Animation is based on 'keyframing'. This is the idea of creating 'key frames' of certain points of motion and letting software algorithms do the frames inbetween. These algorithms are termed 'interpolations'. Linear means just that - the transition from one keyframe to the next is linear. If you start an object at (0,0,0) and move it to (10, 20, 30) using linear interpolation the motion from the start to the new position will be a straight line (linear). Spline means that a type of curve is used for the transition to introduce a more 'natural' motion. In other words, objects usually don't go from 0 to 60 mph instantaneously and from 60 mph to 0 similarly (linearly). They accelerate up to full speed and decelerate to full stop because of inertia (etc., won't get too technical here). For natural motions, splines are better because the curve mimics these affects of motion on real objects. The problem with spline interpolation is that it can be nonintuitive. When working with multiple keyframes, the spline curve is usually based upon several of them which can lead to unexpected results. For instance, although you have a maximum point (say, height) of motion as a keyframe, the combination of previous and following keyframes may cause the curve of the spline to go to a higher maximum (way beyond the maximum height intended). Again, for natural, organic motions spline interpolation is better than linear, but be sure to check the keyframe graphs and adjust the keyframes to get the motion desired.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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