llamamod/notes/mod-fog.md

Draw distance mod (fog, VISUAL)

Code: src/main/java/com/eliasmueller/llamamod/mods/fog/FogMod.java, test src/test/.../mods/fog/FogModTest (needs -Dllamamod.cd=<cd dir>). Data-only (MAP chunks), no exe patch.

Options

Key	Type	Default	Effect
distance	INT % 100–600	250	fog start/end (= far clip) × distance/100
fog	CHOICE scaled / off	scaled	off: fog band shrunk to 1/16 tile, i.e. no fade, hard edge at the draw distance

Engine facts (groove.exe)

• Environment zone (MAP block 4, 90 bytes on disk, 0x5c in memory; loader FUN_0042ac50). Fog values are 20.12 fixed point, 4096 = 1 tile:

  memory	file offset	meaning
  +0x34	0x34	fog mode (3 = vertex fog/darkening; 2 = other mode; read by FUN_00438180)
  +0x3c / +0x40	0x3a / 0x3e	fog start / end for detail "Minimum"
  +0x44 / +0x48	0x42 / 0x46	fog start / end for detail "Maximum" (loader: if +0x44 ≤ +0x3c, both are set to 1.5 × the low pair)
  +0x4c, +0x50	0x4a, 0x4e	colours (ambient → FUN_0041bcf0, backdrop/fog → FUN_00439110)
  +0x58	0x56	music id
  The u16 at +0x38 takes 2 bytes on disk, so all later file offsets are memory − 2.

• Per frame (FUN_0041a480; zone switch FUN_0041e1c0): t = detail setting (0x5848cc: 1.0 Maximum, 0.5 Medium, 0.0 Minimum). Fixed end = lo_end + (hi_end − lo_end)·t → 0x58658c / 0x586590; float far = end·(t + (1−t)·0.5)/4096 → 0x57f90c = 0x57f8fc; fog start likewise → 0x57f8f8; 0x57f904 = 1/(far − start) (fog factor), 0x57f914 = 0.9999999/(far − near) (depth). FUN_00424bb0(near, far) sets the clip range for the rasteriser.
• Everything that limits view distance derives from these zone values:
  • fog: vertex colours are scaled by (far − d)·0x57f904 when d > start (0x421a14 …, 0x557b20 …),
  • far clip plane 0x57f90c,
  • tile renderer FUN_0042bf40 walks the grid cells within camera ± (far + const),
  • strat visibility/activity radius 0x586590 + 0x800 (FUN_00428700, FUN_004361xx; strat opcode 0xf2 also returns it). So objects (and enemies) wake up from further away as well.
• Not changed: the region mask 0x58642c (32 region bits from MAP block 6/7, set by triggers; hides other rooms/areas) – it is a level-design visibility mechanism, not a distance limit.
• Original fog end at Maximum detail, per level: 7–16 tiles (village t1l1m001 8.2–12.8, jungle t1l2m001 8.6–10.5, jungle night chase t1l2m003 6.9, city t1l6m001 14, roller coaster up to 23). At Minimum the float far is only half of the low pair (2.5–4 tiles). Zones with mode 0 carry placeholder values (2.0/3.5/3.0/5.25) and are scaled as well.

Verified in game (apptest, Wine 6, llvmpipe, 2026-11-06)

• Jungle t1l2m001 at 50 % / original / 400 %: at 50 % the spider-web gate at the end of the first path disappears in the dark fog, at 400 % it is a bit clearer than in the original (img/mods/fog-jungle-50-100-400.jpg). This proves the four zone values drive fog + clip.
• Village t1l1m001 (400 %, 300 %, 50 %) and city t1l6m001 (400 %, img/mods/fog-city-400.jpg): levels load and run, no crash, no slowdown noticeable in the harness; the start views are enclosed (village backdrop mountains are a sky texture, the city streets are short), so the difference there is small.
• Not tested: 600 % in large open levels (roller coaster, 23 tiles × 6) – expect more tiles and strats per frame; and the off mode in game (unit test only).

Known limits

• Many areas are enclosed or use region masks, so the gain is mostly visible on long paths and open views (jungle, river, roller coaster). The village backdrop is a texture, not geometry.
• Objects/enemies activate from further away (strat radius = fog end); with large values enemies may start moving before they are in view.