Baking animation of bones affected by a curve (spline IK) is wrong #79670

New Issue

Gabriel · 2020-08-09T17:44:56+02:00

Gabriel commented

2020-08-09 17:44:56 +02:00

System Information
Operating system: windows 10
Graphics card: gforce 1060 6gb

Blender Version
Broken 2.92 alpha
Broken 2.83
Worked: likely never

Short description of error
I have a row of bones, I add the curve modifier, I make an animation with that modifier on, I bake the animation of the bones to be able to send it to unreal or unity and the animation breaks completely

Exact steps for others to reproduce the error
CurveBreackExample.blend

Open file
inspect wrong bake of bone action
to reproduce:
go to object mode
select SkeletonBase object
go to pose mode (all bones should be selected)
Pose > Animation > Bake Action... with following settings:

**System Information** Operating system: windows 10 Graphics card: gforce 1060 6gb **Blender Version** Broken 2.92 alpha Broken 2.83 Worked: likely never **Short description of error** I have a row of bones, I add the curve modifier, I make an animation with that modifier on, I bake the animation of the bones to be able to send it to unreal or unity and the animation breaks completely **Exact steps for others to reproduce the error** [CurveBreackExample.blend](https://archive.blender.org/developer/F9179943/CurveBreackExample.blend) - Open file - inspect wrong bake of bone action - to reproduce: - go to object mode - select `SkeletonBase` object - go to pose mode (all bones should be selected) - Pose > Animation > Bake Action... with following settings: ![image.png](https://archive.blender.org/developer/F9180025/image.png)

Gabriel commented

2020-08-09 17:44:56 +02:00

Added subscriber: @GabrielAntonioDelacruz

Richard Antalik commented

2020-08-11 11:51:15 +02:00

Added subscriber: @iss

Richard Antalik commented

2020-08-11 11:51:15 +02:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Richard Antalik commented

2020-08-11 11:51:15 +02:00

Please upload simple .blend file that demonstrates this issue.

Gabriel commented

2020-08-11 16:25:21 +02:00

CurveBreackExample.rar

[CurveBreackExample.rar](https://archive.blender.org/developer/F8769781/CurveBreackExample.rar)

Campbell Barton commented

2020-08-20 09:58:25 +02:00

Changed status from 'Needs User Info' to: 'Needs Developer To Reproduce'

Philipp Oeser commented

2020-11-03 10:27:53 +01:00

Changed status from 'Needs Developer To Reproduce' to: 'Needs Triage'

Philipp Oeser changed title from ~~Baking animation of bones affected by a curve~~ to Baking animation of bones affected by a curve (spline IK) is wrong

2020-11-03 10:51:20 +01:00

Philipp Oeser commented

2020-11-03 10:51:20 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Philipp Oeser commented

2020-11-03 10:51:54 +01:00

Added subscriber: @lichtwerk

Sybren A. Stüvel self-assigned this 2020-11-05 15:45:58 +01:00

Sybren A. Stüvel commented

2020-11-05 15:50:31 +01:00

I think there are two issues at work here:

The Bake Action code doesn't do visual keying correctly (it seems to be using the wrong matrix math).
There is an issue with the Spline IK causing bones to become the wrong scaling. This is handled already in #81704 (Spline IK "Bone Original" Y-scale Causes Scaling).

For this task I'll focus on the first item, as the 2nd is already handled somewhere else.

I think there are two issues at work here: 1. The Bake Action code doesn't do visual keying correctly (it seems to be using the wrong matrix math). 2. There is an issue with the Spline IK causing bones to become the wrong scaling. This is handled already in #81704 (Spline IK "Bone Original" Y-scale Causes Scaling). For this task I'll focus on the first item, as the 2nd is already handled somewhere else.

Sybren A. Stüvel commented

2020-11-05 18:11:18 +01:00

Added subscriber: @Josephbburg

Sybren A. Stüvel commented

2020-11-05 18:11:18 +01:00

It looks like the visual keying issues I thought were there, aren't. This seems to be caused by the Spline IK constraint somehow. @Josephbburg do you have any insight here?

Here is a simplified file that I made. It has no animation, just a scale on the root bone and a Spline IK on the tip bone.

#79670-spline-scaled-nonanimated.blend

To reproduce:

Ensure all bones are selected; this gives you the "Before" image.
Press {key Ctrl A} and choose "Apply Visual Transform to Pose".
Disable the Spline IK constraint; this gives you the "After" image.

I've expanded my "Copy Visual Transform" add-on to aid in debugging this. It has a button "Bake Selected Bones" and a nice display of the bone matrices .matrix and .matrix_basis.

Even with D9244: Fix #81704 applied, there still seems to be a slight scaling error creeping in. And then at Bone.008 all of a sudden things explode.

It looks like the visual keying issues I thought were there, aren't. This seems to be caused by the Spline IK constraint somehow. @Josephbburg do you have any insight here? Here is a simplified file that I made. It has no animation, just a scale on the root bone and a Spline IK on the tip bone. [#79670-spline-scaled-nonanimated.blend](https://archive.blender.org/developer/F9210676/T79670-spline-scaled-nonanimated.blend) To reproduce: - Ensure all bones are selected; this gives you the "Before" image. - Press {key Ctrl A} and choose "Apply Visual Transform to Pose". - Disable the Spline IK constraint; this gives you the "After" image. ![image.png](https://archive.blender.org/developer/F9210698/image.png) I've expanded my "[Copy Visual Transform](https://raw.githubusercontent.com/sybrenstuvel/random-blender-addons/main/copy_visual_transform.py)" add-on to aid in debugging this. It has a button "Bake Selected Bones" and a nice display of the bone matrices `.matrix` and `.matrix_basis`. ![image.png](https://archive.blender.org/developer/F9210725/image.png) Even with [D9244: Fix #81704](https://archive.blender.org/developer/D9244) applied, there still seems to be a slight scaling error creeping in. And then at `Bone.008` all of a sudden things explode.

Sybren A. Stüvel removed their assignment 2020-11-05 18:15:58 +01:00

Sybren A. Stüvel commented

2020-11-05 18:15:58 +01:00

Added subscriber: @dr.sybren

Joseph Brandenburg commented

2020-11-06 03:11:39 +01:00

Well, here's something interesting -- if you set the bones to use "ALIGNED" scale inheritance and THEN bake using the operator, then there seems to be no issue. This makes sense -- the Spline IK constraint handles the scaling components separately. Y is handled and then XZ is handled, and so "aligned" is the most correct way to bake the scale inheritance. I'm fairly certain this is the cause of the bug. The Y scale sets the length of the bone, which sets the next point on the curve that the constraint uses. Then the bone is rotated to point towards it (basically Damped Track) and scales along the local y axis until the tail reaches that point. So the situation is weird -- the input scale is not directly correlated with the output scale. In face, with sufficiently weird curves, you can get much shorter or longer bones than the y-scale should create. Here's a .blend that shows this problem:
#79670-spline-scaled-nonanimated_Bad.blend

It's a really difficult corner case, it's always weird when increasing the scale results in a shorter bone!

I haven't closely read the code of your addon, but I have a suspicion that it isn't factoring the parent's scale or scale inheritance mode in, at least not the same way the baking operator does, Weird behavior like scale explosion after a few bones is actually normal in my experience -- in fact, I've gotten this behavior in scripts that apply a rotation matrix to each bone in a chain one at a time (In my ChainTools addon, it's "SnapPoseBonesToCurve "). Even if the matrices don't contain any scale, and in fact, even when the matrices are decomposed and recomposed to remove scale, this will still occur. So I think all that is happening there is floating point error. In my experience, Python's values aren't very trustworthy past 3 decimal places. I always round them in my own scripts.

Finally, I don't see this as directly related to Y-Scale Mode bug, but even if it is --if there's still slight scale errors creeping in, it's probably just because the fix isn't perfect. In fact, the .blend file I just uploaded should reveal the error in my fix. I mean to solve it by a brute-force bisection search, but I've been too busy to look into it for the past few weeks 😭 . I don't think they're related, though, because it looks like the baking code is just interpreting scale inheritance incorrectly for this situation.

Well, here's something interesting -- if you set the bones to use "ALIGNED" scale inheritance and THEN bake using the operator, then there seems to be no issue. This makes sense -- the Spline IK constraint handles the scaling components separately. Y is handled and then XZ is handled, and so "aligned" is the most correct way to bake the scale inheritance. I'm fairly certain this is the cause of the bug. The Y scale sets the length of the bone, which sets the next point on the curve that the constraint uses. Then the bone is rotated to point towards it (basically Damped Track) and scales along the local y axis until the tail reaches that point. So the situation is weird -- the input scale is not directly correlated with the output scale. In face, with sufficiently weird curves, you can get much shorter or longer bones than the y-scale should create. Here's a .blend that shows this problem: [#79670-spline-scaled-nonanimated_Bad.blend](https://archive.blender.org/developer/F9214410/T79670-spline-scaled-nonanimated_Bad.blend) It's a really difficult corner case, it's always weird when increasing the scale results in a shorter bone! I haven't closely read the code of your addon, but I have a suspicion that it isn't factoring the parent's scale or scale inheritance mode in, at least not the same way the baking operator does, Weird behavior like scale explosion after a few bones is actually normal in my experience -- in fact, I've gotten this behavior in scripts that apply a rotation matrix to each bone in a chain one at a time (In my ChainTools addon, it's "[SnapPoseBonesToCurve ](https://gitlab.com/josephbburg/blender-chaintools/-/blob/development/ops_poseBone.py#L139)"). Even if the matrices don't contain any scale, and in fact, even when the matrices are decomposed and recomposed to remove scale, this will still occur. So I think all that is happening there is floating point error. In my experience, Python's values aren't very trustworthy past 3 decimal places. I always round them in my own scripts. Finally, I don't see this as directly related to Y-Scale Mode bug, but even if it is --if there's still slight scale errors creeping in, it's probably just because the fix isn't perfect. In fact, the .blend file I just uploaded should reveal the error in my fix. I mean to solve it by a brute-force bisection search, but I've been too busy to look into it for the past few weeks 😭 . I don't think they're related, though, because it looks like the baking code is just interpreting scale inheritance incorrectly for this situation.

John Dupuis commented

2020-12-09 18:14:55 +01:00

Added subscriber: @Yad

Philipp Oeser removed the

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Baking animation of bones affected by a curve (spline IK) is wrong #79670