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Enabling/Disabling the Speed Vector Pass in the Renderlayer causes different IK Bone Motions
Closed, InvalidPublic

Description

System Information
Tested with Ubuntu 14.04 and a GeForce GTX 1080 Ti as well as Windows 10 and a GeForce GTX Titan X

Blender Version
Broken: 2.79b (official release), 2.78c (official release)

Short description of error
Activating / Deactivating of the "Speed Vector Pass" in the renderlayer causes a different motion of a IK Bone.
If the "Speed Vector Pass" is disabled, the rendered results are consistent to the animation shown in the 3D view.
The only difference between the attached images is checking/unchecking the "speed vector pass".


Exact steps for others to reproduce the error
Open the blend file


Render Animation with "Deliver speed vector pass" of the RenderLayer enabled / disabled
Compare images

Event Timeline

Philipp Oeser (lichtwerk) changed the task status from Unknown Status to Invalid.Oct 2 2018, 3:30 PM
Philipp Oeser (lichtwerk) claimed this task.

Hi there!

It seems like you are specifying an IK target bone ("TailTipBone") which itself is a child of the bone with the IK constraint ("TailBone").
And while I am not an expert in rigging, I think this is asking for trouble. This is something like a dependency cycle (TailTipBone affects TailBone via IK constraint, but TailBone also affects TailTipBone through parenting...)

You can notice this even without the Speed Vector pass enabled if you scrub the timeline or go back and forth a frame (results will be different each time).

Now if you ask for a motion pass MOTION_PASS in cycles it will also evaluate motion multiple times and thus the 'faulty' setup in the rig will show...

If you are looking to somewhat delay/smooth the back of the car in your example file, you are better of using other techniques [e.g have another bone follow the path, but with a different offset].

Daring to close this (feel free to reopen if there was a misunderstanding and you still think you've found a bug)

Hi Phillip,

thanks for checking the file.
While I see your point, I'm wondering if "you should not do it this way" is the correct way to handle cases like this.
The fact, that a render pass can influence the motion of a rig is highly non-intuitive.
At least some kind of user notification would be nice.
Maybe the render layer should print an appropriate message if vector pass is enabled and a cycle is detected?

Cheers