Smooth key framing using the image plane

Key framing is an integral part of the animation making process. The user places the camera in a sequence of "key" positions, and the computer produces a set of intermediate camera locations that interpolates between these key frames. Traditional methods treat the camera as just another 3D object in the scene, with intermediate frames produced by interpolating the position and focal point of the camera in space. Unlike a 3D object, however, the camera's role is to project the 3D scene into 2D. The user indirectly controls the projection -- how objects in the scene are placed in the 2D image -- by adjusting the camera parameters for each key frame. Automatic 3D camera interpolation adds yet another layer of indirection. The net effect is that the user must solve a complicated inverse problem in order to move objects across the 2D scene in the desired manner. Figure 1 shows an example where the user wanted the table to move down and across the scene while moving the view to the top of the table. Traditional interpolation (top row) rotates the table out of the view. Correcting this using traditional approaches requires the addition of a substantial number of key frames which in turn reduce the smoothness of the motion.