VISI (MREA Section)

Within MREA resources, the VISI section stores a PVS indexed into an octree. This PVS is used to determine visibility/occlusion between cubic sections of the area and models, scripting entities, and lights in the scene.

Layout

Data Type	Description	Notes
FourCC	VISI magic
u32	VISI Version	0x2 in Metroid Prime.
bool	Unknown 1
bool	Unknown 2
u32	Feature count	Number of static MREA meshes + script entities encoded in octree.
u32	Light count	Number of area lights encoded in octree leaves.
u32	Unknown 3
u32	Entity count	Number of script entities encoded in octree.
u32	Leaf size	Size of octree leaf node in bytes.
u32	Light visibility node count	Count of leaf nodes from light perspectives (matches light count).
u32[entityCount]	Entity array	Array of entity IDs mapping the octree bits to the lower 16-bits of the SCLY editor IDs.
u8[lightVisibilityNodeCount][leafSize]	Light visibility node array	Array of leaf node data indicating visibility of other objects from light's perspective.
float[2][3]	Octree AABB	Total bounds subdivided by octree.
u32	Total visibility bit count	Number of bits per-leaf that indicate visibility from the selected octree region (static + entities + lights).
u32	Total light bit count	Number of bits after visibility bits that indicate in-bounds/out-of-bounds state of each light.
u32	Octree length	Size of octree in bytes.
u8[octreeLength]	Octree data	Byte-packed PVS octree data described below.

Octree Nodes

The octree data is arranged in a Z-major, depth-first hierarchy embedded with variable-length pointers to skip over children, permitting rapid child selection. Each node of the octree starts with a 1-byte header of classification bits. If at least one subdivide bit is set, the node is a branch and contains 1-7 pointers, addressed relative to the position after the pointers. One subdivision will have 1 pointer, two subdivisions will have 3 pointers, and three subdivisions will have 7 pointers. The first child pointer is implied to be 0x0. Pointer data type is specified in the header (8, 16, or 24 bits).

Bit (from LSB)	Description
0	Branch Subdivide X
1	Branch Subdivide Y
2	Branch Subdivide Z
3-4	Node type Out of bounds (not used) End of hierarchy (indicates nothing is visible) Regular node (followed by branch pointers or leaf bits)
5-6	Branch pointer type 16-bit pointers 8-bit pointers 24-bit pointers

Leaf Nodes

Instead of child pointers, leaf nodes contain byte-packed bits indicating visibility. The bits are arranged from LSB to MSB per-byte. First are the bits relating static MREA geometry in-order of the first MREA section. Next are bits relating the scripted entities in the VISI entities table. Finally are the bits relating lights in order of the BABEDEAD section of the MREA. Following these is another set of bits for each light indicating if it's in-bounds of the octree or not.