BTG file format: Difference between revisions

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(Initial version)
 
(Initial version)
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=== Triangle Fans
=== Triangle Fans ===


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Revision as of 17:43, 30 September 2008

Terrain in FlightGear is currently stored using the Binary Terrain Format, in *.btg or *.btg.gz files. These files contain geometry in the form of points, individual triangles, triangle strips and triangle fans.

The authoritative source for the format definition is the class SGBinObject, which is defined in simgear/io/sg_binobj.{hxx,cxx}.

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 are geocentric/cartesian coordinates, with

  • the center of the earth being the origin,
  • the z-axis aligned with the earth axis pointing north,
  • the x-axis going through the Greenwich Meridian at its intersection with the equator, and
  • the y-axis going through the intersection of the meridian at 90 degrees east and the equator.

In order to allow more compact storage, all coordinates are stored as single precision floating point values relative to the center of a bounding sphere, except for the bounding sphere itself, which is stored in double precision format.

All measurements are in meters.

Endianness

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

Header

Byte offset Type Description
0 unsigned short Version (currently 7)
2 unsigned short Magic Number 0x5347 ("SG")
4 unsigned int Creation Time (seconds since the epoch)
8 unsigned short Number of Toplevel Objects

In files before version 7, the number of toplevel objects was interpreted as a signed short, leading to a maximum number of 32767 objects in a single file.

Objects

Each object starts by an object header.

Byte offset Type Description
0 char Object Type (see below)
1 unsigned short Number of Object Properties
3 unsigned short Number of Object Element

The object type can be one of the following:

Code Object Type
0 Bounding Sphere
1 Vertex List
2 Normal List
3 Texture Coordinates List
4 Color List
9 Points
10 Individual Triangles
11 Triangle Strips
12 Triangle Fans

The object header is followed by a list of properties. The number of properties in this list is given by the Number of Object Properties field.

Byte offset Type Description
0 char Property Type (see below)
1 unsigned int Number of Bytes in Property Data
5 bytes Property Data

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

Code Property Type
0 Material
1 Index Types

Bounding Sphere Object

The Bounding Sphere object gives the bounding sphere of the tile, defined by a center and a radius.

The elements of the Bounding Sphere object are bounding spheres. 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 Type Description
0 double X-part of the center
8 double y-part of the center
16 double z-part of the center
24 float radius of the bounding sphere

Vertex List

TODO

Normal List

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Texture Coordinates List

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Color List

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Points

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Individual Triangles

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Triangle Strips

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Triangle Fans

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