends Excalibur Component.
  • Contains a single flag: isActive.
  • Used to identify entities that can reveal fog.

FOW

  • Extends Excalibur System.
  • Queries entities that contain:
    • FogExplorerComponent
    • TransformComponent
    • BodyComponent
  • Maintains an optional reference to the current ExIsoMetricMap.
  • Uses a tick interval to reduce processing frequency.

How it works

  1. The system is created with a reference to the Excalibur World.
  2. registerMap(map) attaches the upper isometric map used for fog.
  3. During update(), the system:
    • returns early when no map is registered.
    • increments a global tick counter and only processes every TIK_INTERVAL updates.
    • iterates over each explorer entity in the query.
    • fetches the explorer's graphics.bounds and compares it against each tile's stored bounds.
    • if an explorer overlaps a fogged tile, and the tile is not already dirty:
      • sets tile.data.fog to false
      • marks the map as dirty by calling this._map.addTag("dirty")
      • pushes the tile onto this._map.dirtyTiles

Map contract

FOW depends on the upper map having these properties:

  • tiles: the isometric tiles to scan.
  • each tile must have:
    • tile.data.get("bounds") — a rectangle with top, bottom, left, right
    • tile.data.get("fog") — a boolean indicating if the tile is currently covered.
  • dirtyTiles: an array of tiles that need redraw.
  • addTag("dirty") triggers the map redraw logic in levelGen.ts.

This contract is implemented by the ExIsoMetricMap type from src/Lib/levelGen.ts.

Integration notes

The active fog explorer is created in src/Actors/walkingPlayer.ts:

  • FowChildActor is a transparent child actor with a FogExplorerComponent(true).
  • It is added as a child of WalkingPlayer.

Typical usage:

  • Add the system to the world: 
    const fogSystem = new FOW(world);
world.addSystem(fogSystem);
  • Register the upper map once it is created: 
    fogSystem.registerMap(upperMap);

Important details

  • TIK_INTERVAL is currently 5. This means the fog check only runs every 5 frames.
  • The implementation uses a global TIK counter shared across the file.
  • If tile.data.get("fog") is already false, the tile is skipped.
  • If this._map.dirtyTiles is undefined, fog clearing is skipped for safety.

Potential improvements

  • Replace the global TIK counter with per-system state.
  • Use a spatial index instead of brute-forcing all tiles.
  • Store explorerBounds and tile bounds in a consistent coordinate space.
  • Avoid scanning all map tiles each update for performance on larger maps.

Related files

  • src/Actors/walkingPlayer.ts — defines FowChildActor and attaches the explorer component.
  • src/Lib/levelGen.ts — builds the isometric map and handles tile redraw via dirty updates.