Issue with bake action and constrained bone under scaled parent #106980

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opened 2023-04-15 09:55:37 +02:00 by RomainTestylier · 3 comments

RomainTestylier commented

2023-04-15 09:55:37 +02:00

System Information
Operating system: Windows 10
Graphics card: NVIDIA GeForce RTX 3060

Blender Version
Broken: 2.83.20, 2.93.16, 3.4.1, 3.5.0

Short description of error
In a rig with a constrained bone with StretchTo or Lookat constraint and a scaled parent, the action baking give a wrong bone rotation and scale.

Exact steps for others to reproduce the error

Using the attached blend file select the initial rig named Armature,
F3 -> Bake Action
Disable the option Only Selected Bones, Enable Visual Keying and Clear Constraint and select Pose as Bake Data
Press OK and the animation is not the same anymore.

**System Information** Operating system: Windows 10 Graphics card: NVIDIA GeForce RTX 3060 **Blender Version** Broken: 2.83.20, 2.93.16, 3.4.1, 3.5.0 **Short description of error** In a rig with a constrained bone with StretchTo or Lookat constraint and a scaled parent, the action baking give a wrong bone rotation and scale. **Exact steps for others to reproduce the error** * Using the attached blend file select the initial rig named Armature, * F3 -> Bake Action * Disable the option Only Selected Bones, Enable Visual Keying and Clear Constraint and select Pose as Bake Data * Press OK and the animation is not the same anymore.

BakedAnimationIssue.blend

884 KiB

RomainTestylier added the

labels 2023-04-15 09:55:38 +02:00

Iliya Katushenock added the

Animation & Rigging

label 2023-04-15 10:23:47 +02:00

Germano Cavalcante added

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labels 2023-04-20 20:03:44 +02:00

Nathan Vegdahl commented

2023-04-28 12:55:50 +02:00

Member

Okay, so this is a tricky one. TLDR; this isn't a bug per se, but a limitation.

The issue is that:

The bone with the Stretch To constraint has a parent with non-uniform scaling. This means that the bone is in a sheared space.
The Stretch To constraint apparently overrides this sheared space, and transforms the constrained bone as if it were in a non-sheared world space. This effectively introduces shear into the constrained bone's transforms, to counteract the shear of the parent space to make it look unsheared in world space.

Together, this means the transform caused by the Stretch To constraint is literally unrepresentable by the loc/rot/scale channels of the bone. They can only represent location, rotation, and scale, not shear. Hence, the shear of the bone (which is counteracting the shear introduced by the parent) gets lost when baking down to those transform channels.

Notably, this issue is not limited to the Stretch To constraint. It can also be caused by the Copy Transforms constraint (I've attached an example blend file of that), and possibly others as well (I haven't checked).

Although this is admittedly a counterintuitive behavior, it's not actually a bug. It's just a natural limitation of how transforms are represented in Blender (and most other software, for that matter). Loc/rot/scale channels can't represent shear, and thus those shear effects get lost when baking down to those channels.

At the moment, the solutions are user-side. Two solutions that come to mind are:

Change your rig to avoid shearing any bones. This is often (but not always) possible. But it does require a bit of knowledge. In the example blends so far, a good solution would be to replace the parent-child relationship with Copy Location and/or Copy Rotation constraints as appropriate.
Don't use Clear Constraints when baking, leaving the full rig (and thus the effect of the constraints) intact. Depending on circumstance, this is not always a feasible solution (e.g. if it's being baked for export to a game engine).

A possible solution in the future might be an option to bake animations down to transform matrices rather than loc/rot/scale channels. That way all transforms--regardless of source--could be fully represented. Although if this is for export that may not fully solve the issue since the software/game engine/whatever that the animation is being imported to might only support loc/rot/scale channels for animation anyway, and then you'd be back to the same problem.

Okay, so this is a tricky one. TLDR; this isn't a bug per se, but a limitation. The issue is that: 1. The bone with the `Stretch To` constraint has a parent with non-uniform scaling. This means that the bone is in a *sheared* space. 2. The `Stretch To` constraint apparently **overrides** this sheared space, and transforms the constrained bone as if it were in a non-sheared world space. This effectively *introduces* shear into the constrained bone's transforms, to counteract the shear of the parent space to make it look unsheared in world space. Together, this means the transform caused by the `Stretch To` constraint is literally *unrepresentable* by the loc/rot/scale channels of the bone. They can only represent location, rotation, and scale, not shear. Hence, the shear of the bone (which is counteracting the shear introduced by the parent) gets lost when baking down to those transform channels. Notably, this issue is not limited to the `Stretch To` constraint. It can also be caused by the `Copy Transforms` constraint (I've attached an [example blend file](/attachments/61da51dc-7ce1-4b2c-881f-35d804d3323a) of that), and possibly others as well (I haven't checked). Although this is admittedly a *counterintuitive* behavior, it's not actually a bug. It's just a natural limitation of how transforms are represented in Blender (and most other software, for that matter). Loc/rot/scale channels can't represent shear, and thus those shear effects get lost when baking down to those channels. At the moment, the solutions are user-side. Two solutions that come to mind are: 1. Change your rig to avoid shearing any bones. This is often (but not always) possible. But it does require a bit of knowledge. In the example blends so far, a good solution would be to replace the parent-child relationship with `Copy Location` and/or `Copy Rotation` constraints as appropriate. 2. Don't use `Clear Constraints` when baking, leaving the full rig (and thus the effect of the constraints) intact. Depending on circumstance, this is not always a feasible solution (e.g. if it's being baked for export to a game engine). A possible solution in the future might be an option to bake animations down to *transform matrices* rather than loc/rot/scale channels. That way all transforms--regardless of source--could be fully represented. Although if this is for export that may not fully solve the issue since the software/game engine/whatever that the animation is being imported to might only support loc/rot/scale channels for animation anyway, and then you'd be back to the same problem.

bake_unrepresentable_transforms.blend

900 KiB

Sybren A. Stüvel commented

2023-05-01 11:31:52 +02:00

Member

I'd like to add a 3rd option to the above suggestions:

There is a bone setting 'Inherit Scale' that can be used to remove sheer. That way you can retain the parent-child relationship and still avoid such issues.

I'd like to add a 3rd option to the above suggestions: ![image](/attachments/a22d705c-43b8-4370-9572-1a48c0778078) There is a bone setting 'Inherit Scale' that can be used to remove sheer. That way you can retain the parent-child relationship and still avoid such issues.

20 KiB

Nathan Vegdahl commented

2023-05-01 11:37:32 +02:00

Member

Closing because it's not a bug. (But the limitation is on our radar.)

Closing because it's not a bug. (But the limitation is on our radar.)

Nathan Vegdahl closed this issue

2023-05-01 11:37:34 +02:00

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