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PSA and PSK binary files contain the raw animation and mesh information used to import skeletal animation art into the Unreal Engine. The PSK file contains the mesh, bone influence indices and weights for each vertex, the bone names, bone hierarchy, and skeletal default pose.

In Mass Effect 3, PSK data is included inside PCC files and can be exported using PCC Editor 2. Exported PSK files may be viewed with blender, Maya, or 3DSMax; editing must be done with one of the latter two tools. This page will be concerned with the specifics of the PSK binary format.

File Structure[]

The PSK file is structured as a collection if header-data pairs, of which there are eight types: Head, Points, Wedges, Faces, Materials, Bones, Weights, and Additional UV maps.

Schematic representation of the PSK file structure.

Psk file format.png

Graphical representation of the various 3D objects.

Psk objects.png

Chunk Header

The Chunk Header contains information about its companion data chunk: id, type, size and count. There are eight possible IDs, corresponding to each of the eight mesh objects:

  • ACTRHEAD: Does not contain a data pair.
  • PNTS0000: Vertices.
  • VTXW0000: Weighted edges (wedges).
  • FACE0000: Triangle faces.
  • MATT0000: Materials.
  • REFSKELT: Bone structure.
  • RAWWEIGHTS: Bone raw weights.
  • EXTRAUVS#: Extra UV maps.

Each header is defined as a 32 byte struct:

Bytes Type Description
20 String Chunk ID
4 Int32 Type Flag
4 Int32 Data Size
4 Int32 Data Count


The Head data chunk is defined by its header as having zero size and zero elements (count). It contains no information, thus it derives that only the header has useful meaning. The header chunk may act as a file-header, but does not necessarily have to be the first chunk in the file.


The Points data chunk stores the vertices (points) of the mesh, arranged as an array of structs with the vertex coordinates.

Bytes Type Description
4 Float X
4 Float Y
4 Float Z


The Wedges data chunk stores weighted edge information. A wedge is a reference to a point with texture coordinates; in other words, the union of a three-dimensional vertex array and its texturing information. It defines a point over the index of points and combines it with a texture coordinate (U,V) and a material. Single points can be shared between different wedges (texture seams). The format of this structure depends on the edge count.

Count lower/equal than 65536d

Bytes Type Description
2 Int16 Point index
2 Int16 Padding
4 Float U
4 Float V
1 Int8 Material index
1 Int8 Reserved
2 Int16 Padding

Count greater than 65536d

Bytes Type Description
4 Int32 Point index
4 Float U
4 Float V
4 Int32 Material index


The Faces data chunk stores the different mesh triangles. Each triangle is defined by a vector struct (3 vertices), filled with the texture of its assigned material and fit by the texture coordinate (U,V). All vectors contained inside a vector array. The data can be formatted as one of two possible structures: Unreal Engine format (FACE0000) or as a Umodel PSK extension (FACE0032).


Bytes Type Description
2 Int16 Wedge 0
2 Int16 Wedge 1
2 Int16 Wedge 2
1 Int8 Material index
1 Int8 Auxiliary material index
4 Int32 Smoothing groups

This variant is used as replacement for FACE0000 section for meshes with more than 64 vertices. This is umodel's psk format extension, Unreal Engine has no support for FACE3200 chunks.

Bytes Type Description
4 Int32 Wedge 0
4 Int32 Wedge 1
4 Int32 Wedge 2
1 Int8 Material index
1 Int8 Auxiliary material index
4 Int32 Smoothing groups


The Material chunk stores the materials used by the mesh:

Bytes Type Description
64 String Name
4 Int32 Texture index
4 Int32 Poly Flags
4 Int32 Auxiliary material
4 Int32 Auxiliary flags
4 Int32 LOD bias
4 Int32 LOD style


The Bones chunk stores information about the bones' position, size, articulations, and structure:

Bytes Type Description
64 String Name
4 Int32 Flags
4 Int32 Children count
4 Int32 Parent index
4 Float Rotation X
4 Float Rotation Y
4 Float Rotation Z
4 Float Rotation W
4 Float Position X
4 Float Position Y
4 Float Position Z
4 Float Length
4 Float Size X
4 Float Size Y
4 Float Size Z


The Weights chunk applies raw weighting to a vertex bone (bone <-> vertex connection with factor 0..1f) to allow the deforming of the mesh by the movement of bones and during the animation.

Bytes Type Description
4 Float Weight
4 Int32 Point index
4 Int32 Bone index

Additonal UV Maps

Additionally, up to three Additional UV (AUV) maps may be stored for each wedge construct:

  • EXTRAUVS0: as the second UV set
  • EXTRAUVS1: as the third UV set
  • EXTRAUVS2: as the fourth UV set

Each AUV type is stored in a separate table or list. The AUV count for each type must be then equal to the wedge count, so that wedge[ i ] gets the AUV[ i ] of each table/type. Because every material defines all textures, these will be accordingly applied to the coordinates defined by the base UV (defined by the wedge) and the AUV sets.

AUV sets are stored as 8 byte structs:

Bytes Type Description
4 Float U
4 Float V