Retrieve Center Pivot of Three or More Objects?



  • Hi,

    I am able to get the center pivot of two objects but for some reason, it needs revision for three or more objects.
    You can see an illustration of the problem here:
    https://www.dropbox.com/s/u76v7zcbv8a38x4/c4d177_center_pivot_three_selected_object.jpg?dl=0

    Here is the working code so far:

    import c4d
    from c4d import gui
    
    # Main function`
    def main():
        #Assuming there are three objects in the scene
        cube01 = doc.SearchObject('cube01')
        cube02 = doc.SearchObject('cube02')
        cube03 = doc.SearchObject('cube03')
    
        cube01_pos = cube01.GetMg().off
        cube02_pos = cube02.GetMg().off
        cube03_pos = cube03.GetMg().off
    
        # Getting the pivot of the two objects. Works as expected
    
        cube_dif =  cube02_pos - cube01_pos
        cube01_02_pos = ((cube_dif/2) + cube01_pos) # Center Pivot
    
        # Getting the pivot of the three objects.
        cube_dif03 = cube01_02_pos - cube03_pos
        cube01_02_03_pos = ((cube_dif03/2) + cube01_02_pos)
    
        print cube01_02_03_pos #Does not match the pivot on the coordinate manager when all three objects are selected.
    
    
    # Execute main()
    if __name__=='__main__':
        main()
    

    Thank you for looking at my problem.



  • Hi,

    the term pivot is a bit ambiguous in this posting, since it refers to a point, while you are showing a frame, a transform matrix, Cinema's little coordinate system widget in your illustration (props by the way for including these).

    The shared frame of N objects is just the arithmetic mean of the frames of these objects. Since you used the term pivot and things will get more complicated for averaging out also rotations (quaternions would be a solution) and scales, I will assume you are just after that the offset of that shared frame. Again in pseudo code:

    pivot = sum([obj.GetMg().off for obj in my_objects]) * (1. / len(my_objects))
    

    Cheers
    zipit



  • @zipit

    Thanks for the response.
    I'm not really sure on the nomenclature but I am referring to the Red,Green,Blue arrows as the pivot.

    Anyhow, for some reason your code does not match the pivot computed by the coordinate manager.
    You can see the illustration here:
    https://www.dropbox.com/s/tzlg3ry03e7tu5p/c4d177_center_pivot_three_selected_object02.jpg?dl=0



  • Hi,

    I am sorry, I wrote this posting without checking it in Cinema. It seemed pretty self explanatory to me, that this is the answer. I am not quite sure why it does not work, I will have to check this myself in Cinema (I do not have the time to do it right now). Possible solutions could be, that Cinema computes the geometric or harmonic mean (which would strike me as odd) or that I made an oopsie in my code (that I do not see right now).

    PS: I pointed that pivot stuff out, because if the object frames (axis) are tilted (in respect to the world frame), then your shared frame will be titled too. Talking about a pivot implies you are not interested in that information. I just wanted to make sure, that this is the case.

    Cheers
    zipit



  • @zipit

    Uhm, yep. For the pivot, I guess I'm also interested on the its "tiltedness". Basically, what the coordinate manager shows (the position and rotation). Sorry for not being clear.

    I would just really like to call on the GetModelingAxis since it corresponds exactly with the coordinate manager but it only works on a selected object not document per se. In other words, it works only on one object but not on multiple objects.

    RE: I do not have the time to do it right now
    No worries. Thanks for the response. Have a great day ahead! :)



  • Hello,

    the axis displayed of a multi-selection is represented by a helper object.

    One can acess that helper object using BaseDocument.GetHelperAxis().

    See Axis and Plane.

    best wishes,
    Sebastian



  • Hi,

    since @s_bach provided a much cleaner solution, I consider this solved and won't look for manual solution.

    Cheers
    zipit



  • @s_bach
    Thanks for the response! Works as expected.

    @zipit
    No worries. Will close this thread.
    I will probably revive this a year from now (lol). I have a feeling that Matrix will haunt me again in other aspects so this would be a nice exercise if that happens.

    Thanks again!


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