Coordinate Systems

Between OpenGL, OSG, and osgEarth, there are several different coordinate systems and reference frames in use and it can get confusing sometimes which is which. Here we will cover some of the basics.

OpenSceneGraph/OpenGL Coordinate Spaces

Here is a brief explaination of the various coordinate systems used in OpenGL and OSG. For a more detailed explaination (with pictures!) we direct you to read this excellent tutorial on the subject:

Model Coordinates

Model (or Object) space refers to the actual coordinates in the geometry (like terrain tiles, an airplane model, etc). In OSG, model coordinates might be absolute or they might be transformed with an OSG Transform.

We will often refer to two types of Model coordinates: world and local.

World coordinates are expressed in absolute terms; they are not transformed. Local coordinates have been transformed to make them relative to some reference point (in world coordinates).

Why use local coordinates? Because OpenGL hardware can only handle 32-bit values for vertex locations. But in a system like osgEarth, we need to represent locations with large values and we cannot do that without exceeding the limits of 32-bit precision. The solution is to use local coordinates. OSG uses a double-precision MatrixTransform to create a local origin (0,0,0), and then we can express our data relative to that.

View Coordinates

View space (sometimes called camera or eye space) express the position of geometry relative to the camera itself. The camera is at the origin (0,0,0) and the coordinate axes are:

+X : Right
+Y : Up
-Z : Forward (direction the camera is looking)

In osgEarth, View space is used quite a bit in vertex shaders – they operate on the GPU which is limited to 32-bit precision, and View space has a local origin at the camera.

Clip Coordinates

Clip coordinate are what you get after applying the view volume (also know as the camera frustum). The frustum defines the limits of what you can see from the eyepoint. The resulting coordinates are in this system:

+X : Right
+Y : Up
+Z : Forward

Clip spaces uses 4-dimensional homogeneous coordinates. The range of values in clip space encompasses the camera frustum and is expressed thusly:

X : [-w..w] (-w = left,   +w = right)
Y : [-w..w] (-w = bottom, +w = top)
Z : [-w..w] (-w = near,   +w = far)
W : perspective divisor

Note that the Z value, which represents depth, is non-linear. There is much more precision closer to the near plane.

Clip space is useful in a shader when you need to sample or manipulator depth information in the scene.