SkeletonIK rotates bones by 180 degrees

Godot version

3.4.stable

System information

Void Linux x86_64 5.13.19, GLES3, Mesa Intel(R) HD Graphics 5500 (BDW GT2)

Issue description

2021-11-11.22-55-40.mp4

Steps to reproduce

Simply move the target up.

Minimal reproduction project

iktest.zip

[TwistedTwigleg]

I downloaded the reproduction project and can confirm there appears to be an issue with the rotations and the bones incorrectly twisting. I'll see if I can figure out what is going on and make a solution.

[TwistedTwigleg]

I've tried to debug this and find a solution, but nothing has worked so far. Will try to look at it again in the future though.

In the meantime, you may be able to use Twisted IK 2 to get around the issue, as long as you are okay with C# for the inverse kinematics solution. It uses a different way of solving rotations, so it shouldn't get the bone twisting issue.

@TwistedTwigleg I was wondering about what's different about SkeletonIK from 3.4 and SkeletonIK3D from master, I've seen lots of renaming without any functional changes, except this one:

-		if (!ci->children.empty()) {
-			/// Rotate basis
-			const Vector3 initial_ori((ci->children[0].initial_transform.origin - ci->initial_transform.origin).normalized());
-			const Vector3 rot_axis(initial_ori.cross(ci->current_ori).normalized());
-
-			if (rot_axis[0] != 0 && rot_axis[1] != 0 && rot_axis[2] != 0) {
-				const real_t rot_angle(Math::acos(CLAMP(initial_ori.dot(ci->current_ori), -1, 1)));
-				new_bone_pose.basis.rotate(rot_axis, rot_angle);
-			}
+		if (!ci->children.is_empty()) {
+			p_task->skeleton->update_bone_rest_forward_vector(ci->bone);
+			Vector3 forward_vector = p_task->skeleton->get_bone_axis_forward_vector(ci->bone);
+			// Rotate the bone towards the next bone in the chain:
+			new_bone_pose.basis.rotate_to_align(forward_vector, new_bone_pose.origin.direction_to(ci->children[0].current_pos));

It appears you forgot to merge this bugfix 92a79ac into 3.x

I didn't test that patch yet, so I can't claim it fixes my issue.

From #53358

I kinda narrowed it down a bit. It seems to be the current_ori calculation in solve_simple_forwards that is causing the issue. It seems it calculates it right, and then immediately calculates it wrong, leading to it jittering back and forth.

I experienced this kind of jittering while animating body, you can notice it in the video above from 19 to 21 seconds.

Ah, I can't even compile that patch because it depends on functions that don't exist in 3.x

[TwistedTwigleg]

I think it might be possible to make a private function that does the same thing as the Basis rotate_to_align function and try that. I thought I tried it, but maybe I am misremembering.

I do agree that it’s something in the rotation and may be related to #53358 but for 3.X. I did try a couple patches for rotation last time I took a look at this issue but none of them solved the issue. I’ll try to convert the rotate_to_align function soon (maybe tomorrow if there is time) and test it to see if it works 👍

update_bone_rest_forward_vector and get_bone_axis_forward_vector also need to be converted, right?

[TwistedTwigleg]

Ah yeah, probably. Not totally sure what the best solution for converting those would be. Maybe the forward directions could be calculated in the SkeletonIK ahead of time as well.

[TwistedTwigleg]

I did a quick test in Godot 4.0, to see if it has the same issue, since it has #53358. It sort of has the issue, but now instead of being above the shoulder that leads to the issue, it's when the rotation moves past the rest position. Here's a video showing what I mean:

Godot_4_SkeletonIK.mp4

This makes me wonder if it's the initial orientation of the skeleton that has something to do with it, given that the issue in Godot 4.0 seems to only occur around the rest position, while the issue in Godot 3.x seems to occur around the rest position flipped on the Y axis.

The new IK system for Godot 4.0 behaves the same as the SkeletonIK node does in 4.0, where it can twist but only around the rest position. I'm not sure what is causing it though, as the new IK system uses a different solve method.