Frame of Reference

Every mathematical vector and transformation is expressed in a certain frame of reference. This is for example handled by the BABYLON.Space.LOCAL and BABYLON.Space.WORLD constants used in the rotate() and translate() functions cited above, or locallyTranslate() and other similar functions.

A frame of reference is in fact a regular transformation matrix, i.e. an association of rotation, translation and scaling operations.

You may need to express a certain vector in a precise frame of reference, be it the one of a mesh or an arbitrary one. In this case, the function BABYLON.Vector3.TransformCoordinates() will come in handy.

Here is how you would compute the up vector (Y+) inside the frame of reference of a mesh:

mesh.computeWorldMatrix();
var matrix = mesh.getWorldMatrix();
var up_local = new BABYLON.Vector3(0,1,0);
var up_global = BABYLON.Vector3.TransformCoordinates(up_local, matrix);

Potential uses of BABYLON.Vector3.TransformCoordinates() may be:

• computing direction and axis vectors for applyImpulse() and other physics-related functions
• setting the position and speed of a mesh relative to another, without the use of parenting (e.g. a spaceship shooting missiles)
• applying a projection matrix to a world position vector to end up with a screen-space position vector