AROT (MREA Section)
Within MREA resources, the AROT section stores an octree which is used to accelerate rendering of areas in back-to-front order.
Octrees are a form of BSP tree that subdivide an area in 3-dimensions. They are structured recursively, starting with a root node and traversing their way to individual leaf nodes. A full-branch octant will split sub-octants across 3-tiers, one for each dimension of euclidean space. Retro uses a Z-major convention with (up to) 8 child-nodes subdivided as: 2 nodes along Z, 4 nodes along Y, and 8 nodes along X. A set of three bit-flags in each node determines which dimensions are subdivided. Nodes that do not subdivide all 3 dimensions will parent less child nodes as a result.
Within each octant is an index to a bitmap relating that AABB-volume to a set of meshes within the MREA; they are sorted and frustum-culled accordingly. Actors, particle effects, and other visual entities are also sorted and frustum-culled according to which octant(s) they intersect.
Layout
| Data Type | Element Count | Description | Notes |
|---|---|---|---|
| Header | 1 | Octree Header | |
| long | round_up(bitmap_bit_count / 32) * bitmap_count | Mesh Bitmap Array | An array of word-packed bitmaps relating octant nodes to sets of MREA meshes |
| long | node_count | Node Indirection Table | Since nodes use variable-length-encoding, there is an indirection table with byte-offsets (relative to first node) |
| Node Entry | node_count | Node Entry Table | Tightly-packed blob of nodes |
Header
| Offset | Size | Data Type | Description | Notes |
|---|---|---|---|---|
| 0x0 | 4 | long | Magic | AROT
|
| 0x4 | 4 | long | Version | Always 0x1 |
| 0x8 | 4 | long | Mesh-Bitmap Count | Count of mesh-bitmaps |
| 0xC | 4 | long | Mesh-Bitmap Bit-Count | Count of bits in each mesh-bitmap (matches MREA's mesh count) |
| 0x10 | 4 | long | Node Count | Number of octant nodes |
| 0x14 | 24 | float2x3 | AABB | AABB that fully-encloses area and serves as the octree's root-shape |
| 0x2C | 20 | Alignment Padding | Allows the following bitmaps to be data-cached with guaranteed 32-byte alignment |
Node Entry
| Offset | Size | Data Type | Description | Notes |
|---|---|---|---|---|
| 0x0 | 2 | short | Bitmap Index | Index of the mesh bitmap defining a set of MREA meshes for this octant node |
| 0x2 | 2 | short | Dimensional Flags | If non-zero, this half-word contains 3-bits specifying which dimensions are subdivided using the children specified (using Z-major convention)
If zero, this node-entry ends here |
| 0x4 | Varies | short[] | Node Child Indices | Variable-length array of node-entry indices that are children of this node-entry (Z-major layout) |