JBA File Structure

1. File Layout
2. File Header
3. Block Header
4. Bone Data
5. Block Data
   1. Keyframe Layout
   2. Keyframe Encoding
6. World Space
7. Bone Names

File Layout

1. File Header
2. Block Header, one per block
3. Run-time initialized values, 4 bytes per block
4. Array of Bone Data structs, 128-byte aligned, length equals number of bones
5. Array of Block Data structs, 128-byte aligned, length equals number of blocks
6. World Space, 128-byte aligned
7. Bone Names

File Header

Offset (h)	Type	Count	Description	Values
00	uint32	1	Run-time initialized value	0
04	float	1	Animation length	N/A
08	float	1	FPS	15.0,30.0
0C	uint32	1	Number of blocks	1-20
10	uint32	2	Run-time initialized values	0
18	uint32	1	Number of bones	N/A
1C	uint32	1	Unknown	0
20	uint32	1	Pointer to keyframes (run-time initialized)	0FFFFFFFh
24	uint32	1	Unknown	0

Block Header

Offset (h)	Type	Count	Description	Values
00	uint32	1	Starting frame	N/A
04	uint32	1	Size of the data structure	N/A

Bone Data

Offset (h)	Type	Count	Description	Values
00	float	3	Translation stride	N/A
0C	float	3	Translation base	N/A
18	float	3	Rotation stride	N/A
2A	float	3	Rotation base	N/A

Block Data

Offset (h)	Type	Count	Description	Values
00	uint32	1	Number of bones	N/A
04	uint32	1	Unknown	0
08	uint32	4 * number of bones	Keyframe layout	N/A
N/A	uint16	N/A	Keyframes (48-bit quaternion + optional 32-bit translation)	N/A

Keyframe Layout

Keyframe layout contains four integer values per bone, where the first value represents a number of rotation poses and the third value represents a number of translation poses (optional).

Keyframes are stored according to this layout:

• Bone 0: keyframe 0 rotation
• Bone 0: keyframe 1 rotation
• Bone 0: keyframe 0 translation (optional)
• Bone 0: keyframe 1 translation (optional)
• Bone 1: keyframe 0 rotation
• Bone 1: keyframe 1 rotation
• Bone 1: keyframe 0 translation (optional)
• Bone 1: keyframe 1 translation (optional)

Keyframe Encoding

Rotation (on disk): Sign(1)-X(15)-Y(16)-Z(16)

Translation (on disk): Z(10)-Y(11)-X(11)

Decoding in Python:

val = ruint32(file)
pos_x = base.x + (val >> 21) * stride.x
pos_y = base.y + ((val >> 10) & 0x7ff) * stride.y
pos_z = base.z + (val & 0x3ff) * stride.z
pos = Vector((pos_x, pos_y, pos_z))

rot_x_raw = ruint16(file)
rot_x = base.x + (rot_x_raw & 0x7fff) * stride.x
rot_y = base.y + ruint16(file) * stride.y
rot_z = base.z + ruint16(file) * stride.z
rot_dot = rot_x * rot_x + rot_y * rot_y + rot_z * rot_z
rot_w = 0.0 if rot_dot > 1.0 else math.sqrt(1.0 - rot_dot)
if rot_x_raw & 0x8000:
    rot_w *= -1.0
rot = Quaternion((rot_w, rot_x, rot_y, rot_z))

Rotations and translations are in the "morpheme" space. Below is its transformation matrix:

1000.0    0.0     0.0   0.0
   0.0    0.0 -1000.0   0.0
   0.0 1000.0     0.0   0.0
   0.0    0.0     0.0   1.0

World Space

Offset (h)	Type	Count	Description	Values
00	uint32	1	Unknown	0
04	float	1	FPS	15.0/30.0
08	float	3	Translation stride	N/A
14	float	3	Translation base	N/A
20	float	3	Rotation stride	N/A
2C	float	3	Rotation base	N/A
38	uint32	1	Number of rotations	N/A
3C	uint32	1	Pointer to rotations (run-time initialized)	0
40	uint32	1	Number of translations	N/A
44	uint32	1	Pointer to translations (run-time initialized)	0
48	uint16	3 * number of rotations	Compressed rotations (run-time initialized?)	N/A
N/A	uint32	Number of translations	Compressed translations, 4-byte aligned	N/A

Bone Names

Offset (h)	Type	Count	Description	Values
00	uint32	1	Number of names	N/A
04	uint32	1	Unknown	N/A
08	uint32	1	Offset to indices	20
0C	uint32	1	Offset to offsets	20+4*(number of names)
10	uint32	1	Offset to names	20+8*(number of names)
14	uint32	number of names	Indices	0-N
N/A	uint32	number of names	Offsets into the names array	N/A
N/A	char	N/A	Null-terminated names	N/A