vtkImageActor test

import vtk
import math

red = [255,0,0]
blue= [0,0,255]
green= [0,255,0]
black= [0,0,0]

class MyImage:
    # create image
    def __init__(self, ren ):
        self.vol = vtk.vtkImageData()
        self.vol.SetDimensions(512,512 ,1)
        self.vol.SetSpacing(1,1,1)
        self.vol.SetOrigin(0,0,0)
        self.vol.SetScalarTypeToUnsignedChar()
        self.vol.SetNumberOfScalarComponents(3)
        self.vol.AllocateScalars()
        self.scalars = vtk.vtkCharArray()
        # initialize the image
        for n in range(512):
            for m in range(512):
                x = 512/2 -n
                y = 512/2 -m
                d = math.sqrt(x*x+y*y)
                if ( d < 100 ):
                    #print "red"
                    col = red
                elif (d< 200 ):
                    col = green
                else:
                    col = blue
                for c in range(3):
                    self.scalars.InsertTuple1( n*(512*3) + m*3 +c, col[c] )
        self.vol.GetPointData().SetScalars( self.scalars )
        self.vol.Update()
        self.ia = vtk.vtkImageActor()
        self.ia.SetInput(self.vol)
        self.ia.InterpolateOff()
        self.vol.Update()

        #ren.AddActor( self.ia )

    def get_actor(self):
        return self.ia

    # modify image
    def modify(self):
        for n in range(512):
            for m in range(512):
                x = 512/2 -n
                y = 512/2 -m
                d = math.sqrt(x*x+y*y)
                if ( d < 66 ):
                    col = black
                    for c in range(3):
                        self.scalars.SetValue( n*(512*3) + m*3 +c, chr(col[c]) )
        self.vol.Update()


if __name__ == "__main__":
    ren = vtk.vtkRenderer()
    renWin = vtk.vtkRenderWindow()
    renWin.AddRenderer(ren)

    iren = vtk.vtkRenderWindowInteractor()
    iren.SetRenderWindow(renWin)
    interactorstyle = iren.GetInteractorStyle()
    interactorstyle.SetCurrentStyleToTrackballCamera()

    camera = vtk.vtkCamera()
    camera.SetClippingRange(-10, 500)
    camera.SetPosition(0, -1, 3000)
    camera.SetFocalPoint(255, 255, 0)

    myim = MyImage(ren)
    ia = myim.get_actor()
    ren.AddActor(ia)

    renWin.Render() # when this Render() is commented out, the results of modify() are visible
    # BUT, when Render() is called here, we don't see the results of modify()

    myim.modify()
    renWin.Render()
    iren.Initialize()
    iren.Start()