thank you for the very quick reply! And yes, you are right. The benefit for the viewers is that they see the drawing develop on an empty canvas. Conversely, if the onion skin reference would be visible to them up front it would make watching the drawing very boring.
I am so glad you are considering this. It would allow for a hybrid animation / whiteboard technique which would be simply fantastic to communicate complex concepts/ideas – even when all the elements must been seen at the same time on the same screen – all the while making it more creative, artful, and fun with the possibility to add real animation.
Researchers and scientists, who are more and more on the look-out for better tools to communicate visually, would be very intrigued by this, I believe.
To my knowledge, a similar feature only exists in the 2D “grease pen” functionality of Blender. Drawing programs, like Procreate or Tayasui (which is much like Animationpaper in its minimalistic elegance) either do not have a good play-back function, or they do not allow for the removal of the reference layer; so the effect can only be achieved in post with a chromakey / green screen video editing program.
None of this is nearly as user-friendly or fun as Animationpaper would make it. So I really keep my fingers crossed that you find a good way to make it work.