Actually, the process is very specific to the object. For example, in Stick Fighter you have to have an object/character comprised of, well, sticks. The tweens are generated by some sort of skeletal animation-like system which tracks how the various sticks would have to rotate to get from the first keyframe to the last. If you were using a different sort of object the process would be entirely different. For example, to generate tweens between two different arbitrary images you would need to do some sort of morphing.
That by the way gets at how, in a general sense, to accomplish something like the Stick Fighter editor. You would create a simple skeletal animation system in which you track various joints, how far apart they are (ie. how long limbs are,) and how they are connected. Then you use that information to determine how the joints are rotating between the keyframes. Finally, you just draw lines connecting the joints to generate the actual images.
For something more complicated that stick figures you may need to actually do this in 3D (depending on the specific animation.) Note I'm not saying you need Blitz3D, although that would make it a lot easier. There are after all examples of doing 3D in 2D, and there's the OpenGL lib. At any rate, you create the animation in 3D but then render out images to assemble the animation strip.
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Oh, and that last post finally cleared up for me what you are trying to accomplish.
That by the way gets at how, in a general sense, to accomplish something like the Stick Fighter editor. You would create a simple skeletal animation system in which you track various joints, how far apart they are (ie. how long limbs are,) and how they are connected. Then you use that information to determine how the joints are rotating between the keyframes. Finally, you just draw lines connecting the joints to generate the actual images.
For something more complicated that stick figures you may need to actually do this in 3D (depending on the specific animation.) Note I'm not saying you need Blitz3D, although that would make it a lot easier. There are after all examples of doing 3D in 2D, and there's the OpenGL lib. At any rate, you create the animation in 3D but then render out images to assemble the animation strip.
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Oh, and that last post finally cleared up for me what you are trying to accomplish.