This is the Indigo Mesh format, which has the extension .igmesh on disk.

See IndigoMesh.h in the SDK for code to read and write this structure.

Notes:
uint32 is an unsigned 32 bit integer.
Byte order is little endian (Intel byte order).
Struct a[9] means that 'a' is an array of 9 'Struct's.

Specification

uint32 MAGIC_NUMBER // = 5456751

uint32 FORMAT_VERSION // = 4

uint32 compression // = 1 if Zstandard compression is used, 0 otherwise.

uint32 data_filtering // = 1 if data is filtered before compression, 0 otherwise.

uint32 num_uv_mappings // Number of UV mappings (UV layers) for this mesh. >= 0

uint32 num_material_names
String used_material_names[num_material_names] // Only used for old <model> scene format, not used for new <model2> scene format where materials are specified in the model2 element. num_material_names may be set to zero.

String format on disk is:

uint32 len // num bytes in string. <= 1024
char string_bytes[len] // data

The string is a Unicode string with UTF-8 encoding.

uint32 num_vert_positions
Vec3f vert_positions[num_vert_positions] // An array of vertex positions. Each vertex position is a single precision, floating-point 3-vector.

uint32 num_vert_normals
Vec3f vert_normals[num_vert_normals] // An array of vertex shading normals. This array can be empty if vertex shading normals are not available. If it is not empty then num_vert_normals should be equal to num_vert_positions.

uint32 uv_layout // Should be either MESH_UV_LAYOUT_VERTEX_LAYER (0) or MESH_UV_LAYOUT_LAYER_VERTEX (1).
Vec2f uv_pairs[num_uv_mappings*n]

An array of UV coordinates. The number of pairs must be a multiple of num_uv_mappings.
This array can be laid out in two possible ways.

MESH_UV_LAYOUT_VERTEX_LAYER UV layout is as follows (in this example num uvs = n, num layers = 2):

uv_0 layer 0
uv_0 layer 1

uv_1 layer 0
uv_1 layer 1

uv_2 layer 0
uv_2 layer 1

...

uv_n layer 0
uv_n layer 1

MESH_UV_LAYOUT_LAYER_VERTEX UV layout is as follows (in this example num uvs = n, num layers = 2):

layer 0 uv_0
layer 0 uv_1
layer 0 uv_2
layer 0 uv_3
...
layer 0 uv_n

layer 1 uv_0
layer 1 uv_1
layer 1 uv_2
layer 1 uv_3
...
layer 1 uv_n

uint32 num_triangles
Triangle triangles[num_triangles]

Where a triangle is defined as:

struct Triangle
{
uint32 vertex_indices[3]; /// Stores indices into the mesh vertex position array.
uint32 uv_indices[3]; /// Stores indices into the mesh uv_pairs array.
uint32 tri_mat_index; /// An index into the materials used by this mesh.
};

uint32 num_quads
Quad quads[num_quads]

Where a quad is defined as:

struct Quad
{
uint32 vertex_indices[4]; /// Stores indices into the mesh vertex position array.
uint32 uv_indices[4]; /// Stores indices into the mesh uv_pairs array.
uint32 mat_index; /// An index into the materials used by this mesh.
};