In opentoonz you can select two drawings (with drawings inbetween) and have the “auto inbewteen” feature automatically create the inbetween drawings for you…
this is very clean and straightforward for simple shapes/ transitions…
you can see in the attachment a simple example
you can see a little more detailed view of this tool starting at 2:58 in this video:
for more complex drawings there is a “vector guided drawing” feature that when combined with onion skin will show you the order and direction of first drawings strokes on the second drawing…follow this guide and the inbetweens are created very smoothly.
I know im probably missing something as i can confidently say i know very little about Toonboom…Is this one of those things hiding in plain sight?
