Unknown issue while rotating bones

Question

I faced a strange behaviour while was making a test skeleton rotation, here come the code:

for (int i = 0; i < bones.size(); ++i) {
    u2long parentlink = bones[i].blenderBone.parent;
    bone* b = &bones[i];
    
    if (!parentlink) {
        b->parent = nullptr;
        b->parentName = "no parent";

        glm::mat4 tailtrans = glm::translate(glm::mat4(1.0f), b->local_tail);
        glm::mat4 headtrans = glm::translate(glm::mat4(1.0f), b->local_head);

        b->tailmatrix = tailtrans * b->bonematrix;
        b->headmatrix = headtrans * b->bonematrix;

        b->renderedmatrix = b->headmatrix;

    }
    else {
        std::optional<bone> bpar = loader->getBone(parentlink);
        std::string parname = bpar->name;
        bones[i].parent = getBone(parname);
        bones[i].parentName = parname;

        glm::mat4 tailtrans = glm::translate(glm::mat4(1.0f), b->local_tail);
        glm::mat4 headtrans = glm::translate(glm::mat4(1.0f), b->local_head);

        b->headmatrix = b->parent->tailmatrix * headtrans * b->bonematrix;
        b->tailmatrix = b->parent->tailmatrix * tailtrans * b->bonematrix;

        b->renderedmatrix = b->headmatrix;
    };
};

local_tail is bone bindspose start at bone space, local_tail is bone bindpose end at bone space, bonematrix is bone bindpose rotation, headmatrix is used for rendering the bones and is matching bindpose model-space matrix for any bone, tailmatrix for further transformations

And that gives me result which I find correct: bindpose

however, if I add a random bone rotation before the cycle:

glm::mat4 r = glm::rotate(glm::mat4(1.0f), 0.89f, glm::vec3(0.0f, 0.0f, 1.0f));
bones[8].bonematrix = bones[8].bonematrix * r;

the rendering is done strange: Wrong, spine

the following bone above the rotated one is rendered at its bindpose position, BUT the rest of the bones above are stacking all positions and rotations offsets correctly, which is the reason I post the question. The issue applies to any other bone connected to its parent: Wrong, right arm

But rotating root bone brings correct result this time: Correct, root

I find this line:

b->tailmatrix = b->parent->tailmatrix * tailtrans * b->bonematrix;

quite suspicious, after having thought all over it seems to me this one:

b->tailmatrix = b->parent->tailmatrix * b->bonematrix * tailtrans;

or better this:

b->tailmatrix = b->headmatrix * tailtrans;

should bring correct results, but swaping to these lines brings chaos, bindpose is a mess, the same mess applies to any bone (except root one) rotation: Bindpose, wrong

Where am I wrong with that code? I bashed this wall for three weeks but when ist almost through my front is almost dead as well :/

(this is blender file parser, the bones not matching the model is a temporary issue with blender "magical" transform axis, all bones above the torso are parented down)

Answer 1

All right, got it done, the line:

b->tailmatrix = b->parent->tailmatrix * tailtrans * b->bonematrix;

I made this line while I was, errmm...guessing, and it managed to give results but now I have understood why it did, in Blender model offset to head and tail of the next bone is done in parent bone's TAIL local space for both of tail and head, but I thought it was different. The next and the last thing to add - apply rotation offset right into this line, for the bone head matrix has rotated offset but unrotated position (armature[3], bindpose, in my case) and tail matrix receives rotated offset as well as updated position:

b->tailmatrix = b->parent->tailmatrix * tailtrans * (b->bonematrix * b->localrot);

b->localrot is rotation offset matrix.

so the code for now looks this way, and the test rotation is done fine:

bones[8].localrot = glm::rotate(glm::mat4(1.0f), 1.0f, glm::vec3(1.0f, 0.0f, 1.0f));

for (int i = 0; i < bones.size(); ++i) {
    bone* b = &bones[i];

    if (!b->parent) {
        glm::mat4 tailtrans = glm::translate(glm::mat4(1.0f), b->local_tail);
        glm::mat4 headtrans = glm::translate(glm::mat4(1.0f), b->local_head);

        b->headmatrix = b->bonematrix * b->localrot * headtrans;
        b->tailmatrix = b->bonematrix * b->localrot * tailtrans;

        b->renderedmatrix = b->headmatrix;

    }
    else {
        glm::mat4 tailtrans = glm::translate(glm::mat4(1.0f), b->local_tail);
        glm::mat4 headtrans = glm::translate(glm::mat4(1.0f), b->local_head);

        b->headmatrix = b->parent->tailmatrix * headtrans * b->bonematrix;
        b->tailmatrix = b->parent->tailmatrix * tailtrans * (b->bonematrix * b->localrot);

        b->renderedmatrix = b->headmatrix;
    };
};

all good.