How Blender's fbx importer is calculating the global transform

Archive Blender Development
1

Hi,
I am trying to host Cycles inside my application. It is a C++ application. I am importing a fbx file using FbxSdk and building Cycles scene. I am able to build Cycles scene and render. The only problem is I could not calculate transforms properly. I could not understand how Blender is calculating global transform. I don’t know python. Can you please help me in this?

In my code, I simply did FbxAxisSystem::MayaZUp.ConvertScene(fbx_scene);
But still the transformations are different from what blender is calculating.

The below is the code from blender. I could understand it upto some extent but i could not understand the method axis_conversion.

Thanks in advance.

   unit_scale = elem_props_get_number(fbx_settings_props, b'UnitScaleFactor', 1.0)
    unit_scale_org = elem_props_get_number(fbx_settings_props, b'OriginalUnitScaleFactor', 1.0)
    global_scale *= (unit_scale / units_blender_to_fbx_factor(context.scene))
    # Compute global matrix and scale.
    if not use_manual_orientation:
        axis_forward = (elem_props_get_integer(fbx_settings_props, b'FrontAxis', 1),
                        elem_props_get_integer(fbx_settings_props, b'FrontAxisSign', 1))
        axis_up = (elem_props_get_integer(fbx_settings_props, b'UpAxis', 2),
                   elem_props_get_integer(fbx_settings_props, b'UpAxisSign', 1))
        axis_coord = (elem_props_get_integer(fbx_settings_props, b'CoordAxis', 0),
                      elem_props_get_integer(fbx_settings_props, b'CoordAxisSign', 1))
        axis_key = (axis_up, axis_forward, axis_coord)
        axis_up, axis_forward = {v: k for k, v in RIGHT_HAND_AXES.items()}.get(axis_key, ('Z', 'Y'))
    global_matrix = (Matrix.Scale(global_scale, 4) @ axis_conversion(from_forward=axis_forward, from_up=axis_up).to_4x4())

def axis_conversion(from_forward='Y', from_up='Z', to_forward='Y', to_up='Z'):
    """
    Each argument us an axis in ['X', 'Y', 'Z', '-X', '-Y', '-Z']
    where the first 2 are a source and the second 2 are the target.
    """
    from mathutils import Matrix
    from functools import reduce

    if from_forward == to_forward and from_up == to_up:
        return Matrix().to_3x3()

    if from_forward[-1] == from_up[-1] or to_forward[-1] == to_up[-1]:
        raise Exception("Invalid axis arguments passed, "
                        "can't use up/forward on the same axis")

    value = reduce(int.__or__, (_axis_convert_num[a] << (i * 3)
                                for i, a in enumerate((from_forward,
                                                       from_up,
                                                       to_forward,
                                                       to_up,
                                                       ))))

    for i, axis_lut in enumerate(_axis_convert_lut):
        if value in axis_lut:
            return Matrix(_axis_convert_matrix[i])
    assert(0)