# Override masks (optional)

Paint in GIMP/Krita as **equirectangular** grayscale PNGs, any size (2:1), 8- or 16-bit.
Mid-gray (32768 in 16-bit, 128 in 8-bit) = neutral / no change. Transparent or missing = neutral.

| File | Effect (white = +1, black = −1) |
|---|---|
| `land_hint.png` | bias toward land (+) or sea (−) on top of the sketch |
| `mountain_hint.png` | extra uplift (+) |
| `o2_zones.png` | O₂ modifier: 1 + 0.5·v (world.toml `[masks] o2_range`) |
| `gravity_zones.png` | gravity modifier: 1 + 0.7·v; v < 0 = low-g zone (relief ×, spires) |
| `lock.png` | where brighter than 75% grey (> +0.5), coastline follows land_hint strictly |

Rebuild: `.venv/bin/python mapgen.py build` (masks are part of the cache key).