Simple LookAt for a bone

panicq

Hey,

How can I make a bone look at toward a target (in world space) ? I tried using SetBoneGlobalPoseOverride passing a transform like this:

Transform t = skeleton.GetBoneGlobalPose(id); t = t.LookingAt(target, t.basis.y); //target in world space.

but it is always off (changing the up for another base isn't working either).

Thanks in advanced.

TwistedTwigleg

You need to convert the target position from world-space to bone-space. The IK demo I made on the Godot Demo repository shows how to do this, but the relevant bit of code is as follows (taken from ik_look_at.gd):

# Convert our position relative to the skeleton's transform.
var target_pos = skeleton_to_use.global_transform.xform_inv(global_transform.origin)

panicq

@TwistedTwigleg Thanks for your answer but I don't get it, here is what I have in my update (not working).

Transform t = skeleton.GetBoneGlobalPose(id);
 target = skeleton.GlobalTransform.XformInv(target);
 t = t.LookingAt(target, t.basis.y);
skeleton.SetBoneGlobalPoseOverride(id, t,1.0f, true);

Could it be a gltf import/export issue ?

TwistedTwigleg

I'm not sure, looking at the code it should work. The only thing I can think of that might be causing the issue is using t.basis.y for the up vector, but I'm not positive that would be causing the issue either. In the IK demo, the code uses Vector3.UP/Vector3.RIGHT/Vector3.FORWARD, but again, I'm not sure if using t.basis.y is the issue. I might try replacing t.basis.y with Vector3.UP as a test and see if anything changes. It might help narrow down where the issue is at the very least.

While I think it is unlikely that the issue related to gltf, it is possible. If you can, maybe try testing the script on a skeleton from a different 3D model format and see if that fixes the issue?

panicq

@TwistedTwigleg Thanks for your answer. I cannot use a global axis as the up of the bone changes all the time and in some situations the maths won't work (the up would be aligned with the bone). I managed to achieve what I need by creating my own basis (see below), but I'm really perplexed about how LookingAt works...it has some unexpected behaviors, I may take a look at the source when I have time. I'm not marking this thread as "answered" (because first I don't know how to do that haha) and I still hope for an explanation about this :D

    Vector3 localTarget = skeleton.GlobalTransform.XformInv(target.GlobalTransform.origin);

    Transform t = skeleton.GetBoneGlobalPose(bone);

    t.basis.y = localTarget - t.origin;
    t.basis.x = t.basis.y.Cross(t.basis.z);
    t.basis.z = -t.basis.y.Cross(t.basis.x);
    
    //t = t.LookingAt(localTarget, t.basis.z);
   // t.basis = t.basis.Rotated(t.basis.x, Mathf.Deg2Rad(-90));

    t.basis = t.basis.Orthonormalized();
    skeleton.SetBoneGlobalPoseOverride(bone, t,1.0f,true);

Megalomaniak

@panicq said: I'm not marking this thread as "answered" (because first I don't know how to do that haha) and I still hope for an explanation about this :D

That's because you started a new discussion not a question. To start a new question you need to click on the little arrow to the right of the new discussion button and pick the question option from the menu that appears.

I've changed this topics type so you can mark posts as answer when you do get a reply that is a satisfactory answer or if you figure it out and write the solution here yourself.

panicq

Okay thanks, I think I have the start of an answer, in the sources SetLookAt affects the Z axis of the basis, my bones aren't pointing in that direction by default, therefore I have to recreate a basis for the Y axis... Do you know what is the default bone basis in godot (or the recommended one) ? Also, SetLookAt is very confusing as the eye parameter is actually a position, the method creates the direction itself !

Thanks in advance !

TwistedTwigleg

I would have to look at the source code to know for sure. There probably is a way to shift the axes of a Basis around, so you could swap Y and Z, but I'm not sure what it is right off. All of the bones I have been using have been using Z as the up access of the bone, which is probably why I haven't had any issues with LookAt.

panicq

Thanks for your answer, here is another related question:

I'm using SetBoneGlobalPoseOverride in order to apply my lookat transform. I have a chain of two bones, and unfortunately when I'm applying the look at to the two bones the last one offsets... When I'm working with only one bone (whatever which one it is in the chain) it is looking at the target fine. I'm getting the global pose of the bone in order to calculate my second lookat to be sure that the parents transformation has been applied. Any idea ? Note that it works on the first frame...

dir = localTarget - skeleton.GetBoneGlobalPose(boneA).origin;
    SetBoneLookAt(boneA, dir);

    dir = localTarget - skeleton.GetBoneGlobalPose(boneB).origin;
    SetBoneLookAt(boneB, dir);

EDIT: it looks like SetBoneGlobalPoseOverride is breaking the parent/child relationship between the bones oO ??

TwistedTwigleg

@panicq said: EDIT: it looks like SetBoneGlobalPoseOverride is breaking the parent/child relationship between the bones oO ??

Yeah, unfortunately if you use SetBoneGlobalPoseOverride with a weight of 1, the bone will no longer receive transform changes from the parent bone(s). In Godot 3.1, setting the bone rotation/position of a node would update the positions and rotations of child bones, however this was changed in Godot 3.2. Unfortunately, it means this functionality needs to be implemented by the programmer.

The code I used in the IK demo for handling the positioning of child nodes is below:

bone_trans.origin = last_bone.origin - last_bone.basis.z.normalized() * bones_in_chain_lengths[i-1]

Where last_bone is the parent bone to the bone being currently processed, and bones_in_chain_lengths holds all of the lengths of each bone. It is not ideal, but it works.