Skeletal animation, transformation multiplication

Question

I have implemented a skeletal animation system where I seem to be missing one last detail for it to work properly.

I have made an animation which only a part of the character has bones. In this image The stickman has a waving arm, but the arm waves at the wrong place compared to the rest of the stickman. (You barely see it between his legs)

I will try to outline the basics of my matrix computation to see if I am doing something wrong.

Computation of bone specific absolute and relative animation matrix (based on my keyframe matrix data):

        if (b == this->root) {
                b->absoluteMatrix = M;
            } else {
                b->absoluteMatrix = b->parent->absoluteMatrix * M;
            } 

            b->skinningMatrix = b->absoluteMatrix * inverse(b->offsetMatrix);       

            if (this->currentAnimationTime == 0) {
                cout << "Bone '" << b->name << "' skinningMatrix:\n";
                printMatrix(b->skinningMatrix);

                cout << "Bone '" << b->name << "' absoluteMatrix:\n";
                printMatrix(b->absoluteMatrix);

                cout << "Bone '" << b->name << "' offsetMatrix:\n";
                printMatrix(b->offsetMatrix);

                cout << "---------------------------------\n";
            }

skinningMatrix which I send to the GPU. This prints the following:

where offsetMatrix is a transform that transforms from mesh space to bone space in bind pose.

In my shader I then do:

layout(location = 0) in vec4 v; // normal vertex data
newVertex = (skinningMatrix[boneIndex.x] * v) * weights.x;
newVertex = (skinningMatrix[boneIndex.y] * v) * weights.y + newVertex;
newVertex = (skinningMatrix[boneIndex.z] * v) * weights.z + newVertex;

Any hints on what could be wrong with my computations?

Answer 1

I am currently working through skeletal animation myself, and the only thing I noticed which may be an issue is with how you use the offset matrix from ASSIMP. The matrix in question is a matrix which "transforms from mesh space to bone space in bind pose".

To my knowledge this matrix is intended to be used 'as-is', which will essentially take your vertices into the bones local space, which you will than multiply by a 'new' global joint pose which will take the vertices from bone space to model space.

When you inverse the matrix, you are transforming the vertices into model space again, and than with your current animation frames global joint pose, pushing the vertices even further.

I believe your solution will be to remove the inverting of your offset matrix, which will result in your vertices moving from 'model-joint-model'.