# LSN-0022: Tilemap Empty Cell Semantics and Glyph Convergence

## Context
During the initial runtime design, `tile_id = 0` was used as a sentinel value for "empty" or "missing" tiles in tilemaps. However, as the asset banking and packer systems evolved, it became clear that `0` should be a valid index for any asset bank, including glyph banks. This conflict was formally tracked in `AGD-0015`.

## Lessons Learned

### 1. Explicit Presence Over Magic Values
Modeling absence with a magic value (like `tile_id = 0`) creates friction when that value is also a valid member of the domain (index 0). The project moved towards explicit presence checks.

### 2. Convergence During Parallel Implementation
The resolution of the tilemap empty cell semantics (moving to explicit presence) was crystallized during the implementation of the `glyph` system. Instead of maintaining two different ways of handling "empty" slots, the tilemap logic was harmonized with the newer, cleaner approach used for glyphs and other render paths.

### 3. Impact on Render Pipeline
By allowing `tile_id = 0` to be a valid tile, the render pipeline now relies on a separate bit or `Option`-like structure to determine if a cell should be drawn. This makes the code more robust and the asset indices more intuitive.

## References
- Discussion: `DSC-0016`
- Agenda: `AGD-0015` (Tilemap Empty Cell vs Tile ID Zero)
- Implementation: Glyph system integration