PXD

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PXD Files

Hedgehog Engine 2 games from 2016 Olympics to Shadow Generations contain skeleton and skeletal animation files with the file extensions .skl.pxd and .anm.pxd respectively, stored in a BINA container. Skeletons contain pose data that usually result in a T-Pose, bone and parent indices, and bone names. Animations contain playback metadata such as frame rate and frame count, track count, and either compressed animation pose data using ACL compression, or raw uncompressed pose data that use bone and frame indices to specify keyframed transforms, with linear interpolation between keyframes. For both skeletons and animations, position and rotation data for each bone is an absolute transform relative to the parent position, whereas scales are inherited from their respective parents locally, and have no effect on their positions.

.anm.pxd Structure

Animation files are stored in a BINA container. Below is the contents stored within the DATA node beginning at file offset 0x40:

Header:

struct Header {
    char[4] magic;  // 'NAXP'
    uint32_t version;   // Always 0x200
    uint8_t additive;      // 0x01 if additive, else 0x00
    uint8_t compressed;    // 0x08 if ACL compressed, 0x00 if uncompressed
    char[6];            // 0x00 align to 8 bytes
    uint64_t metadata_offset;    // Offset to metadata, always 0x18
    float32 duration;         // Duration of the animation in seconds, calculated as ((frame_count-1) / FPS)
    uint32_t frame_count;   // Frame count
    uint32_t track_count;   // Bone count
    uint64_t skel_anim_offset;  // Offset for character's skeletal animation, always 0x40 if compressed, 0x38 if uncompressed
    uint64_t root_anim_offset;  // Offset for root motion animation, 0x00 if no root motion is present, aligned to 16 bytes
    if (compressed)
        char[8];    // Likely alignment to 16 bytes for ACL data. Always 0x00 if compressed
};

ACL Data:

All Hedgehog Engine 2 games that utilize the ACL library appear to use v2.0.0. See the ACL docs for more info. If compressed == 0x08, skeletal animations and root motion animations are compressed.

Compressed Data:

Below is a high level overview of the stored ACL chunk data:

uint32_t acl_chunk_size;  // Total size of chunk
int32_t acl_hash;   // Hash
uint32_t acl_tag;   // Identifies ACL buffer type, always 0xAC11AC11 to mark compressed_tracks
// https://github.com/nfrechette/acl/blob/976ff051048477f2281c7d3609fddf0b3cba2c2d/includes/acl/core/buffer_tag.h#L49

uint16_t acl_version;   // ACL Version Enum Identifier. 0x07 indicates v2.0.0, the only version observed in any Hedgehog Engine 2 game.
// https://github.com/nfrechette/acl/blob/976ff051048477f2281c7d3609fddf0b3cba2c2d/includes/acl/core/compressed_tracks_version.h#L71

char acl_padding;   // always 0x00
uint8_t acl_track_type;    // ACL Track Type Enum. 0x0C indicates qvvf, the only track type observed in any Hedgehog Engine 2 game. 
// https://github.com/nfrechette/acl/blob/976ff051048477f2281c7d3609fddf0b3cba2c2d/includes/acl/core/track_types.h#L68

uint32_t track_count;   // Bone count if skeletal animation, 0x01 if root motion. Should match header track_count if skeletal. 
uint32_t sample_count;   // Frame count, should match header frame_count.
float32 sample_rate;   // Playback FPS of animation, should equal header ((frame_count - 1) / duration)
char[acl_chunk_size - 0x1C] acl_data;   // String of compressed ACL data

Raw/Decompressed Track List:

_TODO...

Uncompressed Animation Data:

_TODO...

.skl.pxd Structure

Skeleton files are stored in a BINA container. Below is the contents stored within the DATA node beginning at file offset 0x40:

_TODO...