# Update a custom uniform in a modified shader
**URL:** https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366
**Category:** Support
**Created:** [July 17, 2019, 12:35pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366 "2019-07-17T12:35:19Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Jellby](https://discourse.vtk.org/u/Jellby)
#### Post date: [July 17, 2019, 12:35pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/1 "2019-07-17T12:35:19Z")
I have modified the default `vtkOpenGLPolyDataMapper` along the lines of [https://github.com/dmreagan/vtk-shaders](https://github.com/dmreagan/vtk-shaders) (case 1). I have also added a new uniform, which I can set as it is done for `diffuseColor` in [https://github.com/Kitware/VTK/blob/master/Rendering/OpenGL2/Testing/Python/TestUserShader2.py](https://github.com/Kitware/VTK/blob/master/Rendering/OpenGL2/Testing/Python/TestUserShader2.py). However, as far as I can see, that sets a hardcoded fixed value for `diffuseColor`, and I would like something I can change “live” (with e.g. a slider on a GUI). This works fine for default uniforms like Opacity: I just call `actor.GetProperty().SetOpacity(value)` and the mapper udates automatically. How can I do the same with a custom uniform?
This was already asked in [http://vtk.1045678.n5.nabble.com/Update-Custom-Uniforms-for-Shaders-td5739937.html](http://vtk.1045678.n5.nabble.com/Update-Custom-Uniforms-for-Shaders-td5739937.html), but the “solution” is basically the same code as above, it seems to be giving a fixed value to `diffuseColor`.
---
### Author:  [@dmreagan](https://discourse.vtk.org/u/dmreagan)
#### Post date: [July 25, 2019, 4:18pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/2 "2019-07-25T16:18:22Z")
In the example from _TestUserShader2.py_, the value of `diffuseColor` is set inside the callback, as follows:
```python
@vtk.calldata_type(vtk.VTK_OBJECT)
def vtkShaderCallback(caller, event, calldata):
program = calldata
if program is not None:
diffuseColor = [0.4, 0.7, 0.6]
program.SetUniform3f("diffuseColorUniform", diffuseColor)
```
…
```python
mapper.AddObserver(vtk.vtkCommand.UpdateShaderEvent, vtkShaderCallback)
```
`vtkShaderCallback` runs each time an `UpdateShaderEvent` is observed, and each time `diffuseColorUniform` is being set to [0.4, 0.7, 0.6]. So you want to assign `diffuseColor`'s value somewhere outside of `vtkShaderCallback`, like in your GUI slider callback. This way, each time `vtkShaderCallback` runs, it will fetch the new value for `diffuseColor` and set the uniform accordingly.
---
### Author:  [@Jellby](https://discourse.vtk.org/u/Jellby)
#### Post date: [July 25, 2019, 4:38pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/3 "2019-07-25T16:38:20Z")
Thanks. I have solved it by subclassing `vtkOpenGLPolyDataMapper`:
```
class AOIMapper(vtk.vtkOpenGLPolyDataMapper):
def __init__ (self, *args, **kwargs):
super(). __init__ (*args, **kwargs)
self.MyCustomUniform = 0.0
self.AddShaderReplacement(...)
self.AddObserver(vtk.vtkCommand.UpdateShaderEvent, self.update_shader)
@vtk.calldata_type(vtk.VTK_OBJECT)
def update_shader(self, caller, event, program):
if program is not None:
program.SetUniformf("MyCustomUniform", self.MyCustomUniform)
```
And then in the rest of the code, something like:
```
actor.GetProperty().SetOpacity(opacity_value)
actor.GetMapper().MyCustomUniform = custom_value
```
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [July 25, 2019, 4:47pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/4 "2019-07-25T16:47:29Z")
Are you aware of the recently introduced [vtkUniforms](https://vtk.org/doc/nightly/html/classvtkUniforms.html) class that allows you to get/set custom uniforms anytime, anywhere, without worrying about adding callback functions, etc. Uniforms are accessible via [vtkShaderProperty::Get…CustomUniforms()](https://vtk.org/doc/nightly/html/classvtkShaderProperty.html) methods.
@Simon are there some simple standalone examples?
---
### Author:  [@Jellby](https://discourse.vtk.org/u/Jellby)
#### Post date: [July 25, 2019, 4:57pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/5 "2019-07-25T16:57:29Z")
How recently? I need it to work with VTK 8.1.2 (pip version). I remember I’ve seen the stuff, but either it wasn’t working or I couldn’t figure out how.
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [July 25, 2019, 5:25pm UTC](https://discourse.vtk.org/t/update-a-custom-uniform-in-a-modified-shader/1366/6 "2019-07-25T17:25:16Z")
It was added a few months ago, so it is not included in VTK 8.1.2.
If you need a Python distribution that contains recent VTK then you can use Python included in a recent Preview Release of [3D Slicer](https://download.slicer.org/). VTK is already bundled and you can pip install any other packages.