Mirrored from http://www.mralligator.com/q3/

This document describes the Quake 3 BSP file format. This is an unofficial document. Quake 3 is a registered trademark of id Software, which does not sponsor, authorize, or endorse this document.
This document describes the Quake 3 BSP file format as the author understands it. While every effort has been made to ensure that the contents of this document are accurate, the author does not guarantee that any portion of this document is actually correct. In addition, the author cannot be held responsible the consequences of the any use or misuse of the information contained in this document.
Copyright © 2000 Kekoa Proudfoot. All rights reserved.

 
The layout of an IBSP file. An IBSP file consists of a header followed by a number of lumps. The header contains a directory which identifies the locations and sizes of the lumps. 
Data types
Quake 3 BSP files contains only four basic data types. They are:
Type  Description 

ubyte  unsigned byte 
int  4byte integer, littleendian 
float  4byte IEEE float, littleendian 
string[n]  string of n ASCII bytes, not necessarily nullterminated 
All data in a BSP file is organized into records composed of these four data types.
The header record looks like this:
string[4] magic Magic number. Always "IBSP". int version Version number. 0x2e for the BSP files distributed with Quake 3. direntry[17] direntries Lump directory, seventeen entries.
Each direntry locates a single lump in the BSP file:
direntry
int offset Offset to start of lump, relative to beginning of file. int length Length of lump. Always a multiple of 4.
Lumps
There are 17 lumps in a Quake 3 BSP file. In the order that they appear in the lump directory, they are:
Index  Lump Name  Description 

0  Entities  Gamerelated object descriptions. 
1  Textures  Surface descriptions. 
2  Planes  Planes used by map geometry. 
3  Nodes  BSP tree nodes. 
4  Leafs  BSP tree leaves. 
5  Leaffaces  Lists of face indices, one list per leaf. 
6  Leafbrushes  Lists of brush indices, one list per leaf. 
7  Models  Descriptions of rigid world geometry in map. 
8  Brushes  Convex polyhedra used to describe solid space. 
9  Brushsides  Brush surfaces. 
10  Vertexes  Vertices used to describe faces. 
11  Meshverts  Lists of offsets, one list per mesh. 
12  Effects  List of special map effects. 
13  Faces  Surface geometry. 
14  Lightmaps  Packed lightmap data. 
15  Lightvols  Local illumination data. 
16  Visdata  Clustercluster visibility data. 
string[length] ents Entity descriptions, stored as a string.
The length of the entity string is given by the size of the lump itself, as specified in the lump directory.
The meanings, formats, and parameters of the various entity descriptions are currently outside the scope of this document. For more information about entity descriptions, see the documentation to Q3Radiant, the Quake 3 level editor.
string[64] name Texture name. int flags Surface flags. int contents Content flags.
Planes
The planes lump stores a generic set of planes that are in turn referenced by nodes and brushsides. There are a total of length / sizeof(plane) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
plane
float[3] normal Plane normal. float dist Distance from origin to plane along normal.
Note that planes are paired. The pair of planes with indices i and i ^ 1 are coincident planes with opposing normals.
int plane Plane index. int[2] children Children indices. Negative numbers are leaf indices: (leaf+1). int[3] mins Integer bounding box min coord. int[3] maxs Integer bounding box max coord.
Leafs
The leafs lump stores the leaves of the map's BSP tree. Each leaf is a convex region that contains, among other things, a cluster index (for determining the other leafs potentially visible from within the leaf), a list of faces (for rendering), and a list of brushes (for collision detection). There are a total of length / sizeof(leaf) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
leaf
int cluster Visdata cluster index. int area Areaportal area. int[3] mins Integer bounding box min coord. int[3] maxs Integer bounding box max coord. int leafface First leafface for leaf. int n_leaffaces Number of leaffaces for leaf. int leafbrush First leafbrush for leaf. int n_leafbrushes Number of leafbrushes for leaf.
If cluster is negative, the leaf is outside the map or otherwise invalid.
int face Face index.
Leafbrushes
The leafbrushes lump stores lists of brush indices, with one list per leaf. There are a total of length / sizeof(leafbrush) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
leafbrush
int brush Brush index.
Models
The models lump describes rigid groups of world geometry. The first model correponds to the base portion of the map while the remaining models correspond to movable portions of the map, such as the map's doors, platforms, and buttons. Each model has a list of faces and list of brushes; these are especially important for the movable parts of the map, which (unlike the base portion of the map) do not have BSP trees associated with them. There are a total of length / sizeof(models) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
model
float[3] mins Bounding box min coord. float[3] maxs Bounding box max coord. int face First face for model. int n_faces Number of faces for model. int brush First brush for model. int n_brushes Number of brushes for model.
Brushes
The brushes lump stores a set of brushes, which are in turn used for collision detection. Each brush describes a convex volume as defined by its surrounding surfaces. There are a total of length / sizeof(brushes) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
brush
int brushside First brushside for brush. int n_brushsides Number of brushsides for brush. int texture Texture index.
Brushsides
The brushsides lump stores descriptions of brush bounding surfaces. There are a total of length / sizeof(brushsides) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
brushside
int plane Plane index. int texture Texture index.
Vertexes
The vertexes lump stores lists of vertices used to describe faces. There are a total of length / sizeof(vertex) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
vertex
float[3] position Vertex position. float[2][2] texcoord Vertex texture coordinates. 0=surface, 1=lightmap. float[3] normal Vertex normal. ubyte[4] color Vertex color. RGBA.
Meshverts
The meshverts lump stores lists of vertex offsets, used to describe generalized triangle meshes. There are a total of length / sizeof(meshvert) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
meshvert
int offset Vertex index offset, relative to first vertex of corresponding face.
Effects
The effects lump stores references to volumetric shaders (typically fog) which affect the rendering of a particular group of faces. There are a total of length / sizeof(effect) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
effect
string[64] name Effect shader. int brush Brush that generated this effect. int unknown Always 5, except in q3dm8, which has one effect with 1.
Faces
The faces lump stores information used to render the surfaces of the map. There are a total of length / sizeof(faces) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
face
int texture Texture index. int effect Index into lump 12 (Effects), or 1. int type Face type. 1=polygon, 2=patch, 3=mesh, 4=billboard int vertex Index of first vertex. int n_vertexes Number of vertices. int meshvert Index of first meshvert. int n_meshverts Number of meshverts. int lm_index Lightmap index. int[2] lm_start Corner of this face's lightmap image in lightmap. int[2] lm_size Size of this face's lightmap image in lightmap. float[3] lm_origin World space origin of lightmap. float[2][3] lm_vecs World space lightmap s and t unit vectors. float[3] normal Surface normal. int[2] size Patch dimensions.
There are four types of faces: polygons, patches, meshes, and billboards.
Several components have different meanings depending on the face type.
For type 1 faces (polygons), vertex and n_vertexes describe a set of vertices that form a polygon. The set always contains a loop of vertices, and sometimes also includes an additional vertex near the center of the polygon. For these faces, meshvert and n_meshverts describe a valid polygon triangulation. Every three meshverts describe a triangle. Each meshvert is an offset from the first vertex of the face, given by vertex.
For type 2 faces (patches), vertex and n_vertexes describe a 2D rectangular grid of control vertices with dimensions given by size. Within this rectangular grid, regions of 3×3 vertices represent biquadratic Bezier patches. Adjacent patches share a line of three vertices. There are a total of (size[0]  1) / 2 by (size[1]  1) / 2 patches. Patches in the grid start at (i, j) given by:
i = 2n, n in [ 0 .. (size[0]  1) / 2 ), and
j = 2m, m in [ 0 .. (size[1]  1) / 2 ).
For type 3 faces (meshes), meshvert and n_meshverts are used to describe the independent triangles that form the mesh. As with type 1 faces, every three meshverts describe a triangle, and each meshvert is an offset from the first vertex of the face, given by vertex.
For type 4 faces (billboards), vertex describes the single vertex that determines the location of the billboard. Billboards are used for effects such as flares. Exactly how each billboard vertex is to be interpreted has not been investigated.
The lm_ variables are primarily used to deal with lightmap data. A face that has a lightmap has a nonnegative lm_index. For such a face, lm_index is the index of the image in the lightmaps lump that contains the lighting data for the face. The data in the lightmap image can be located using the rectangle specified by lm_start and lm_size.
For type 1 faces (polygons) only, lm_origin and lm_vecs can be used to compute the worldspace positions corresponding to lightmap samples. These positions can in turn be used to compute dynamic lighting across the face.
None of the lm_ variables are used to compute texture coordinates for indexing into lightmaps. In fact, lightmap coordinates need not be computed. Instead, lightmap coordinates are simply stored with the vertices used to describe each face.
ubyte[128][128][3] map Lightmap color data. RGB.
Lightvols
The lightvols lump stores a uniform grid of lighting information used to illuminate nonmap objects. There are a total of length / sizeof(lightvol) records in the lump, where length is the size of the lump itself, as specified in the lump directory.
Lightvols make up a 3D grid whose dimensions are:
nx = floor(models[0].maxs[0] / 64)  ceil(models[0].mins[0] / 64) + 1
ny = floor(models[0].maxs[1] / 64)  ceil(models[0].mins[1] / 64) + 1
nz = floor(models[0].maxs[2] / 128)  ceil(models[0].mins[2] / 128) + 1
lightvol
ubyte[3] ambient Ambient color component. RGB. ubyte[3] directional Directional color component. RGB. ubyte[2] dir Direction to light. 0=phi, 1=theta.
Visdata
The visdata lump stores bit vectors that provide clustertocluster visibility information. There is exactly one visdata record, with a length equal to that specified in the lump directory.
visdata
int n_vecs Number of vectors. int sz_vecs Size of each vector, in bytes. ubyte[n_vecs * sz_vecs] vecs Visibility data. One bit per cluster per vector.
Cluster x is visible from cluster y if the (1 << y % 8) bit of vecs[x * sz_vecs + y / 8] is set.
Note that clusters are associated with leaves.

At some point I put together a page that describes triangle meshes and other q3 leaf elements. I forget the exact reason I created that page, but you might find it interesting.
Feel free to ask for clarification, but please accept my apologies if I can't find the time to answer.
Keywords: quake 3 quake3 q3 arena quake3arena q3arena map bsp file spec specs format