I don't know what to do anymore so I'll give it another shot:

This is the data I get from my FBX file (scaling omitted):

shoulder (root):

T: (-2.0, 0, 0)

R: (0, 0, -180.0)

elbow:

T: (-6.0, 0, 0)

R: (0, 0, -180.0)

wrist:

T: (-10.0, 0, 0)

R: (0, 0, -90.0)

I am pretty sure those are the transformations when the skin was bound to the skeleton. It looks to me that there is no child-parent relationship, the data for each joint is global (in Model space.)

From what I've learned I need to compute the inverse bindpose matrix for each joint, this only has to be done once. Is the info above all I need to calculate those matrices ? How do I do that ?

***

Buckeye, I've read your article a few times and am still having trouble, I'm assuming the offset matrix is the same as the inverse bindpose matrix I'm trying to compute:

"offsetMatrix = MeshFrame.ToRoot * Inverse( bone.ToParent * parent.ToParent * ... * root.ToParent )"

"MeshFrame.ToRoot is the transform for moving the mesh into root-frame space" - is that the matrix that goes from Model space to the root joint's space ?

What is root.ToParent ? I thought roots don't have parents.

***

Also, considering that my data is not in a child-parent relationship will that equation still work ? Do I first have to put the bind pose in a child-parent relationship myself ?

I also noticed that when I get the data for the keyframes (poses) there is no child-parent relationship either. I use a parent-relative SRT for the joint poses, does that mean I have to construct child-parent relationships myself ? That's kind of bugging me now.

I think the problem I've been having is that the tutorials I've read assume you can get the data exactly how it's supposed to be, and I'm fairly new to FBX as well as animation. I'm sorry if my questions are kinda dumb.

Thanks for helping.

Hi again,

I can't for the life of me figure out why this isn't working. What I'm doing doesn't seem complicated at all, yet it will not work.

I am basing this off the last post by Buckeye in this thread:

http://www.gamedev.net/topic/589351-animation-bones-matrices-skeleton-how-does-it-work-/

I simplified everything and removed loops and recursions.

Here are the bones in their bind pose:

And here s the code:

 
 
// all rotations are done around the y-axis
float3 axis={0.0f, 1.0f, 0.0f};

// parent bone bind pose local matrix (relative to model space)
translation=matrixTranslation(0.0, 0.0, 0.0); // no translation
q=quaternionRotationAxis(axis, -45.0);
rotation=matrixRotationQuaternion(q);
parent_local=rotation*translation;

// child bone bind pose local matrix (relative to parent bone)
translation=matrixTranslation(2.0, 0.0, 2.0);
q=quaternionRotationAxis(axis, 45.0f);
rotation=matrixRotationQuaternion(q);
child_local=rotation*translation;

// parent bone inverse bind pose matrix
parent_bone.invbind=matrixInverse(parent_local);

// child bone inverse bind pose matrix
child_bone.invbind=matrixInverse(child_local*parent_local);
 
 

Alright, that's half the job. I compute those only once when I create my skeleton.

Now I'll make a pose (or keyframe) using SRT (actually RT since I have no scaling):

 

// parent bone pose will be same as the bind pose
parent_pose.translation={0.0, 0.0, 0.0}; // same as bind pose
q=quaternionRotationAxis(axis, -45.0); // same as bind pose
parent_pose.rotation=matrixRotationQuaternion(q);

// child bone pose will be no rotation relative to parent
child_pose.translation={2.0, 0.0, 2.0}; // same as bind pose
q=quaternionRotationAxis(axis, 0.0); // this is different from bind pose
child_pose.rotation=matrixRotationQuaternion(q);
 
 

And finally here is how I create the final (or skinning) matrices:

// parent bone final matrix
parent_slerp=parent_pose.rotation*parent_pose.translation; // this would normally be a slerp
parent_final=parent_bone.invbind*parent_slerp;

// child bone final matrix
child_slerp=child_pose.rotation*child_pose.translation; // this would normally be a slerp
child_final=child_bone.invbind*(child_slerp*parent_slerp);
 
 

And here it is, with the child bone not in the correct position (it is pointing in the correct direction though.)

What am I doing wrong ? This is killing me.

Thanks for everything.

Buckeye, that really clears up a lot for me.

I was indeed including the root transform in my calculations, and I also didn't realize the root should be transformed by the world matrix.

I'm still having a problem though and I think it's related to your first observation:

"First: it's not clear how you are rendering the hierarchy. Consider how your bind pose should look. The root is rotated -45 and the child is rotated +45. The child should therefore be in line with the root. But it appears both have a local rotation of +45 (assuming left-hand rule)."

I really don't know what you mean by that. To me the rotation of the child in reference to the root being 45 deg makes it impossible to be in line (meaning they both point in the same direction.) I must not understand something.

Here is simplistic bind pose:

...with a hierarchy of pelvis->spine->neck. Notice the pelvis and spine are facing opposite directions and the spine and neck are facing the same direction.

pelvis.local=identity

spine.local=matrixRotationY(180.0); // rotate around y-axis, no translation

neck.local=matrixTranslation(0, 0, 2); // translation along z-axis, no rotation

Isn't that correct ? If it isn't then it's definitely my problem.

Thanks so much for helping, I'm going to have to get you an Amazon gift card or something :)

And this is what happens:

All I'm doing is changing the rotation for the neck from 0 to 45. It's pointing in the direction it's supposed to but it's translated wrong.

Here is my exact code for calculating the final matrices

 
// using the inverse bind as computed like this (see wireframe image in post 15):

skeleton.joints[0].invbind=matrixIdentity(); // pelvis
skeleton.joints[1].invbind=matrixInverse(matrixRotationY(pi)); // spine
skeleton.joints[2].invbind=matrixInverse(matrixTranslation(0.0f, 0.0f, 2.0f)); // neck
 
// pose T part
 
pose->joints[0].lclT=float3(0.0f, 0.0f, 0.0f);
pose->joints[1].lclT=float3(0.0f, 0.0f, 0.0f);
pose->joints[2].lclT=float3(0.0f, 0.0f, 2.0f);

// pose R part
 
quaternionIdentity(&pose->joints[0].lclR);
quaternionRotationAxis(&pose->joints[1].lclR, &axis, pi); // 180 deg about y-axis
quaternionRotationAxis(&pose->joints[2].lclR, &axis, pi*0.25f); // 45 deg about y-axis
 
//    bone chain

matrixIdentity(&transforms[0]);
parent=transforms[0];

for(int i=1;i<3;i++){

 matrixTranslation(&translation,&pose->joints[i].lclT);
 matrixRotationQuaternion(&rotation,&pose->joints[i].lclR);
 matrixMultiply(&local,&rotation,&translation);
 matrixMultiply(&transforms[i],&local,&parent);
 parent=transforms[i];

}

 //    finalize transformations

 for(int i=1;i<3;i++){
  matrixMultiply(&transforms[i],&skeleton.joints[i].invbind,&transforms[i]);
 }
 

I've been working with this for a month and keep getting similar results no matter how I do it.

Maybe I'm just not cut out for this

Hey Buckeye, thanks a lot for the encouragement, it goes a long way.

I worked it out with pencil and paper and it seems to work like it should.

Now I'm working in a Windows program I made that will dump all the values in text to the screen, this way I can really see what's going on (It's easier to follow than breakpoints). To keep it simple I'm using two points to represent each bone.

I also switched to the D3DXMATRIX routines to eliminate the possibility that my routines are wrong.

I'll let you know what happens, and if I can't get it right I will use consistent terminology to show what I'm doing. I really feel like I understand this now but I'm still having some trouble implementing it. I think it's the matrix order.

Thanks for everything.

Hi Buckeye,

I'm really at a loss. Either I am doing something fundamentally wrong, or the algorithm doesn't work. I did what you said and made everything clear, I also put everything in a single function that has no dependencies (except D3DX).

It should calculate the correct final (skinning) matrices. As a test I multiply the two blue points (left half of diagram) with joint_c.final, they should result in the two blue points on the right half of the diagram but they don't.

Here is the code:

 
struct Joint // or Bone or Frame
{
 D3DXMATRIX offset; // pre-computed offset (or inverse bind)
 D3DXMATRIX local; // transforms to parent's space (in bind pose)
 D3DXMATRIX anim; // normally in SRT struct but here for simplicity
 D3DXMATRIX combined; // anim*local * parent.anim*parent.local * ...(up to root)
 D3DXMATRIX final; // final or skinning matrix
};
 
void Test()
{
 D3DXMATRIX R,T,temp;

 Joint joint_a; // root
 Joint joint_b; // child of joint_a
 Joint joint_c; // child of joint_b

 // local transforms

 D3DXMatrixIdentity(&joint_a.local);

 D3DXMatrixRotationY(&R, -D3DX_PI*0.5f);
 D3DXMatrixTranslation(&T, 2.0f, 0.0f, 0.0f);
 joint_b.local=R*T;

 D3DXMatrixTranslation(&T, 0.0f, 0.0f, 2.0f);
 joint_c.local=T;

 // offset (inverse bind)

 temp=joint_a.local;
 D3DXMatrixInverse(&joint_a.offset, NULL, &temp);

 temp=joint_b.local*joint_a.local;
 D3DXMatrixInverse(&joint_b.offset, NULL, &temp);

 temp=joint_c.local*joint_b.local*joint_a.local;
 D3DXMatrixInverse(&joint_c.offset, NULL, &temp);

 // joint animation

 D3DXMatrixIdentity(&joint_a.anim);
 D3DXMatrixIdentity(&joint_b.anim);

 // we only rotate (animate) joint_c with a 90deg rotation
 D3DXMatrixRotationY(&joint_c.anim, D3DX_PI*0.5f);

 // compute combined matrices

 joint_a.combined=joint_a.anim*joint_a.local;

 joint_b.combined=(joint_b.anim*joint_b.local)*joint_a.combined;

 joint_c.combined=(joint_c.anim*joint_c.local)*joint_b.combined;

 // final matrices

 joint_a.final=joint_a.offset*joint_a.combined;
 joint_b.final=joint_b.offset*joint_b.combined;
 joint_c.final=joint_c.offset*joint_c.combined;

 // transform (morph) some vertices

 D3DXVECTOR3 va,vb;

 va.x=2.0f; va.y=0.0f; va.z=2.0f;
 vb.x=2.0f; vb.y=0.0f; vb.z=4.0f;

 D3DXVec3TransformCoord(&va, &va, &joint_c.final);
 D3DXVec3TransformCoord(&vb, &vb, &joint_c.final);
}
 

It's only about 20 lines of code with basic matrix math. I really don't know what I am doing wrong. If you don't want to go through my code maybe you can just look at the diagram and write some code the way you would do it.

I don't know what else to try at this point.

Thanks a lot.