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UE3 Home > Technical Artist > Binary Format Specifications for Skeletal Animation Source Files

Binary Format Specifications for Skeletal Animation Source Files

Binary Format Specifications for Skeletal Animation Source Files

Skeletal animation and mesh source binaries

As exported by ActorX from Max or Maya, the PSA and PSK binary files contain the raw animation and mesh information used to import skeletal animation art into the Unreal engine. Specifications are given here to aid anyone interested in expanding the format or writing 3rd party utilities for it (skin painter programs, exporters for other 3d packages, etc. )

PSK format:

This contains the mesh, bone influence indices and weights for each vertex, the bone names, bone hierarchy, and skeletal default pose.

PSK format, Version 1.0 (note: all headers are of type VChunkHeader). In general the file is constructed from header, data pairs. The header specifies (in the Datacount field) the number of data entries that follow.

Order of appearance	Description
General Header	When the `Typeflag` value equals decimal `1999801` or lower it denotes this version 1.0 layout of the PSK file.
Points Header	Header specifying number of points, defined in `Datacount`.
Points Data	Array with VPoint .
Wedges Header	Header specifying amount of wedges.
Wedges Data	VVertex wedges array (a wedge consists of a UV pair with an index into the 3d points array.)
Faces Header	Header specifying amount of faces.
Faces Data	VTriangle faces array.
Materials Header	Header specifying amount of materials.
Materials Data	VMaterial Materials array.
Bones Header	Header specifying amount of bones.
Bones Data	VBone bones array.
Influences Header	Header specifying amount of bone influences.
Influences Data	VRawBoneInfluence array of Influences.

PSA format:

PSA files contain the bone names, one or more sequence names, and for each sequence, its frame number N, and tracks of N rotations and N translations for each bone.

PSA format, Version 1.0. (note: all headers are of type VChunkHeader). Just like the PSK file the PSA file is constructed from header, data pairs.

Order of appearance	Description
General Header	When the `Typeflag` value equals decimal `20090127` (for Maya) `2003321` (for 3dsMax) or higher it denotes this version 1.0 layout of the PSA file.
Bones Header	Header specifying amount of bones.
Bones Data	FNamedBoneBinary bone names array.
Animations Header	Header specifying amount of animation sequence info chunks.
Animations Data	AniminfoBinary array of animation sequence info chunks.
Raw keys Header	Header specifying amount of raw keys.
Raw keys Data	VQuatAnimKey array.
Scale keys Header	Header specifying amount of scale keys.
Scale keys Data	VScaleAnimKey array.
Curve keys Header	Header specifying amount of curve keys(# of blendshapes). (Only supported if you exported from Maya)
Curve keys Data	VBlendCurve array. (Only supported if you exported from Maya)

The raw key array holds all the keys for all the bones in all the specified sequences, organized as follows:

For each AnimInfoBinary's sequence there are [Number of bones] times [Number of frames keys] in the VQuatAnimKeys, laid out as tracks of [numframes] keys for each bone in the order of the bones as defined in the array of FnamedBoneBinary in the PSA.

For each VBlendCurve includes BlendCurveName[128] and float arrays (equal to [Number of frame keys]) of weight values .

Animation data inside the engine

Once the data from the PSK (now digested into native skeletal mesh) and PSA (digested into a native animation object containing one or more sequences) are associated together at runtime, bones are linked up by name. Any bone in a skeleton (from the PSK) that finds no partner in the animation sequence (from the PSA) will assume its reference pose stance ( as defined in the offsets & rotations that are in the VBones making up the reference skeleton from the PSK)

The native animation and mesh code data as imported into the editor's Animation Browser, takes the shape of UKX package files.

C++ structures used

The relevant structs and classes are outlined in this header file:

https://udn.epicgames.com/pub/Three/BinaryFormatSpecifications/UnrealAnimDataStructs.h

Note that all structures are assumed padded to multiples of DoubleWord (32 bit) sizes by VC++ - when writing custom code, putting #pragma pack(push,1) around the definitions will break psa/psk binary compatibility. The default packing can be forced with #pragma pack(push,4). Fortunately the 'wasted' space is not an issue because this is for the raw data only. The same padding is assumed in the editor's import code as well as in the Max/Maya exporters.