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Is lightweighted and very easy to implement. Its strength is that it is modular and consists of three parts:

• Animation Engine
• Sequencing tools
• BitmapFilter comfort

If you need just movieclip tweening, it is enough to use just the Animation Engine. Your project compiles faster and also has a smaller size.

Animation Engine - provides movieclip tweening. You can tween a movieclip property or properties with one single command.

Sequencing tools - make it simple to build also a complex sequence of movieclip tweening.

BitmapFilter comfort - provides a simplified interface for applying the bitmap filters.

Although there are also others animation packages, I think the Zigo engine is the most convenient one for an actionscript programmer. Its a pity that it does not have an AS3 version yet.