BTK stores texture matrix animation data.

JSYSTEM header

The file starts with a standard JSYSTEM header, using a magic of J3D1btk1. BTK files have one chunk, the TTK1 chunk.

TTK1 Chunk

Offset	Type	Name	Description
0x00	Chunk Header	Standard JSYSTEM chunk header
0x08	Loop Mode	Loop Mode
0x09	int	Rotation Frac	Multiplier for fixed-point decimal
0x0A	short	Duration	The number of frames in the animation
0x0C	short	Keyframe Count	The total number of keyframes, times 3
0x0E	short	Scale Table Count	The number of entries in the scale table
0x10	short	Rotation Table Count	The number of entries in the rotation table
0x12	short	Translation Table Count	The number of entries in the translation table
0x14	int	Animation Data Table Offset	Offset to the start of the animation data table
0x18	int	Remap Table Offset	Not used at runtime
0x1C	int	Name Table Offset	Contains the material names to bind against
0x20	int	Texture Coordinate Index Table Offset
0x24	int	Center Coordinate Table Offset
0x28	int	Scale Table Offset
0x2C	int	Rotation Table Offset
0x30	int	Translation Table Offset
0x34	byte[0x28]	Post Matrix Data	A similar set of tables, for Post Matrices. Has never been observed to exist in the wild.
0x5C	MatrixMode	Matrix Mode	Determines whether texture coordinates should be calculated with the Basic or Maya algorithm. This field is not read in Sunshine's copy of JSYSTEM, hardcoding the Basic mode, and their tooling seems to have output uninitialized data instead of a valid enum value. It is explicitly checked to equal exactly 0x01 in Wind Waker onwards, likely for this reason.

MatrixMode

MatrixMode is an enum as follows:

Value	Name	Meaning
0x00000000	Basic Mode
0x00000001	Maya Mode

The table data is packed in Struct-of-Arrays fashion, meaning that to read the Nth animation, one should read the Nth item in the material name, texture coordinate index, material center, and animation data tables.

Material Name and Texture Coordinate Index

The material name and texture coordinate index tell you how to bind the animation to a model. Animations are always bound by material name and texture coordinate index. The material name table is a JUTNameTab table. The table entries are laid out as follows:

Offset	Type	Name	Description
0x00	GXTexCoordID (byte)	Texture Coordinate Index

Center Coordinate Table

The center coordinate is used by the Basic Matrix Mode algorithm. The table entries are laid out as follows:

Offset	Type	Name
0x00	float	S
0x04	float	T
0x08	float	Q

Animation Data

The animation data itself is stored in an identical form to BCK, so see the BCK page for details. Note that as this is acting on texture coordinates rather than world positions, the coordinates S, T, Q are used instead of X, Y, Z.

Calculating the SRT Texture Matrix

The SRT texture matrix can be calculated by either the Basic or Maya matrix modes.

Basic Mode

The Center S, Center T, Scale S, Scale T, Rotation Q, Translation S and Translation T inputs are used. The final matrix is a 3x2 matrix, which is as follows:

00 = Scale S * cos(Rotation Q)
01 = Scale S * -sin(Rotation Q)
02 = Translation S + Center S + (Scale S * (sin(Rotation Q) * Center T - cos(Rotation Q) * Center S))

10 = Scale T * sin(Rotation Q)
11 = Scale T * cos(Rotation R)
12 = Translation T + Center T + (Scale T * (-sin(Rotation Q) * Center S + cos(Rotation Q) * Center T))

Maya Mode

The Scale S, Scale T, Rotation Q, Translation S and Translation T inputs are used. The final matrix is a 3x2 matrix, which is as follows:

00 = Scale S * cos(Rotation Q)
01 = Scale T * -sin(Rotation Q)
02 = Scale S * ((0.5 * -cos(Rotation Q)) - (0.5 * sin(Rotation Q) - 0.5) - Translation S)

10 = Scale S * sin(rotation)
11 = Scale T * cos(rotation)
12 = Scale T * ((0.5 * -cos(Rotation Q)) + (0.5 * sin(Rotation Q) - 0.5) + Translation T) + 1

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