vtkActor do not rotate as I want

Support
1

The code to reproduce my problem is:

import vtkmodules.all as vtk

cylinder = vtk.vtkCylinderSource()
cylinder.SetCenter(0, 10, 0)
cylinder.SetRadius(5)
cylinder.SetHeight(10)
cylinder.SetResolution(100)
cylinder.Update()

cylinderMapper = vtk.vtkPolyDataMapper()
cylinderMapper.SetInputData(cylinder.GetOutput())
cylinderActor = vtk.vtkActor()
cylinderActor.SetMapper(cylinderMapper)
cylinderActor.GetProperty().SetOpacity(0.3)


cylinderActor.SetOrigin(0, 10, 0)

cylinderActor.RotateX(90)

#######################################################
cylinderActor.RotateY(90) # line makes me confuzed
#######################################################

def Line(p0, p1, c):

    line = vtk.vtkLineSource()
    line.SetPoint1(p0[0], p0[1], p0[2])
    line.SetPoint2(p1[0], p1[1], p1[2])
    line.Update()

    mapper = vtk.vtkPolyDataMapper()
    mapper.SetInputData(line.GetOutput())
    actor = vtk.vtkActor()
    actor.SetMapper(mapper)
    actor.GetProperty().SetColor(c[0], c[1], c[2])
    return actor

x = Line(p0=[0, 0, 0], p1=[20, 0, 0], c=[1, 0, 0])
y = Line(p0=[0, 0, 0], p1=[0, 20, 0], c=[0, 1, 0])
z = Line(p0=[0, 0, 0], p1=[0, 0, 20], c=[0, 0, 1])

renderer = vtk.vtkRenderer()
renderer.AddActor(cylinderActor)
renderer.AddActor(x)
renderer.AddActor(y)
renderer.AddActor(z)


renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(renderer)
iren = vtk.vtkRenderWindowInteractor()
iren.SetRenderWindow(renWin)
iren.SetInteractorStyle(vtk.vtkInteractorStyleTrackballCamera())
iren.Initialize()
iren.Start()

The final shown figure is:

image

where red line indicates x axis, green line indicates y axis, and blue line indicates z axis.

I find the cylinderActor.RotateY(90) do not make any change for the final result.

If cylinderActor.RotateY(90) is added, I expect the final shown figure is:

image

Why cylinderActor.RotateY(90) don’t rotate the cylinder along the y axis?

2

Consider VTK’s code for vtkProp3D::RotateY():

//------------------------------------------------------------------------------
// Rotate the Prop3D in degrees about the Y axis using the right hand
// rule. The axis is the Prop3D's Y axis, which can change as other rotations
// are performed.  To rotate about the world Y axis use RotateWXYZ (angle, 0,
// 1, 0). This rotation is applied before all others in the current
// transformation matrix.
void vtkProp3D::RotateY(double angle)
{
  this->IsIdentity = 0;
  this->Transform->PreMultiply();
  this->Transform->RotateY(angle);
  this->Modified();
}

The “PreMultipy()” is the important thing: it causes a rotation around the actor’s original Y axis, not the world Y axis.

To rotate around world Y axis, it is necessary to use RotateWXYZ(), because it uses "PostMultiply(). However, this will not rotate around the actor’s Origin. Instead, it will rotate around the world origin. In general, I would not recommend using this method onvtkActor.

//------------------------------------------------------------------------------
// Rotate the Prop3D in degrees about an arbitrary axis specified by the
// last three arguments. The axis is specified in world coordinates. To
// rotate an about its model axes, use RotateX, RotateY, RotateZ.
void vtkProp3D::RotateWXYZ(double degree, double x, double y, double z)
{
  this->IsIdentity = 0;
  this->Transform->PostMultiply();
  this->Transform->RotateWXYZ(degree, x, y, z);
  this->Transform->PreMultiply();
  this->Modified();
}

So here is my advice for doing rotations: use RotateX() and RotateY() and RotateZ(), but remember that these rotations are applied in the reverse ordering from what you expect.

1 Like