orientation in openGl

Question

Hi

Could someone explain to me what are the up front and right vectors of an object and how are they used ?

Answer 1

Are you referring to how vectors in Object or Model space are used? Each object or model has its own coordinate space. This is necessary since the points in the model will be relative to the models origin. This makes it possible to work with arbitrary models in larger worlds. You would perform certain operations on the model (like Rotation) before moving the model in the World (translation). If I understand your question correctly, you are referring to a set of vectors that define the models position in the world. These up, front and right vectors would be what you would use to possibly determine which way the model was facing or moving.

I hope this helps if anything to formulate your question a bit more.

This Gamedev question might be of help glMultMatrix, how does it work?

Answer 2

Those vectors usually refer to world-space transformations of the local body axes of the model in question.

Usually a model is defined with respect to some local coordinate system whose origin is at the center of mass, centroid, or some other convenient location from which to construct the object's geometry. This local coordinate system has its own x, y, and z axes with x = [1, 0, 0]', y = [0, 1, 0]', and z = [0, 0, 1]'. The coordinates of each vertex in the model are then defined with respect to this local frame. Usually the origin is chosen so that the "forward" direction of the model is aligned with this local x, the "left" direction is aligned with local y, and "up" is aligned with local z (though any right-handed system will do.

The model is placed into the world via the modelview matrix in OpenGL. When the model's vertices are sent to the GPU, they are transformed from their local space (aka "object" space or "model space" or "body space") to world space by multiplying them by the modelview matrix. Ignoring scaling, the upper left 3x3 block in the modelview matrix is an orthonormal rotation matrix that defines the projection of the body axes into the world frame, assuming the model is placed at the world origin. The modelview matrix is augmented into a 4x4 by adding the translation between the model and world origins in the upper right 3x1 block of the modelview matrix.