AROT (MREA Section)

Left: Recursive subdivision of a cube into octants. Right: The corresponding octree.

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) 0x4 — Z subdivided 0x2 — Y subdivided 0x1 — X subdivided 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)