# Question about BozoShader example
**URL:** https://discourse.vtk.org/t/question-about-bozoshader-example/14324
**Category:** Support
**Created:** [July 30, 2024, 1:16pm UTC](https://discourse.vtk.org/t/question-about-bozoshader-example/14324 "2024-07-30T13:16:19Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@whereismymoney](https://discourse.vtk.org/u/whereismymoney)
#### Post date: [July 30, 2024, 1:16pm UTC](https://discourse.vtk.org/t/question-about-bozoshader-example/14324/1 "2024-07-30T13:16:19Z")
The VTK BozoShader example, [https://examples.vtk.org/site/Cxx/Shaders/BozoShader](https://examples.vtk.org/site/Cxx/Shaders/BozoShader), is another truly excellent example, and I am studying it with great interest. I would like to add one line
```auto
actor->GetProperty()->EdgeVisibilityOn();
```
but this results in errors such as
```auto
ERR| vtkOpenGLPolyDataMapper (0x3a85730): Could not set shader program
```
Any advice on how I can switch on EdgeVisibility? Rather than just adding the one line I mentioned above, should I create a new actor using the same PolyData, and have just the edges visible for it?
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [July 30, 2024, 10:47pm UTC](https://discourse.vtk.org/t/question-about-bozoshader-example/14324/2 "2024-07-30T22:47:14Z")
Hello,
It’s likely `EdgeVisibilityOn()` sets its own shader, hence the error (my two cents on this). So, you likely need to add edge visibility to the shaders of the BozoShader example. Please, take a look at this [Shader-Based Wireframe Drawing (cgg-journal.com)](https://cgg-journal.com/2008-2/06/index.html) This is old, but there is a straightforward shader code in the bottom of the page.
I hope this helps,
PC