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keyframe interpolation "Elastic" always starts down on the Y-Axis
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Description

System Information
Operating system: Linux-5.4.0-65-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.102.04

Blender Version
Broken: version: 2.92.0 Beta, branch: master, commit date: 2021-01-31 13:31, hash: rBa9b53daf23ba
Worked: (newest version of Blender that worked as expected)

Short description of error
when having two keyframes and setting the interpolation of the first one to elastic the curve always goes down on the Y- Axis first.
This leads to having to add another keyframe like this:


Maybe I am missing something and this is not a bug, but setting the Amplitude of the elastic interpolation to a negative value is where I would flip that behaviour, but the amplitude is limited to positive values only. Allowing negative values for the amplitude would fix this.

Exact steps for others to reproduce the error
Add 2 keyframes to cube, set interpolation of the first keframe to elastic, try to flip the elastic curve to start upwards bound on the Y-Axis...

Event Timeline

tt (ttt) created this task.Feb 6 2021, 6:55 PM

I believe what you want to adjust is the Period. If you just flip the sign it will flip the direction.


Falk David (filedescriptor) changed the task status from Needs Triage to Needs Information from User.Feb 9 2021, 12:20 PM

indeed this solves this issue. Somehow I figured it has to be the amplitude that needs to be flipped, scaling down the period seems counter intuitive, as in my mind it would flip the curve along the x axis and the elastic interpolation always starts at the keyframe and goes on to the right and there is nothing to the left of it where that flipped period would appear in a maths graph when manipulating the x expansion, but in a keyframe editor it kind of makes sense...
scaling the amplitude seemed more intuitive as it was manipulating the y axis already... can't we have negative values on both amplitude and period?

I am not sure about the negative amplitude. Usually, it's an absolute value. "The amplitude is the amount of the maximum deflection in the 𝑦 direction". This would have to be discussed with the UI team.

Closing this for now since it's not a bug.