Unexpected transformationMatrix and offset Matrix with Skeletal Animation

Question

What I use:

• Assimp to import .fbx files from blender
• OpenGL for rendering
• glm lib for handling matrices and vectors

I am trying to make skeletal animation work. I dont read the .fbx file directly into the Program but I convert it to a binary at first. I tried to understand how transformationMatrix and offsetMatrix is supposed to work. This is what I understood: In order to later be able to run an animation, we need to find a way to make a moving Bone affect its vertices but also the bones connected to it. So the idea is to use transformation matrices, which describe the coordinate system of a bone or a node and we multiply along these paths to a bone and then multiply by is offsetMatrix to be back in objectSpace. I think by now I tried every possible combination of multiplying these but I always get something wrong. Then I looked at the values a couple of times and to me it is not obvious at all how this should work. Correct me if I am wrong but my expectation is that when in BindPose and using the offsetMatrices and transformationMatrices from the Assimp import I must result with an identity Matrix, because I want my model in bind pose just as without those Matrices. These are the transformation Matrices from all nodes up to the first bone:

The Root Node is identity

Armature mTransformationMatrix:

100 0 0 0 
0 -1.6e-05 100 0
0 -100 -1.629e-05 0
0 0 0 1

Bone mTransformationMatrix:

1 0 0 0
0 1.6e-07 -1 0
0 1 -1.6e-07 0
0 0 0 1  

I expect those two to result with something like an identity*100 when multiplied.

mOffsetMatrix of the corresponding Bone:

0.38 0 0 0 
0 0 -1 0 
0 0.28 0 0
0 0 1.67 1

In my opinion this doesnt help me at all. So either my expectaiton to result with an identity matrix are wrong or the offsetMatrix is.

In case you consider it important what my model looks like:

Edit: I forgot to mention: to read in the aiMatrix4x4 I use

static inline glm::mat4 mat4_cast(const aiMatrix4x4& m) { return glm::transpose(glm::make_mat4(&m.a1)); }

But the transformationMatrices I wrote in this report are directly from the Scene so only the offsetMatrix is transposed. But this doesn't change a thing.

Answer 1

I found the problem. The problem is that I also have to transform from meshSpace back to object space. The offset Matrix is the inverse of the tranformation Matrices multiplied and also multiplied with the inverse of the transformation Matrix to the Mesh which is on a different node.