# Artifacts when using Gouraud or Phong interpolation
**URL:** https://discourse.vtk.org/t/artifacts-when-using-gouraud-or-phong-interpolation/913
**Category:** Support
**Created:** [May 6, 2019, 3:43pm UTC](https://discourse.vtk.org/t/artifacts-when-using-gouraud-or-phong-interpolation/913 "2019-05-06T15:43:11Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Gilthalas](https://discourse.vtk.org/u/Gilthalas)
#### Post date: [May 6, 2019, 3:43pm UTC](https://discourse.vtk.org/t/artifacts-when-using-gouraud-or-phong-interpolation/913/1 "2019-05-06T15:43:11Z")
I am getting odd artifacts when looking at a 3D OBJ with vtk8.2 (even though we did not see this in our tool using vtk6.3). When I choose flat shading it looks fine (but we really want the option to show both as before). Any thoughts on what may be causing this?
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### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [May 6, 2019, 6:02pm UTC](https://discourse.vtk.org/t/artifacts-when-using-gouraud-or-phong-interpolation/913/2 "2019-05-06T18:02:04Z")
It looks like some of the triangles are not oriented properly. You may be able to fix it using [vtkPolyDataNormals](https://vtk.org/doc/nightly/html/classvtkPolyDataNormals.html). How did you generate this polydata?
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### Author:  [@Gilthalas](https://discourse.vtk.org/u/Gilthalas)
#### Post date: [May 6, 2019, 6:08pm UTC](https://discourse.vtk.org/t/artifacts-when-using-gouraud-or-phong-interpolation/913/3 "2019-05-06T18:08:01Z")
I agree - but things looked just fine using vtk6.3.
I accidentally ran this with a 8.90 build of vtk earlier, so I’m working on getting 8.2 built so I can test that way.
This polydata was generated by another tool (I can’t say too much since the work is proprietary).