PSK & PSA file formats

A PSK file stores shape and bone tree data for a skeletal mesh. A PSA file stores animation data for a particular skeletal bone tree. The two file types share a similar format and purpose, which is why they are described in the same article.

Shared binary format

Generally PSK and PSA files consist of a number of binary chunks with a common header. The first chunk is empty and serves as the file header.

Chunk layout

General chunk layout, offsets and lengths in decimal numbers

Byte Offset	Byte Length	Type	Description
0	20	Char[20]	Chunk ID.
20	4	Int	Type flags. If this value is decimal 1999801 or smaller, you are seeing the version of PSK/PSA files described here.
24	4	Int	Data size - how long each of the following data records are.
28	4	Int	Data count - how many data records will follow this header.
32	data size	binary	First data record in the chunk.
32 + data size	data size	binary	Second data record in the chunk.
...
32 + (data count - 1) * data size	data size	binary	Last data record in the chunk.

A chunk may be empty, which is indicated by a data count of zero. In that case, data size may be zero as well. This is particularly true for the file header chunk.

PSK file layout

A PSK file specifies the skeletal mesh in its reference pose. PSK files seem to always consist of the following types of chunks in the specified order:

File header 

Chunk ID is "ACTRHEAD", type flags value is decimal 1999801 or lower for the file version described in this article. Data size and data count are zero, denoting an empty chunk body.

Points 

Lists the vertex points the mesh consists of. Data set type is Point.

Wedges 

Lists the wedges, which basically assign a material UV coordinate to a particular mesh point. Data set type is Vertex.

Faces 

List of triangle faces, combining three wedges to a triangle. Data set type is Triangle.

Materials 

List of materials used by the mesh. Data set type is Material.

Reference bones 

List of skeletal bones, their reference position and how the skeletal bone tree looks. Data set type is Bone.

Influences 

Defines how points are affected when bones move. Data set type is RawBoneInfluence.

PSA file layout

A PSA file specifies animation data for a skeletal bone tree. PSA files seem to always consist of the following types of chunks in the specified order:

File header 

Chunk ID is "ANIMHEAD", type flags value is decimal 1999801 or lower for the file version described in this article. Data size and data count are zero, denoting an empty chunk body.

Bone names 

List of skeletal bones, their reference position and how the skeletal bone tree looks. Data set type is Bone. (Same as for PSK files.)

Animation infos 

Specifies the animations contained in this file, including name, group, length and affected bones. Data set type is AnimInfo.

Animation keys 

Specifies the position, orientation and time index for a single bone. Data set type is QuatAnimKey.

Individual data structures

This section lists the data structures involved in the file format. Generally, all multi-byte basic types are considered to be in little endian byte order.

Byte

An 8 bit unsigned integer value corresponding to the UnrealScript type byte.

Short

A 16 bit integer value, usually assumed to be unsigned.

Int

A 32 bit integer value corresponding to the UnrealScript type int, but usually assumed to be unsigned.

Float

A 32 bit floating point value corresponding to the UnrealScript type float or generally a single-precision floating-point number.

Char[n]

A series of exactly n bytes that are interpreted as a zero-terminated ASCII string filled with additional zero bytes to the required length.

Vector

An Euclidian vector denoting a position in 3D space, corresponding to the UnrealScript struct type Vector. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name
0	4	Float	X
4	4	Float	Y
8	4	Float	Z

Quat

A unit quaternion denoting an orientation or rotation in 3D space, corresponding to the UnrealScript struct type Quat. Length: 16 bytes.

Byte Offset	Byte Length	Type	Name
0	4	Float	X
4	4	Float	Y
8	4	Float	Z
12	4	Float	W

ChunkHeader

The chunk header structure. Length: 32 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	20	Char[20]	ChunkID	The chunk name. The Unreal Engine does not really care about this value.
20	4	Int	TypeFlags	The Unreal Engine does not really care about this value either, even though it supposedly specifies a file version identifier.
24	4	Int	DataSize	The size of each data record potentially following this header.
28	4	Int	DataCount	The number of data records following this header.

Point

Specifies a skeletal mesh vertex point. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	12	Vector	Point	The location of the point in the skeletal mesh's reference pose.

Vertex

Associates skeletal mesh points with a UV coordinate of a material. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	2	Short	PointIndex	Index of the point in the points list.
2	4	Float	U	U coordinate in the material.
6	4	Float	V	V coordinate in the material.
10	1	Byte	MatIndex	Index of the material in the materials list. (seems unused, probably overridden by face's own MatIndex)
11	1	Byte	Reserved	Padding byte to get to a multiple of four bytes for the structure.

Triangle

Combined three vertices to a triangle face. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	3x 2	3x Short	WedgeIndex	Indices of the three vertices in the wedges list.
6	1	Byte	Matindex	Index of the face's material in the materials list.
7	1	Byte	AuxMatIndex	Index of an additional material in the materials list.
8	4	Int	SmoothingGroups	Bit field of smoothing groups this face belongs to.

Triangle

Combined three vertices to a triangle face. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	3x 2	3x Short	WedgeIndex	Indices of the three vertices in the wedges list.
6	1	Byte	MatIndex	Index of the face's material in the materials list.
7	1	Byte	AuxMatIndex	Index of an additional material in the materials list. (seems unused)
8	4	Int	SmoothingGroups	Bit field of smoothing groups this face belongs to. (Does the engine actually care about this for skeletal meshes?)

Material

Specifies a surface material. Length: 88 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	64	Char[64]	MaterialName	Name of the material. The Unreal Engine will use this to find a texture or other material with this name in any package.
64	4	Int	TextureIndex	Should be the same as this material's index in the materials list. (But it's probably a good idea to not rely on the value being correct.)
68	4	Int	PolyFlags	Poly flags for all faces with this material. (seems unused)
72	4	Int	AuxMaterial	??? (seems unused)
76	4	Int	AuxFlags	??? (seems unused)
80	4	Int	LodBias	??? (seems unused)
84	4	Int	LodStyle	??? (seems unused)

JointPos

A helper struct for storing the bone position. Length: 44 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	16	Quat	Orientation	Rotation of the bone, relative to its parent.
16	12	Vector	Position	Location of the bone, relative to its parent.
28	4	Float	Length	??? (seems unused)
32	4	Float	XSize	??? (seems unused)
36	4	Float	YSize	??? (seems unused)
40	4	Float	ZSize	??? (seems unused)

Bone

Specifies a node in the skeletal bone tree for both the skeletal mesh and its animations. Length: 120 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	64	Char[64]	Name	Name of the bone.
64	4	Int	Flags	??? (seems unused)
68	4	Int	NumChildren	Number of child nodes in the skeletal bone tree.
72	4	Int	ParentIndex	Index of this bone's parent in the bone list. The root bone must be the first in the bone list and have ParentIndex zero.
76	4	JointPos	BonePos	Bone position information.

RawBoneInfluence

Specifies the influence of a bone's movement (through animation or otherwise) on a skeletal mesh point's position. Length: 12 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	4	Float	Weight	How strongly the bone's movement affects the point's location. Values are in the range (0.0; 1.0], i.e. zero weights should not appear in the influences list, because they are meaningless.
4	4	Int	PointIndex	Index of the affected point in the points list.
8	4	Int	BoneIndex	Index of the affecting bone in the bones list.

AnimInfo

Specifies an animation sequence. Length: 168 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	64	Char[64]	Name	Name of the animation sequence.
64	64	Char[64]	Group	Group name for the animation sequence. While an animation sequence can be part of multiple groups in the Unreal Engine, the PSA format allows only a single group name to be specified here. You can use the AnimIsInGroup() UnrealScript function in Unreal Engine 2 to check if the currently playing animation in a specified channel is part of a particular animation group. You can use the GetAnimGroup() UnrealScript function in Unreal Engine 1 to get the group name of the currently playing animation.
128	4	Int	TotalBones	Total number of bones involved in this animation.
132	4	Int	RootInclude	??? (seems unused)
136	4	Int	KeyCompressionStyle	??? (related to animation compression)
140	4	Int	KeyQuotum	??? (related to animation compression)
144	4	Float	KeyReduction	??? (related to animation compression)
148	4	Float	TrackTime	??? (seems unused as it's overridden by AnimRate * NumRawFrames)
152	4	Float	AnimRate	Default animation speed in frames per second.
156	4	Int	StartBone	??? (something about partial animations, seems unused)
160	4	Int	FirstRawFrame	First entry in the animation keys list for this animation.
164	4	Int	NumRawFrames	Number of entries in the animation keys list for this animation.

QuatAnimKey

Specifies the position, orientation and time index for a single bone in an animation sequence. Length: 32 bytes.

Byte Offset	Byte Length	Type	Name	Description
0	12	Vector	Position	Position of the bone at this time index, relative to the parent bone.
12	16	Quat	Orientation	Orientation of the bone at this time index, relative to the parent bone.
28	4	Float	Time	The duration until the next key.

External links

• Binary format specifications for skeletal and vertex animation source files