Skinned mesh file format (.bsm2)

Simple binary file for animated skinned meshes. Contains vertex buffer, index buffer (triangle indices), geometry subsets (ranges in index buffer), material names, animations, and optional bone hierarchy info for physics meshes.

File structure

uint32 number of vertices

uint32 number of indices

uint32 number of geometry subsets

uint32 number of bones

uint32 number of material names

uint32 number of animations

for each vertex

float3 position

int1010102 normal // in normalized signed 1010102 format

int1010102 tangent // tangent in x,y,z, sign mult of bitangent in w

uint16 u, v // texture coord in normalized uint16 format

uint8 bones[4] // bone indices

uint8 weights[4] // bone weights in normalized uint8

for each index

uint16 index

for each geometry subset

uint32 first index

uint32 number of indices

uint32 material index

for each material name

string name

for each animation

string name

uint64 duration in ns

uint64 keys interval in ns

uint32 number of keys

for each animation key

for each bone

float3x4 bone transformation matrix

optional hierarchy info for physics meshes:

uint8 parent bone index[number of bones]

float4x4 inverse base pose matrix[number of bones]

string bone names[number of bones]

There string stored as:

uint32 string size

char characters[string size] // without 0 terminator

For regular animated meshes (without hierarchy info in file) all bone transformation matrices include inverse base pose transform and all parent bones transforms. For physics meshes (with hierarchy info) bone transformation matrices are local and don't include parent and inverse base pos transforms.

Tooling

.bsm2 files are created from .fbx files by fbx_bsm2 tool: fbx_bsm2 [-hierarchy] [-last_bones] path/to/file.fbx will output path/to/file.bsm2.

Options:

• -hierarchy - include hierarchy info for physics.
• -last_bones - output full bone chains. Without this conversion will include only bones, which influence mesh vertices. Sometimes bones at the ends of hierarchy chains doesn't influence mesh, but they may be useful for physics simulation.