This quick tutorial shows how to create automatic animation in OpenToonz using stroke-by-stroke method.
In short: make sure that you have a sequence of vector frames (or, create an empty one). Then switch to Brush Tool. On Tool Options Panel you will see a “Range” dropdown – change it to anything other than “Off” (obvious hint: it will define interpolation that will be used for tweening).
Important note: This feature requires OpenToonz 1.2.0 or higher.