Potential inconsistency in gravity and external force integration

[NeffSoft]

Hello , and thank you so much for making this plugin! It it very easy to use, and creates beautiful results, and you are very generous to release it to us!

I am very new to using the plugin, and work with our lead animator to set it up. It is likely there is no problem and we are learning how to use it, so advice is welcome!

We noticed something odd with external forces when the time dilation scaled is changed. Some forces appeared to change in strength based on a slow time scale (1/10) vs normal time scale, and the resulting shape of the simulated bones would change. E.g. in slow motion, the gravity seemed weaker.

I examined the code and found some different integration schemes, and I couldn't identify why they are different:

Simulate:
Bone.Location += 0.5 * GravityInSimSpace * DeltaTime * DeltaTime;

ExternalForce_Basic:
Bone.Location += BoneForce * ForceRate * Node.DeltaTime;
ExternalForce_Gravity:
Bone.Location += 0.5f * Force * ForceRate * Node.DeltaTime * Node.DeltaTime;
ExternalForce_Curve:
Bone.Location += BoneForce * ForceRate * Node.DeltaTime;
ExternalForce_Wind:
Bone.Location += WindDirection * ForceRate * RandomizedForceScale * Node.DeltaTime;

It seems to me that when updating the position with an acceleration factor, the scaling time factor should not be squared?

We set the gravity external force to 0, and used ExternalForce_Basic as our gravity, and it fixed the issue we were having! So for now we can get by, but I would like to know what the distinction is, or perhaps at least report a potential issue.

Thank you!

[pafuhana1213]

Hi @NeffSoft,
thank you for your detailed report! Since you're right, I've reviewed the Gravity implementation.
#185

please try new gravity method of v.1.20. thanks!