Hi werzaque, thanks for the detailed explanation. The setup you described is actually correct. It should read the cel in Frame 1, read the cel in Frame 12 (offset 45px), and generate proper in-between frames that smoothly interpolate the movement.
Since I’m currently away from my computer, I can’t test this exact case right now, but based on recent reports from other users, this behavior may be related to Aseprite version changes.
Tweencel was developed and tested on Aseprite v1.3.14-beta1, and some changes introduced in v1.3.15 may affect how cel positions are read by scripts. If you’re on 1.3.15 or higher, this could explain why the interpolation is incorrect.
When I get back to my machine, I’ll test your exact scenario and update Tweencel if needed.
In the meantime, could you let me know which version of Aseprite you’re using? That will help me confirm whether it’s a version-related issue.
Thanks again for reporting this, your feedback is a huge help.