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Occlusion culling, per frame

How RAC1 builds its visibility mask each frame from the camera position and tests every terrain fragment, tie and moby against it before any frustum or distance test. ReRAC's reading of the NTSC-U code (SCUS_971.99), verified in the disassembly of the four functions below and against the data of all 19 levels. The data: OCCLUSION.md (formats). RENDERER.md's summary (a 1,024-bit mask per 4-unit cell, read by the renderer cores) is this system.

Games. RAC1. Not measured in the sequels.

Functions (NTSC-U and PAL)

Role NTSC-U (boot) PAL
Per frame, by mode 0x1F2C10 (level 01: 0x2193F8) UpdateOcclusion func_001F2FB8
Build the mask 0x1F2820 BuildOcclVisibility func_001F2BC8
Cell lookup 0x1F2690 ParseOcclGrid func_001F2A38
Neighbour lookup 0x1F2768 GetOcclGridFromPair func_001F2B10

1. When

The frame render (DrawWorld, RENDER_PIPELINE.md) calls UpdateOcclusion near its top, before the sky and all world passes, whatever the pass switches. The mode is set at level load: 2 (active) when the level has a grid and mappings, else 0 (off). The debug menu calls it "occlusion" with off, freeze and active; retail never changes it.

2. Building the mask

mode 0: mask = all visible
mode 1 (freeze): mask unchanged
mode 2:
  c = (int)(camera × 0.25) per axis
  if the cell exists: mask = its mask; remember it as "previous"
  else:
    if fallback == 0: for each axis try the nearer neighbour, then the
        other; OR the hits; if any, mask = that union (remembered)
    if still nothing:
      fallback 1: all visible
      fallback 2 with an octant override: the octant's mask
      otherwise: the previous mask, or all visible if there is none
        or the debug camera is on
  set bit 1023
  • Outside the grid the game keeps the last mask it used, so a camera that leaves the playable space can hide things in direct view; the game's camera never goes there.
  • The fallback flag is 0 in play; the freeze game modes set 2 for the next frame, a camera placement path sets 1, the end of every frame clears it.
  • The previous mask is never reset at level load in the code found (a port should reset it).

3. The test

Each renderer core copies the 0x80-byte mask to the scratchpad and, for each object with occlusion word w, skips it when mask[w >> 8] & (w & 0xFF) is 0:

Core Where in its loop
TfragProc first test; occluded fragments never reach the lighting list
TieProc first real test (after a DMA wait), before draw distance; both tie passes
MobyProc after the moby's state check, before the mode skip and the sphere tests; an occluded moby also gets +0x31 (drawn) cleared and no animation job
Shrubs none

Occluded mobys keep updating: the update loop does not read +0x31.

Open

  • The initial previous mask and the mask before the first frame.
  • Why the camera placement path sets fallback 1; the gameplay meaning of the octant override.
  • RAC2–RAC4: not measured.

Sources: ReRAC (https://github.com/re-rac/rerac, ISC, Copyright (c) 2026 ReRAC contributors), docs/plan/occlusion_culling.md, docs/formats/occlusion_rac1.md; PAL names from games/rac1/config/overlays/us_map.tsv and RENDERER.md.