# lighting bug with Gauraud shading of 'inverted' models

**URL:** https://discourse.paraview.org/t/lighting-bug-with-gauraud-shading-of-inverted-models/881
**Category:** ParaView Support
**Created:** [November 10, 2018, 7:08pm UTC](https://discourse.paraview.org/t/lighting-bug-with-gauraud-shading-of-inverted-models/881 "2018-11-10T19:08:47Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![Marcin\_Krotkiewski](https://discourse.paraview.org/user_avatar/discourse.paraview.org/marcin_krotkiewski/32/2185_2.png) [@Marcin\_Krotkiewski](https://discourse.paraview.org/u/Marcin_Krotkiewski)
#### Post date: [November 10, 2018, 7:08pm UTC](https://discourse.paraview.org/t/lighting-bug-with-gauraud-shading-of-inverted-models/881/1 "2018-11-10T19:08:47Z")

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Hi,

I came across this problem: when I transform my model with Z scale = -1, my transformed model is much darker when I use Gauraud shading, compared to Flat shading. Two pictures and the VTK model attached. The ‘Flat’ inverted model looks similar to the original. Not sure, but I guess this could be a VTK problem.

Any ideas? Should I report it there instead?

Thanks!

Marcin

 ![flat](https://discourse.paraview.org/uploads/default/original/1X/d3e07a70125be408cc7c64554030c1cbfe2dd3a1.png)  
 ![gauraud](https://discourse.paraview.org/uploads/default/original/1X/9cd9ee441e495dc0a2497b0618ddf1972bd31ddd.png)  
[Untitled.vtk](https://discourse.paraview.org/uploads/default/original/1X/c97d00edc3a848f3fbfddc4ceba21a2b2f9512a6.vtk) (893.2 KB)

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### Author: ![Dave\_DeMarle](https://discourse.paraview.org/user_avatar/discourse.paraview.org/dave_demarle/32/27_2.png) [@Dave\_DeMarle](https://discourse.paraview.org/u/Dave_DeMarle)
#### Post date: [November 11, 2018, 2:07pm UTC](https://discourse.paraview.org/t/lighting-bug-with-gauraud-shading-of-inverted-models/881/2 "2018-11-11T14:07:09Z")

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It is dark because the negative scaling flipped all of the normals, see if the generate surface normals filter flips them back or use the caclculator filter to do so.

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### Author: ![Marcin\_Krotkiewski](https://discourse.paraview.org/user_avatar/discourse.paraview.org/marcin_krotkiewski/32/2185_2.png) [@Marcin\_Krotkiewski](https://discourse.paraview.org/u/Marcin_Krotkiewski)
#### Post date: [November 12, 2018, 8:17am UTC](https://discourse.paraview.org/t/lighting-bug-with-gauraud-shading-of-inverted-models/881/3 "2018-11-12T08:17:11Z")

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You’re right. If I first flip the model, and then compute the normals - the lighting is correct. But if I flip the result of the Generate Normals filter, light is wrong.