BCO (File Format)
BCO is a collision file format used in Mario Kart Double Dash!!.
File Format
Header
The format begins with the following header:
| Offset | Type | Description |
|---|---|---|
| 0x00 | Char[4] | File identifier. Always 0003 in ASCII. |
| 0x04 | UInt16[2] | Number of root quadtree nodes. |
| 0x08 | Int32[2] | Quadtree origin. |
| 0x10 | Int32[2] | Quadtree node size. |
| 0x18 | UInt16 | Number of collision attributes. |
| 0x1A | UInt16 | Unknown. Only seen as 0. |
| 0x1C | UInt32 | Triangle index array offset. |
| 0x20 | UInt32 | Triangle array offset. |
| 0x24 | UInt32 | Position array offset. Stores the positions of the triangles. Each position consists of three floats. |
| 0x28 | UInt32 | Collision attribute array offset. |
Quadtree
The quadtree is stored directly after the header. Triangles are organized in a quadtree. Each node in the quadtree is either a leaf or an internal nodes. A leaf points to triangles and internal nodes points to 4 sub nodes.
The game's collision works in 2D, which is why a quadtree is used and not an octree. The order which the triangles are sort is important, because the first triangles a kart hits is the one which is collided. Therefore the highest triangle center should be stored first.
Each quadtree node has the following structure:
| Offset | Type | Description |
|---|---|---|
| 0x00 | Byte | Number of triangles. |
| 0x01 | Byte | Unknown. Only seen as 0. |
| 0x02 | UInt16 | Index of first child node. |
| 0x04 | UInt32 | Triangle array index. |
Triangle
Each triangle has the following structure:
| Offset | Type | Description |
|---|---|---|
| 0x00 | Int32[3] | Position indices. Links to the three positions of the triangle. |
| 0x0C | Float | Negative dot product between triangle center and triangle normal. Unknown usage. |
| 0x10 | Int16[3] | Triangle normal. Base 10 fixed-point number with scale 10 000. |
| 0x16 | UInt16 | Collision attribute. Stores the collision type. |
| 0x18 | Byte | Min max look up. |
| 0x19 | Byte | Unknown. |
| 0x1A | UInt16[3] | Triangle neighbor indices. Stores triangle indices for neighbors for edge AB, BC respectively CA. 0xFFFF means no neighbor triangle. |
| 0x20 | UInt16 | Unknown. |
| 0x22 | Byte | Splash object index. Unknown if there are more usages for it. |
| 0x23 | Byte | Enables splash. Unknown if there are more usages for it. |
- Collision attributes
A collision attribute is a 16-bit number which specifies how entities interact when colliding. The number consists of two bytes. The high byte represents the basic type, and the low byte represent the variant. The basic types are shown below:
| Attribute | Name | Description |
|---|---|---|
| 0x00 | DIRT | Off-road. |
| 0x01 | ROAD | Road. |
| 0x02 | WALL | Wall. |
| 0x03 | GRASS | ? |
| 0x04 | ICE | ? |
| 0x05 | RESQ | ? |
| 0x06 | SLIP | ? |
| 0x07 | DASHJ | ? |
| 0x08 | DASHB | ? |
| 0x09 | CANNON | ? |
| 0x0A | VALLEY | Fall boundary. |
| 0x0B | ? | ? |
| 0x0C | ? | ? |
| 0x0D | ? | ? |
| 0x0E | RESQD | ? |
| 0x0F | RESQS | Lava/Water |
| 0x10 | ? | ? |
| 0x11 | BEACH | ? |
| 0x12 | OUTF | ? |
| 0x13 | ? | Heavy off-road. |
| 0xFF | NONE | ? |
Collision Attribute
The collision attribute structure stores more information about each collision attribute used in triangles. Each entry has the following structure:
| Offset | Type | Description |
|---|---|---|
| 0x00 | UInt16 | Collision attribute. |
| 0x02 | UInt16 | Unknown. |
| 0x04 | UInt32 | Unknown. |
| 0x08 | UInt32 | Unknown. |
Tools
The following tools can handle BCO files:
- mkdd-collision, by RenolY2
