Annoying Dots in Snapshots on Volume Render

Support
1

I have an issue on taking snapshots on volume render visualization.
The following is my procedure.
[1] add renderer
ren1 = vtk.vtkRenderer()
ren1.AddVolume(volume)
ren1.SetBackground(0,0,0)
ren1.GetActiveCamera().Azimuth(theta)
ren1.ResetCamera()
ren1.ResetCameraClippingRange()

[2] add render window
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren1)
renWin.SetSize(600, 600)

[3] add windowto_image_filter
windowto_image_filter = vtk.vtkWindowToImageFilter()
windowto_image_filter.SetInput(renWin)
windowto_image_filter.SetScale(1)
windowto_image_filter.SetInputBufferTypeToRGBA()
windowto_image_filter.ReadFrontBufferOff()
windowto_image_filter.Update()

[4] save to image
writer = vtk.vtkPNGWriter()
writer.SetFileName(pngname)
writer.SetInputConnection(windowto_image_filter.GetOutputPort())

In the VTK interactive window, I got the following result

图片
图片710×741 46.1 KB

but the generated PNG image, I got the following ( notice lots of dots, needed to get rid of )

图片
图片710×741 49.4 KB

Does anyone have idea what these dots are? And how can they be removed???

Thanks in advance

2

I doubt if rendering process was not finished when taking snapshots PNG images.

I tried to add renWin.Start() and renWin.WaitForCompletion() in [2],
but it didn’t remove those annoying dots.