Graphics question about how to compute the tranf. Matrix between keyframes.

Hello to all,

Im currently making a small skeletal animation project in OpenGL and I use blender to export meshes, animations etc. I need a little help about how blender calculates the intermediate bone positions ( between keyframes ).

In the image bellow there is an armature deforming a mesh. I have a keyframe in frame 0 and one in frame 2. Blender calculates the frame 1 using an interpolation algorithm. (In action editor menu I’ve used linear interpolation.)

My question is: If I extract (using python) the matrix for keyframe 0 and the matrix for keyframe 2 WHAT is the algorithm to calculate the matrix that transforms the bone in frame 1??

I know this question is not Blender related. Its more into graphics. Thanks in advance.

I think it is probably a rotational matrix about the joint