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name: fog-of-war-js-ts description: Fog of war, field of view, line of sight, and tile visibility in JavaScript or TypeScript games — roguelikes, RTS, top-down, or strategy maps. Covers efficient FOV algorithms (recursive and symmetric shadowcasting, DDA raycasting, 2D visibility polygons), update scheduling and multi-viewer reference counting, typed-array and bitset state, canvas and WebGL fog rendering, memory and large-map scaling, hot-loop geometry math, and visibility correctness. Trigger when implementing, reviewing, or optimizing such code — even when the user only says "fog of war", "reveal the map", "what can this unit see", "shadowcasting", or "visibility grid" without mentioning performance — the naive raycast-per-cell-every-frame approach is usually what needs replacing. Prefer these patterns when writing new visibility code or refactoring slow or buggy fog.

dot-skills Fog of War (JavaScript/TypeScript) Best Practices

Performance and correctness guide for fog of war and field-of-view systems in JS/TS games, distilled from the canonical FOV literature (Björn Bergström, Albert Ford, Adam Milazzo), Red Blob Games, rot.js, and the MDN/WebGL rendering APIs. Contains 44 rules across 8 categories, ordered by impact, to guide writing, reviewing, and refactoring visibility code.

When to Apply

Reference these guidelines when:

• Implementing field of view, line of sight, or tile visibility for a grid or continuous map
• Building or refactoring a fog-of-war display (unexplored / explored / visible layers)
• Diagnosing slow fog (recompute every frame), visual artifacts (flicker, light leaks), or memory blowups on large maps
• Scaling visibility to many units (RTS) or to large/streaming worlds
• Choosing how to store and render the visibility/explored state

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	FOV / Visibility Algorithm	CRITICAL	fov-
2	Update Scheduling & Incremental Recompute	CRITICAL	update-
3	State Representation & Data Structures	HIGH	state-
4	Rendering the Fog Layer	HIGH	render-
5	Memory & Allocation	MEDIUM-HIGH	mem-
6	Multi-Viewer & Map Scaling	MEDIUM	scale-
7	Geometry & Hot-Loop Math	MEDIUM	geo-
8	Correctness & Visual Artifacts	LOW-MEDIUM	correct-

Quick Reference

1. FOV / Visibility Algorithm (CRITICAL)

• fov-recursive-shadowcasting - Use recursive shadowcasting, not ray-per-cell FOV
• fov-symmetric-shadowcasting - Prefer symmetric shadowcasting for consistent visibility
• fov-octant-transforms - Transform octants with a lookup table, not eight loops
• fov-radius-bounded-scan - Bound the scan to the sight radius and map edges
• fov-single-ray-los - Use a single line-of-sight ray for point-to-point checks
• fov-dda-continuous - Traverse continuous space with a DDA grid walk
• fov-visibility-polygon - Compute a visibility polygon for smooth 2D fog

2. Update Scheduling & Incremental Recompute (CRITICAL)

• update-recompute-on-move - Recompute field of view only when the viewer moves
• update-dirty-flag - Track a dirty flag per viewer and a map version stamp
• update-refcount-visibility - Count viewers per tile for incremental multi-viewer updates
• update-delta-not-clear - Emit visibility deltas instead of clear-all-recompute
• update-debounce-map-edits - Batch map edits and recompute affected viewers once
• update-merge-explored - Merge visible into explored as you reveal, never rebuild it

3. State Representation & Data Structures (HIGH)

• state-typed-arrays - Store fog state in typed arrays, not arrays of objects
• state-flat-1d-index - Index a flat buffer with y*width+x, not nested arrays
• state-three-state-encoding - Encode the three fog states as bit flags in one byte
• state-bitset-layers - Use a bitset for boolean visibility layers
• state-row-major-iteration - Iterate row-major to match the buffer's memory layout
• state-no-string-keys - Avoid string-keyed maps for per-tile visibility

4. Rendering the Fog Layer (HIGH)

• render-offscreen-fog-layer - Render fog to a separate offscreen layer
• render-dirty-region-only - Repaint only the dirty fog region
• render-imagedata-not-fillrect - Build fog as one ImageData, not per-tile fillRect
• render-lowres-soft-upscale - Render soft fog at tile resolution and upscale on the GPU
• render-webgl-texsubimage - Upload only the dirty rect of the fog texture in WebGL
• render-fade-alpha-lerp - Animate fog reveal by lerping alpha, not recomputing FOV

5. Memory & Allocation (MEDIUM-HIGH)

• mem-reuse-buffers - Allocate fog buffers once and reuse them
• mem-clear-with-fill - Clear the visible buffer with fill, not reallocation or a loop
• mem-generation-stamp - Use a generation stamp to skip the per-frame clear
• mem-bitpack-explored - Pack the explored layer into bits for memory and saves
• mem-no-hot-loop-closures - Hoist allocations out of the FOV scan loop

6. Multi-Viewer & Map Scaling (MEDIUM)

• scale-chunk-large-maps - Chunk large maps and keep only active chunks resident
• scale-shared-team-visibility - Share one refcounted visibility buffer per team
• scale-spatial-partition-edits - Find edit-affected viewers with a spatial index
• scale-gameplay-vs-render-culling - Separate gameplay visibility from on-screen render culling
• scale-cap-recompute-budget - Cap FOV recomputes per frame with a time budget

7. Geometry & Hot-Loop Math (MEDIUM)

• geo-squared-distance - Compare squared distances to avoid per-cell sqrt
• geo-avoid-modulo-deindex - Track x and y directly instead of modulo-deindexing
• geo-integer-slope-shadows - Use rational slopes to avoid floating-point drift
• geo-avoid-trig-in-loop - Precompute directions instead of calling trig per cell
• geo-radius-shape-choice - Choose the radius metric for the shape you want

8. Correctness & Visual Artifacts (LOW-MEDIUM)

• correct-symmetric-walls - Light wall tiles consistently to avoid flickering faces
• correct-corner-peeking - Handle diagonal wall corners deliberately
• correct-permissiveness-model - Pick one permissiveness model and apply it everywhere
• correct-explored-not-overwritten - Never let the visible pass overwrite the explored layer

How to Use

Read individual reference files for the full explanation and incorrect/correct code:

• Start with fov- and update- for the two biggest wins: a correct algorithm and recomputing only on change.
• Section definitions - Category structure, impact levels, and the execution-lifecycle ordering.
• Rule template - Template for adding new rules.

Reference Files

File	Description
references/_sections.md	Category definitions and impact ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information