Bone roll calculation flips local axes at wrong rotation angle #38407

New Issue

Gaia Clary · 2014-01-30T09:46:57+01:00

Gaia Clary commented

2014-01-30 09:46:57 +01:00

System Information
windows

Blender Version
Broken: hash b'6c32930'
Worked: (optional)

Short description of error

The problem is that the axes flipping only happens when the ongoing rotation delta
passes 90 degrees, hence when you rotate 3 times by 60 degree you get
a different result compared to rotating by 180 degrees in one step.

Exact steps for others to reproduce the error

See axis_flip3.blend (changed to blend file with correct text)
Initially: Bone is set to 0.0,local Z points upwards (aligned with global Z)

start rotating around y (use "r" "y")
pass very slowly over the range around 90 degree
you will see the local x axis flips 3 times during the transition (i believe it should only flip once)
continue rotation until you are close to 180 degree

Now bone roll is ~ 0 degree, the local Z axis points upward,
the local X axis points backwards to positive Y.
Actually this result looks good to me.

revert the rotation using CTRL +Z
rotate around y by 60 degree (use "r" "y" 60)
rotate around y by 60 degree (use "r" "y" 60)
rotate around y by 60 degree (use "r" "y" 60)

Now bone roll is ~ 180 degree,
local X axis has not flipped
I believe this result is wrong!

**System Information** windows **Blender Version** Broken: hash b'6c32930' Worked: (optional) **Short description of error** The problem is that the axes flipping only happens when the ongoing **rotation delta** passes 90 degrees, hence when you rotate 3 times by 60 degree you get a different result compared to rotating by 180 degrees in one step. **Exact steps for others to reproduce the error** See [axis_flip3.blend](https://archive.blender.org/developer/F75658/axis_flip3.blend) (changed to blend file with correct text) Initially: Bone is set to 0.0,local Z points upwards (aligned with global Z) - start rotating around y (use "r" "y") - pass very slowly over the range around 90 degree - you will see **the local x axis flips 3 times during the transition** (i believe it should only flip once) - continue rotation until you are close to 180 degree Now bone roll is ~ 0 degree, the local Z axis points upward, the local X axis points backwards to positive Y. Actually this result looks good to me. - revert the rotation using CTRL +Z - rotate around y by 60 degree (use "r" "y" 60) - rotate around y by 60 degree (use "r" "y" 60) - rotate around y by 60 degree (use "r" "y" 60) Now bone roll is ~ 180 degree, local X axis has not flipped I believe this result is wrong!

Gaia Clary commented

2014-01-30 09:46:57 +01:00

Changed status to: 'Open'

Gaia Clary commented

2014-01-30 09:46:57 +01:00

Added subscriber: @GaiaClary

Bastien Montagne commented

2014-01-30 11:59:41 +01:00

Added subscribers: @brecht, @mont29

Bastien Montagne commented

2014-01-30 11:59:41 +01:00

@GaiaClary: Didn’t @brecht just addressed this issue in its 6c329304c1 commit?

@GaiaClary: Didn’t @brecht just addressed this issue in its 6c329304c1 commit?

Gaia Clary commented

2014-01-30 12:01:51 +01:00

No, i detected this after 6c329304c1 was applied. so either it is a follow up issue, or i didn't see it before. But its an issue i believe ...

No, i detected this after 6c329304c1 was applied. so either it is a follow up issue, or i didn't see it before. But its an issue i believe ...

Brecht Van Lommel commented

2014-01-31 09:36:10 +01:00

The problem is around 90° rotations, the roll is basically undefined then and the result is influenced by float precision errors. It should handle that better than jumping around randomly.

Bastien Montagne was assigned by Sergey Sharybin

2014-02-17 09:56:14 +01:00

Sergey Sharybin commented

2014-02-17 09:56:14 +01:00

Added subscriber: @Sergey