Glimmr Canvas Animation
version 1/111030 by Erik Temple
Chapter: Easing (Tweening)
The term “easing” refers to motions in which the acceleration changes over time. The term may seem strange, but think of it in terms of easing into a chair–your body moves more slowly as you approach the cushion. Easing equations can imitate this sort of gradual slowing as the motion reaches the endpoint, and they can also do much more, such as imitate the bouncing of a ball, or overshoot the endpoint and snap back.
Easing is closely related to the concept of "tweening". Briefly, tweening is short for “in-betweening”. In traditional animation, animations were organized around key frames–-the critical points in any sequence. These keyframes were drawn first, and then the “betweens” were filled in to connect those key moments smoothly; because they were less critical, they were often filled in by the lower ranks of the animation team. In computer animation, keyframes are defined by the user, and the tweening is done automatically by the software.
Glimmr Canvas Animation (GCA) is no different, though it doesn’t really use the concept of the keyframe. Where programs like Flash have a master timeline punctuated by user-defined keyframes, each GCA animation track can be thought of as a self-contained timeline separating two keyframes–the starting and ending points of that particular motion or effect. GCA builds sophisticated and customizable tweening into nearly all of the preset animation types via the use of “easing” equations that interpolate movement between the starting and ending points.
The most common use of easing is in moving objects across the screen, but easing equations can be used on any continuous field of values, not just coordinates in space. For example, when used on a zooming track: instead of proceeding from 50% of actual size to 100% by equal intervals, the zoom might be “eased” using a cubic ease-out equation so that the size change starts out rapid, increases slightly in speed for the first 1/2 or so of the duration, and then slows comparatively quickly. (For just such an effect, see the Maps of Murder example included with this extension.)
The best way to start exploring the effects that easing can have on your animations is probably to try out the "Eased Movements" example included with this extension. You might also want to check out these online resources: