How to reproduce BUG UE-51931

Development Programming & Scripting
1
  1. Create a new project (blank is fine)
  2. Create an animation blueprint and open it (TutorialTPP skeleton is fine)
  3. Add a FABRIK node
  4. Set FABRIK transform space to bone space
  5. Set FABRIK effector to head
  6. Set FABRIK tip to hand_r
  7. Set FABRIK root to upperarm_r
  8. Promote the Alpha pin to a variable
  9. Plug the FABRIK node into the final pose
  10. Save and compile
  11. Change the value of the Alpha variable from 1.0 to 0.0
  12. Save and compile
  13. Select the FABRIK node and BOOM!

I’m not sure you have to do all that exactly like I did but that’s what I did to reproduced the error consistently.

This may not be easily reproducible but this is a show-stopping bug.

Hope that helps,

2

Looks like it has to do with that alpha value being zero. It seems I can work around the issue by setting it to 0.0001. I can see in the code where it’s asserting and it doesn’t make much sense. The alpha has nothing to do with the IsValidIndex() code.

3

I got the same error another way now too. I duplicated a FABRIK node that worked fine and pinned them both into a blend by bool (input coming from cached pose). Alphas were both at 1.0 this time.

4

The Bug tracker says it was fixed in 4.18.1.

5

Hi,

I can confirm that this issue was fixed in 4.18.1, but if you have any other questions regarding the issue feel free to reopen this thread and I will get back to you.

-Thank you

6

Nope. I just finished building with the latest and the problem is gone. I’m happy. Seems you caught it before my post here.