How to change roll of bone?

I have some animations that work for this avatar I built. I found another avatar with the exact same bone hierarchy but the bones had different roll values. In the animation this resulted in twisted limbs. I solved this in Blender by unparenting the armature, changing the roll of the bones (some to 0, some to -180 or 180) then re-parenting. Is there a way to do this with BabylonJS code?

Basically bones are simple matrices. So it should be “the same”. You have to edit every bone and set the matrix to apply the the correct roll

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So I just have to alter the bone’s local matrix once?


Actually I think it’s the difference matrix that should be changed, not the local matrix, because if you just change the local matrix then those changes will be overwritten when you animate the bone…

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This is good catch:)

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Hi @ozRocker just checking in, were you able to solve your problem? :smiley:

Not yet. Unfortunately the playground example doesn’t work for me. Applying a new rotation matrix to my avatar screws up scale and position

What about doing it like below to update the existing base matrix instead of replacing it? (You might have to start an animation to see the effect).

const scale = new BABYLON.Vector3();
const rotationQuaternion = new BABYLON.Quaternion();
const position = new BABYLON.Vector3();

const differenceMatrix = bone._baseMatrix;
differenceMatrix.decompose(scale, rotationQuaternion, position);

const rotation = rotationQuaternion.toEulerAngles();
rotation.y += Math.PI;
BABYLON.Quaternion.FromEulerVectorToRef(rotation, rotationQuaternion);

BABYLON.Matrix.ComposeToRef(scale, rotationQuaternion, position, differenceMatrix);

I’ve implemented those changes and I can partially get it working.
You can see my efforts here. This first playground is using the GLB with the original roll values. I’m attempting to untwist the bones
In the second playground I’m using a GLB where I corrected the roll values in Blender and it works perfectly, so this is how it should look

This looks better but there is still some serious warping going on. If you change the animation to “sitting.glb” you’ll notice it clearly.

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I get a load error trying to use these PG links but I downloaded the models and then the code woks fine so maybe the server settings are blocking it somehow? It looks better but IDK why it isn’t working - I ran out of ideas for now but if I think of something I’ll give it another try. :slight_smile:

EDIT: rock on they’re loading in the PG’s now. :+1:

Whoops! Sorry about the CORS errors. I just added an .htaccess that should fix that

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