# How to get a tangent vector for a vertex in VTK9.0?

**URL:** https://discourse.vtk.org/t/how-to-get-a-tangent-vector-for-a-vertex-in-vtk9-0/4600
**Category:** Support
**Created:** [November 8, 2020, 12:23pm UTC](https://discourse.vtk.org/t/how-to-get-a-tangent-vector-for-a-vertex-in-vtk9-0/4600 "2020-11-08T12:23:04Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![Jone7](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/j/e36b37/32.png) [@Jone7](https://discourse.vtk.org/u/Jone7)
#### Post date: [November 8, 2020, 12:23pm UTC](https://discourse.vtk.org/t/how-to-get-a-tangent-vector-for-a-vertex-in-vtk9-0/4600/1 "2020-11-08T12:23:04Z")

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Hi,  
I want to use a MERL material on a polydata, but I’m confused about how to get a tangent vector for a vertex in vertex shader.  
Thanks!

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<div class="post-metadata">

### Author: ![Jone7](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/j/e36b37/32.png) [@Jone7](https://discourse.vtk.org/u/Jone7)
#### Post date: [November 9, 2020, 10:39am UTC](https://discourse.vtk.org/t/how-to-get-a-tangent-vector-for-a-vertex-in-vtk9-0/4600/2 "2020-11-09T10:39:41Z")

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I found vtkPolyDataTangents in VTK, but an error occurred “Texture coordinates are requested to generate tangents.”, is there any way to get the tangent without providing Tcoords?