Difference between revisions of "AROT (MREA Section)"

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(AROT header)
 
 
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[[File:Octree2.svg|thumb|right|400px|Left: Recursive subdivision of a cube into [[wikipedia:octant (solid geometry)|octant]]s. Right: The corresponding octree.]]
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Within [[MREA (File Format)|MREA]] resources, the '''AROT section''' stores an [[wikipedia:Octree|octree]] which
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is used to accelerate rendering of areas in back-to-front order.
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Octrees are a form of [[wikipedia:Binary Space Partitioning|BSP tree]] that subdivide an area in 3-dimensions.
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They are structured recursively, starting with a ''root'' node and traversing their way to individual ''leaf'' nodes.
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A full-branch octant will split sub-octants across 3-tiers, one for each dimension of euclidean space.
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Retro uses a Z-major convention with (up to) 8 child-nodes subdivided as: 2 nodes along Z, 4 nodes along Y,
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and 8 nodes along X. A set of three bit-flags in each node determines which dimensions are subdivided.
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Nodes that do not subdivide all 3 dimensions will parent less child nodes as a result.
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Within each octant is an index to a [[wikipedia:Bit array|bitmap]] relating that AABB-volume to a set of meshes within the MREA; they
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are sorted and frustum-culled accordingly. Actors, particle effects, and other visual entities are also sorted and
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frustum-culled according to which octant(s) they intersect.
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=== Layout ===
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{| class="wikitable"
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! Data Type
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! Element Count
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! Description
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! Notes
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|-
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| [[#Header|Header]]
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| 1
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| Octree Header
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|
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|-
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| long
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| round_up(bitmap_bit_count / 32) * bitmap_count
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| Mesh Bitmap Array
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| An array of word-packed bitmaps relating octant nodes to sets of MREA meshes
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|-
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| long
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| node_count
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| Node Indirection Table
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| Since nodes use variable-length-encoding, there is an indirection table with byte-offsets (relative to first node)
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|-
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| [[#Node Entry|Node Entry]]
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| node_count
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| Node Entry Table
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| Tightly-packed blob of nodes
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|}
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=== Header ===
 
=== Header ===
  
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| long
 
| long
 
| Mesh-Bitmap Bit-Count
 
| Mesh-Bitmap Bit-Count
−
| Count of bits in each mesh-bitmap
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| Count of bits in each mesh-bitmap (matches MREA's mesh count)
 
|-
 
|-
 
| 0x10
 
| 0x10
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| AABB
 
| AABB
 
| AABB that fully-encloses area and serves as the octree's root-shape
 
| AABB that fully-encloses area and serves as the octree's root-shape
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|-
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| 0x2C
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| 20
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|
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| Alignment Padding
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| Allows the following bitmaps to be data-cached with guaranteed 32-byte alignment
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|}
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=== Node Entry ===
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{| class="wikitable"
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! Offset
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! Size
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! Data Type
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! Description
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! Notes
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|-
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| 0x0
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| 2
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| short
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| Bitmap Index
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| Index of the mesh bitmap defining a set of MREA meshes for this octant node
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|-
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| 0x2
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| 2
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| short
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| Dimensional Flags
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| If non-zero, this half-word contains 3-bits specifying which dimensions are subdivided using the children specified (using Z-major convention)
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<ul>
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<li>0x4 — Z subdivided</li>
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<li>0x2 — Y subdivided</li>
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<li>0x1 — X subdivided</li>
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</ul>
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'''If zero, this node-entry ends here'''
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|-
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| 0x4
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| Varies
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| short[]
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| Node Child Indices
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| Variable-length array of node-entry indices that are children of this node-entry (Z-major layout)
 
|}
 
|}

Latest revision as of 23:59, 18 October 2018

File:Octree2.svg
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)
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