BTG file format: Difference between revisions

These files contain geometry in the form of points, individual triangles, triangle strips and triangle fans.

== General Structure ==

Each BTG-File contains a header followed by a variable number of objects.

Each object is introduced by a header indicating the type of object, the number of associated properties and the number of elements contained in the object.

=== Coordinates ===

All coordinates in the file (except for texture coordinates) are [[Geographic Coordinate Systems#Cartesian Geocentric Coordinates|cartesian geocentric coordinates]].

All measurements are in meters.

Texture coordinates are used to map textures onto 3D faces.

For this each corner vertex of a face is associated with the location of the texture pixel to be shown at this corner.

=== Endianness ===

The files are stored in Little Endian, i.e. the least-significant byte is stored first.

=== Alignment ===

Some of the structures used in this file format are not properly aligned, i.e. not all fields start at an offset which is a multiple of their size.

Therefore special care is to be taken when translating these definitions into record types, e.g. C or C++ <code>struct</code>, as the compiler must be instructed not to enforce proper alignment.

== Header ==

|-

! Byte offset

Each object starts by an object header.

|-

! Byte offset

The object type can be one of the following:

|-

! Code

The number of properties in this list is given by the '''Number of Object Properties''' field.

|-

! Byte offset

The property type can be one of the following (see the object type descriptions for more details):

|-

! Code

Even though more than one bounding sphere may be stored in such an object, all except for the last one found in the file are ignored.

|-

! Byte offset

=== Vertex List ===

The Vertex List object contains a list of vertices to be referenced by other geometry primitives.

Each of the vertices gets a unique index, starting with 0 for the first vertex found in the file and incrementing by 1 for each further vertex.

|-

! Byte offset

Each vertex is a triple of single precision floating point values:

|-

! Byte offset

Each of the normals gets a unique index, starting with 0 for the first normal found in the file and incrementing by 1 for each further normal.

|-

! Byte offset

Each normal is a triple of signed bytes, interpreted as fixed point values:

|-

! Byte offset

Each of the coordinates gets a unique index, starting with 0 for the first coordinate found in the file and incrementing by 1 for each further coordinate.

|-

! Byte offset

Each texture coordinate is a pair of single precision floating point values:

|-

! Byte offset

Each of the colors gets a unique index, starting with 0 for the first color found in the file and incrementing by 1 for each further color.

|-

! Byte offset

Each color definition is a 4-tuple of single precision floating point values:

|-

! Byte offset

=== Geometry Objects ===

All geometry objects are structured the same way, with each element consisting of a list of index-tuples referencing vertex, normal, color and texture coordinate lists.

The meaning of the bits is given in the following table:

|-

! Bit Number

==== Points ====

In case of the Points object type, each of the elements represents a single point, e.g. a point lightsource.

==== Individual Triangles ====

In case of the Triangle object type, three succeeding elements are combined to form the vertices of a single triangle.

The vertices of triangle <code>i</code> (starting with 0 for the first triangle) are given by elements <code>3*i</code>, <code>3*i+1</code> and <code>3*i+2</code>.

==== Triangle Fans ====

Triangle Fans are similar to Triangle Strips.

However, in case of triangle fans, all triangles share the first element and the other two triangle vertices are provided in pairs of elements, representing a fan of triangles around a single vertex.