如何在vtk中缩放PolyData而不转换它？

Question

I am using VTK in python to import .stl files. then what i want to do is to scale down the mesh and making it smaller without changing the orientation matrix.

I tried vtkTransform with a scale tuple but the problem is the scaled polydata is getting rotated.

Here is the code:

def scaleSTL(filenameSTL, opacity=0.75, scale=(1,1,1), mesh_color="gold"):
    colors = vtk.vtkNamedColors()

    reader = vtk.vtkSTLReader()
    reader.SetFileName(filenameSTL)
    reader.Update()

    transform = vtk.vtkTransform()
    transform.Scale(scale)

    transformFilter = vtk.vtkTransformPolyDataFilter()
    transformFilter.SetInputConnection(reader.GetOutputPort())
    transformFilter.SetTransform(transform)
    transformFilter.Update()

    mapper = vtk.vtkPolyDataMapper()
    mapper.SetInputConnection(transformFilter.GetOutputPort())

    actor = vtk.vtkActor()
    actor.SetMapper(mapper)
    actor.GetProperty().SetColor(colors.GetColor3d(mesh_color))
    actor.GetProperty().SetOpacity(opacity)

    return actor

def render_scene(my_actor_list):
    renderer = vtk.vtkRenderer()
    for arg in my_actor_list:
        renderer.AddActor(arg)
    namedColors = vtk.vtkNamedColors()
    renderer.SetBackground(namedColors.GetColor3d("SlateGray"))

    window = vtk.vtkRenderWindow()
    window.SetWindowName("Oriented Cylinder")
    window.AddRenderer(renderer)

    interactor = vtk.vtkRenderWindowInteractor()
    interactor.SetRenderWindow(window)

    # Visualize
    window.Render()
    interactor.Start()

if __name__ == "__Main__":

    filename = "400_tri.stl"
    scale01 = (1, 1, 1)
    scale02 = (0.5, 0.5, 0.5)
    my_list = []
    my_list.append(scaleSTL(filename, 0.75, scale01, "Gold"))
    my_list.append(scaleSTL(filename, 0.75, scale02, "DarkGreen"))
    render_scene(my_list)

I used my mesh file kidney.stl (yellow one) but what i getting is the scaled and rotated mesh. i set opacity to 0.75 to see both meshes. In the picture below you can see that the green one is completely moved but i want to scale so the green one is completely inside the original yellow mesh.

Answer 1

Simple answer (no explanation) can be found here: Scaling 3D models, finding the origin

That is because the scaling transformation is defined simply as multiplying the coordinates by a given factor (see eg https://www.tutorialspoint.com/computer_graphics/3d_transformation.htm ). This intrinsically means that it is done with respect to a certain reference point. Your transform.Scale() call will use the origin (0,0,0) as this reference point and since your object is apparently not centered around origin, you get the translation (not rotation as you claim btw).

To get a locally centered scaling, you need to choose a reference point R on your object around which you want to scale (in your case, since you want the scaled object to be inside the original, you want some kind of center - since the object is "almost convex", centroid - average of all points - could be good enough). Translate the object by -R to align it with the coordinate system, scale and then translate back by +R.

Try a little exercise to visualize this: simple 2D example - draw yourself a square made of points with coordinates (2,2), (2,3), (3,3), (3,2) and "scale it by 2" - you get (4,4), (4,6),(6,6), (6,4) - draw it as well. Now try the alternative - first translate by the square's center (2.5,2.5), you get (-0.5,-0.5), (-0.5,0.5), (0.5,0.5), (0.5,-0.5) (draw it), scale by two, you get (-1,-1), (-1, 1), (1,1), (1,-1) (draw) and finally translate back by 2.5: (1.5, 1.5), (1.5,3.5), (3.5,3.5), (3.5, 1.5) and draw - see the difference?