Skitters and jacks: interactive 3D positioning tools

Let <italic>scene composition</italic> be the precise placement of shapes relative to each other, using affine transformations. By this definition, the steps of scene composition are the selection of objects to be moved, the choice of transformation, and the specification of the parameters of the transformation. These parameters can be divided into two classes: <italic>anchors</italic> (such as an axis of rotation) and <italic>end conditions</italic> (such as a number of degrees to rotate). I discuss the advantages of using Cartesian coordinate frames to describe both kinds of parameters. Coordinate frames used in this way are called <italic>jacks</italic>. I also describe an interactive technique for placing jacks, using a three-dimensional cursor, called a <italic>skitter</italic>.