KCL (File Format)

KCL Files are collision files used in Mario Kart Wii and many other games. They have been used since at least Mario Kart DS. The documentation here is specifically for the Mario Kart Wii implmentation, although other implmentations are likely to be similar.
The KCL files contain simplified versions of model files in order to allow rapid collision detection. They use an octree for efficient spatial indexing, as well as storing the actual triangles of the models in a more rapidly accessible format.

Available editors:

• SZS Modifier
• CTools

File Format

The basic file format consists of a header, and four data sections.

Header

The header is a 0x3c byte structure as follows.

Offset	Type	Description
0x00	u32	Offset to section 1
0x04	u32	Offset to section 2
0x08	u32	Offset to section 3
0x0c	u32	Offset to section 4
0x10	single	Unknown (300.0 typical)
0x14	single3	Spatial grid first coordinate
0x20	u32	X mask
0x24	u32	Y mask
0x28	u32	Z mask
0x2c	u32	Coordinate shift
0x30	u32	Y shift
0x34	u32	Z shift
0x38	single	Unknown (250.0 typical)

All offsets are relative to the start of the file.
The meaning of the shift and mask values is explained in section 4.

Section 1 - Verticies

Section 1 is simply a large array of verticies, stored as 3 successive singles for x, y and z. The length of this array is not stored, but can usually be calculated by subtracting the section 1 offset from the section 2 offset and dividing by 0xc.

Section 2 - Normals

Section 2 is much the same as section 1, in that it is a large array of normals. Again the values are stored as 3 successive singles for x, y and z. The length of this array is not stored, but can usually be calculated by subtracting the section 2 offset from the section 3 offset + 0x10 and dividing by 0xc.

Section 3 - Triangles

The third section is the section containg the actual model information. The offset to this section is stored as 0x10 less than the actual location of the data, because this section is one indexed in section 4. The strcutre of each entry in this section is a 0x10 byte strcutre given below.

Offset	Type	Description
0x00	single	Length
0x04	u16	Position index
0x06	u16	Direction index
0x08	u16	Normal A index
0x0a	u16	Normal B index
0x0c	u16	Normal C index
0x0e	u16	Collision flags

All indicies in this section are 0 indexed. The position index is an index for section 1, and the others are indicies to section 2. The exact manner in which the values are used for collision detection is unknown, however a method for converting this form of triangle to a set of three coordinates is outlined below. The coordinate system is right handed.

Vertex1 = Position
Vertex2 = Position + Cross(Direction, NormalA) * (Length / Dot(NormalA, NormalC))
Vertex3 = Position + Cross(NormalB, Direction) * (Length / Dot(NormalB, NormalC))

A method for converting three verticies into the KCL form is given below. This method assumes the verticies are arranged anti clockwise when viewed from the collidable side.

Position = Vertex1
Direction = Cross(Normalize(Vertex2 - Vertex1), Normalize(Vertex3 - Vertex1))
Length = Dot(Normalize(Vertex2 - Vertex1), Direction)
NormalA = Cross(Direction, Normalize(Vertex2 - Vertex1))
NormalB = Cross(Normalize(Vertex3 - Vertex1), Direction)
NormalC = Cross(Direction, Normalize(Vertex3 - Vertex2))

Section 4

Section 4 is a spatial index. It is a series of u32 values, followed by lists of u16s. It subdivides three dimensional space using an octree, and indicates which triangles from section 3, if any, appear in each cube of the space. Given a coordinate (x,y,z) in world space, in order to find which triangles are at in range of that location, the spatial gird first coordinate is subtracted from the coordinate. If the value is negative, it is not colliding. If the value is positive, it is rounded to a u32, and then each component is ANDed with the mask. If the value is non zero, it is also not colliding. If not, the the original u32 components are all shifted right by coordinate shift. The y and z coordinates are then shifted left by their shift values. The resulting components are ORed with each other to produce an index into the octree.
The octree is then to be followed until a triangle list is found. At each stage, if the top bit of the current u32 is set, then the remaining 31 bits are an offset to a list of u16 triangle indicies. Each of these is a 1 based index to section 3, which is a triangle that must be checked by an object at the original location. The list is 0 terminated. If the top bit is not set, then the remaining 31 bits are an offset to 8 more of the u32s in the octree. The index into these 8 values is calculated by getting the next least significant bit in the u32 of each component, and then shifting z left by 2, y left by 1 and oring them all together. The procedure then repeats with the value at that offset.

Mario Kart Wii Collision Flags

This section has an own page now: KCL flag