Thank you, José!
I think the part I am most interested in is your Step 1. Can you please expand it, and explain how to create frames and use onionskin and any of the other techniques you skipped?
I am currently looking into Synfig, and will take a look at your links when I have more time.
It seems to me that creating one frame at a time is very laborious. This is just the kind of labor that computers can handle!
I’m a beginner in animation. I’ve tried several tools and techniques, but none are the kind of tool I can easily imagine to generate prototype/pilot animations using the power of the computer. Here is how it would work, as I see it:
Step 1: you would start a scene by sketching (freehand or with drawing tools, bitmap or vector, I don’t care, but vector objects would be easiest to interpolate by computer) some objects that will be in the scene.
Step 2: you grab some lines with the mouse and start pulling to move and distort one or more of the objects, or making them visible or hidden, depending on shift keys. Frames get generated as you go, and you can click a key to see them loop up to speed and click other keys to speed up or slow down the translation or distortion. You can even grab the lines that make up text characters, move them around, twist them, etc.
Step 3: now that you have a working sketch for the scene, you go back and add effects that require more user input (this could be programming, like with GreenSock, or preferably something else).
Step 4: on to the next scene, until done.
There should also be an option to have a really low frame rate, and automatically fade-transition from one frame to the next. This is mechanical, so it can be also done by the computer.
With a system like this, I could quickly sketch out an animation and even use it to plan out timings and layout for a more detailed animation using Unity, Mo.js, Web GL, Adobe After Effects, or who knows what real modern animation tool.