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Tweening is a term used in several software packages which have animation features. It refers to the automatic interpolation of frames based on the content of two keyframes. Programs that offer animation tend to use the same technique as is used in motion pictures: namely the creation of multiple frames each containing different content. When the images are played back rapidly, they appear to contain a moving sequence rather than simply a succession of images.

Creating the content of each frame manually is very time-consuming but can sometimes be the only way of building some animation sequences. For most types of animation, tweening can be used to speed up the process of creating animation. Instead of creating the content of all frames manually, only the content of important frames is created. These frames are referred to as keyframes and the frames in-between them can be generated automatically by most animation software.

In Adobe Flash CS4, there are two types of tweening: motion tweening and shape tween. Shape tweening is a rudimentary two-dimensional morphing facility and will be discussed in another article. Motion tweening is used for most types of animation in Flash. This includes not only movement but fades, zooms, rotation, colour changes, panning and swivelling.

In order to create a motion tween, the artwork to be animated has to be converted into a symbol. This is a mechanism used by Flash to encapsulate artwork which can then be treated as a single unit rather the components which make up the symbol. One result of this encapsulation is that multiple instances of the same symbol can then be used within the same animation and each symbol can be given different attributes. Symbol instances can of course be placed in the same frame but, more usefully, they can be placed in different keyframes. Flash will use tweening to generate the content of the frames between the keyframes. The basic process of creating an animation using motion tweening is as follows.

1. Create a keyframe where the animation needs to start.

2. In this keyframe, create the artwork that you wish to animate.

3. Convert the artwork into a symbol.

4. The artwork will automatically be replaced by an instance of the symbol you have just created.

5. Insert a keyframe where you want the animation to end.

6. A second instance of the symbol will be automatically inserted into this keyframe.

7. Assign a motion tween to the first of the keyframes.

8. Assign the required attributes to the symbol instance in your first keyframe.

9. Assign the required attributes to the symbol instance in your second keyframe.

The attributes that you assign to the symbol instance in your first keyframe will gradually change to become those that you assigned to the symbol instance in your second keyframe. If you want the artwork to move, then the position of the symbol instance would be different. For a zoom, their sizes would be different. For a fade, their opacity (Alpha is the term used in Flash) would be different, and so forth.

The author is a trainer and developer with Macresource Computer Solutions, an independent computer training company offering Adobe Flash CS4 Classes in London and throughout the UK.

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