There are two main ways to keep track of rigid body motions. Most current texts on robotics use the passive approach, where the rigid body has a coordinate frame embedded in it, and then its position and orientation is given by the coordinate transformation from the world frame to a frame moving with the body. When there are several bodies and when bodies also carry several different frames, it can be hard to account for all the different frames. In the lesser-known active view, there is a single fixed coordinate frame. The position and orientation of a rigid body is specified by the transformation, which moves the body from its home position to its current position. This active view is completely equivalent to the traditional passive view, so readers familiar with the traditional approach need not change their habits. However, newcomers to the subject may find it simpler to learn robotics using the active view since there is a greater emphasis on the bodies and their motion and, consequently, less emphasis on coordinate frames and changes of coordinates.
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