Get the world position of a vertex in glTF animation?

glTF glTF: Support
1

Hi,
Given:

"meshes": [
    {
      "name": "my_mesh",
      "primitives": [
        {
          "attributes": {
            "POSITION": 0,
            "JOINTS_0": 1,
            "WEIGHTS_0": 2
          },
          "indices": 3,
          "material": 0
        }
      ]
    }
  ],

...
attribute vec4 a_joint;
attribute vec4 a_weight;

uniform mat4 u_jointMat[2];
...
void main(void)
{
    mat4 skinMat =
        a_weight.x * u_jointMat[int(a_joint.x)] +
        a_weight.y * u_jointMat[int(a_joint.y)] +
        a_weight.z * u_jointMat[int(a_joint.z)] +
        a_weight.w * u_jointMat[int(a_joint.w)];
    vec4 worldPosition = skinMat * vec4(a_position,1.0);
    ...
}

the worldPosition should be the vertex world position, but I have a doubt on the meaning of u_jointMat.
In the tutorial page, it is described as

jointMatrix(j) =
  globalTransformOfJointNode(j) *
  inverseBindMatrixForJoint(j);

So I could write:

 mat4 skinMat =
        a_weight.x * globalTransformOfJointNode((a_joint.x) *  inverseBindMatrixForJoint((a_joint.x) +
        a_weight.y * globalTransformOfJointNode((a_joint.y) *  inverseBindMatrixForJoint((a_joint.y) +
        a_weight.z * globalTransformOfJointNode((a_joint.z) *  inverseBindMatrixForJoint((a_joint.z) +
        a_weight.w * globalTransformOfJointNode((a_joint.w) *  inverseBindMatrixForJoint((a_joint.w);
    vec4 worldPosition = skinMat * vec4(a_position,1.0);

with a_position the vertice position taken from the mesh attribute POSITION above, a_weight the weight from WEIGHT_0 at the proper offset and a_joint the joint id from JOINT_0 at the proper offset.

Nonetheless I don’t get the proper world vertex position.

My global matrices are gathered like so:
From parent to childs,
local_matrix = T * R * S, each taken from the animation channel at the proper animation step
and the global
this_global = parent_global_matrix * local_matrix

Reading the vertices info from Blender show a correct result but not in my code. At all.

What’s missing ?
If you need more info I will happily expand to clarify.
Note: I am writing a high level code in C++ to print the info I need, it is not OpenGL related.