[vtkusers] Rendering artifact at opacity<1

[marcin.krotkiewski]

Dear developers,

I am observing strange rendering artifacts on my triangulated surface 
when I use transparency. The artifacts are observed at certain angles 
only. I've attached 3 pictures:

  - opacity_1.png shows the model as it looks without transparency
  - opacity_09.png shows the model at an angle chosen such that the 
artifact on the surface is visible.
  - opacity_0.9_behind.png shows the same model with transparency 
rotated around to a view that does not show the artifact

I am using vtk 8.0 with the OpenGL 2 rendering backend.

Is this a bug, or am I doing sth wrong?

Thanks!

Marcin

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