IdaJS
    Preparing search index...

    Exterior Decors in LBA2/Ida

    This document explains how exterior decor objects work in the engine, how they are loaded, how visibility is toggled, and what modders must edit when changing island content.

    This is about exterior 3D island decors (T_DECORS), not scene actors.

    • Actors are loaded from scene files/scripts.
    • Exterior decors are loaded from island resources (.ILE/.OBL) per cube.

    Exterior decor objects are stored in island/cube data and rendered by the exterior engine path.

    Relevant code:

    • SOURCES/3DEXT/LOADISLE.CPP (LoadIsland, LoadCube)
    • SOURCES/3DEXT/DECORS.CPP (FixeObjetsDecorsInvisibles, AffichageObjetDecorsZBuf)
    • SOURCES/3DEXT/LBA_EXT.H (T_DECORS, flags)

    When a cube is loaded, its own DOB chunk is loaded into ListDecors:

    • LoadCube() sets NbObjDecors and ListDecors from cube data.

    Implication:

    • Decor content is tied to the currently loaded exterior cube.
    • If you modify visuals in one cube, other cubes are unaffected unless they share the same mapped cube data.

    3) Visibility control is encoded in decor data (Beta >> 16)

    Each decor can encode a game-variable condition in the high 16 bits of Beta:

    • numvar = ptrobj->Beta >> 16

    Engine rule (from FixeObjetsDecorsInvisibles):

    • numvar > 0: hide if ListVarGame[numvar] != 0
    • numvar < 0: hide if ListVarGame[-numvar] == 0
    • numvar == 0: no variable-based visibility condition

    Example (specific to Citadel citabau cube 42): both +83 and -83 exist for opposite variants, where var 83 toggles spaceship vs occupied/barbed-wire set.

    For Citadel island cube 42, traces showed:

    • body=101 with numvar=+83 (spaceship set)
    • multiple occupied props with numvar=-83

    Behavior:

    • ListVarGame[83] == 0 -> +83 visible, -83 hidden (spaceship visible)
    • ListVarGame[83] != 0 -> +83 hidden, -83 visible (occupied props visible)

    So this swap is done by decor visibility logic, that allows us to enrich the same island model without need to load another island model.

    These are separate systems:

    1. Island file selection (e.g. citadel vs citabau) in InitGrilleExt:
    • SOURCES/EXTFUNC.CPP

    The only 2 places in the game where the whole island model needs to be switched is citadel (initial island model when we have storm) vs citabau, and the volcano island before the ceremony vs after the ceremony. A corresponding API exist in IdaJS to force those models (see the storm sample). In all other cases, we can reuse the same island model and just swap specific decor subsets through game variables.

    1. Decor variant selection inside the loaded cube data:
    • SOURCES/3DEXT/DECORS.CPP (Beta >> 16 + ListVarGame)

    So you can be on the same island model and still swap specific decor subsets through game variables.

    From SOURCES/3DEXT/LOADISLE.CPP and SOURCES/3DEXT/LBA_EXT.H:

    • Island map index is a 16x16 table (IsleMapIndex) in .ILE.
    • Each map cell points to a cube entry (or empty).
    • Cube data is chunked with step 6:
      • INF: cube infos
      • DOB: decor objects (T_DECORS list)
      • GRD: ground polys
      • TXD: texture defs
      • Y: heightmap
      • LUM: lightmap

    To add/change decor objects, you edit cube DOB data.

    To add a new logical cube layout location, you edit:

    • cube data entries (all required cube chunks), and
    • 16x16 map index cells to reference the new cube index.
    • MAX_CUBES_PER_ISLE = 20 (SOURCES/3DEXT/LBA_EXT.H)
    • MAX_OBJ_DECORS = 200 (SOURCES/3DEXT/LBA_EXT.H)

    In SOURCES/DEFINES.H:

    • MIN/MAX_ISLE_OBJ_MEM
    • MIN/MAX_CUBE_INFOS_MEM
    • MIN/MAX_LIST_DECORS_MEM
    • MIN/MAX_MAP_PGROUND_MEM
    • MIN/MAX_LIST_TEXDEF_MEM
    • MIN/MAX_MAP_SOMMETY_MEM
    • MIN/MAX_MAP_INTENSITY_MEM

    If you increase cube/decor complexity, you may need both:

    • bigger data resources, and
    • increased engine limits/memory pools.

    GRM (scene fragments) is a separate brick-overlay system and is not the same mechanism as T_DECORS visibility flags.

    For detailed GRM internals, life opcodes/functions, persistence, and IdaJS usage, see:

    A practical way to force decor variant swaps is to set the controlling game variable after scene load.

    Example (Citadel occupation switch variable 83):

    scene.addEventListener(scene.Events.afterLoadScene, (sceneId, loadMode) => {
      if (loadMode === scene.LoadModes.NewGameStarted) return;

      const gameStore = useGameStore();

      const VAR_CITADEL_OCCUPATION_DECORS = 83;
      const OCCUPIED_SET = 8; // nonzero => hide +83, show -83
      const NORMAL_SET = 0;   // zero => show +83, hide -83

      // Optional guard for target scenes only.
      // if (!isCitadelExterior(sceneId)) return;

      scene.setGameVariable(
        VAR_CITADEL_OCCUPATION_DECORS,
        gameStore.showOccupiedSet ? OCCUPIED_SET : NORMAL_SET
      );

      // ... more code
    });

    Notes:

    • Only zero/nonzero matters for this visibility rule.
    1. Identify target cube and island (with runtime trace).
    2. Trace conditional decors (numvar, body, visible/hidden state), using the debug tracing mechanism (LBA_IDA_TRACE_DECORS env variable, see below).
    3. Find the controlling variable(s) used by that cube.
    4. Switch in IdaJS afterSceneLoad event when needed
    5. If editing map assets:
      • edit cube DOB decor list,
      • keep Beta >> 16 conditions consistent,
      • verify body IDs exist in the island .OBL bank.
    6. If adding cubes:
      • add cube chunks,
      • update 16x16 map index,
      • verify engine limits and memory pools.

    • Decor not appearing:

      • wrong body ID in Body & 0xFFFF
      • wrong sign on numvar (+N vs -N)
      • controlling game variable not in expected state
      • map cell pointing to different cube index than expected

    • Works in one area but not another:

      • you edited one cube only; neighboring map cells may reference another cube

    • Crash/memory issues after data growth:

      • exceeded MAX_* limits or memory pools

    SOURCES/3DEXT/DECORS.CPP includes optional trace controlled by:

    • LBA_IDA_TRACE_DECORS=1 (enable)
    • LBA_IDA_TRACE_DECORS=0 (disable)

    It logs, per visibility pass:

    • island/cube
    • each conditional decor: index, body, numvar, var value, applied rule, final state
    • summary counts

    This is useful to compare save states and confirm which variable drives each decor set.

    Settings

    Member Visibility

    On This Page

    Exterior Decors in LBA2/Ida