# Efficient use of vtkPolyDataNormals to show smooth surfaces
**URL:** https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635
**Category:** Support
**Tags:** python
**Created:** [February 7, 2023, 9:51am UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635 "2023-02-07T09:51:39Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@sotodela](https://discourse.vtk.org/u/sotodela)
#### Post date: [February 7, 2023, 9:51am UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/1 "2023-02-07T09:51:39Z")
Hi,
I am using vtkLookupTable() and vtkUnsignedCharArray() to re-color a mesh. The Array that contains the colors is modified several times interactively. The problem I have is that to show the actor with a smooth surface I am using vtkPolyDataNormals, as shown below, but I have to create vtkPolydataNormals() everytime I need to recolor the actor. I can create the mapper only once and modify it but it has not been possible to do the same with the vtkPolydataNormals().
Is there a way to achieve a smooth surface in an efficient manner?
```auto
def Recolor_Actor(self, mesh):
normals = vtkPolyDataNormals()
normals.SetInputData(mesh)
normals.SetFeatureAngle(80)
normals.AutoOrientNormalsOn()
normals.Update()
self.mapper.SetInputData(normals.GetOutput())
self.mapper.ScalarVisibilityOn()
actor.SetMapper(self.mapper)
```
Thanks!
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [February 7, 2023, 1:09pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/2 "2023-02-07T13:09:31Z")
Re-applying vtkPolyDataNormals shouldn’t be necessary. Can you show the code where you apply the lookup table?
---
### Author:  [@sotodela](https://discourse.vtk.org/u/sotodela)
#### Post date: [February 7, 2023, 1:27pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/3 "2023-02-07T13:27:45Z")
Hi, thank you for you reply, this in an example of where I apply the lookup table:
```
lut = self.CreateLUTtable(self.min, self.max)
self.colors_init.SetNumberOfComponents(3)
self.colors_init.Fill(255)
for h in range(self.radius_list.GetNumberOfIds()):
dcolor = 3 * [0.0]
index_id = self.radius_list.GetId(h)
lut.GetColor(self.efieldnorms[index_id], dcolor)
color = 3 * [0.0]
for j in range(0, 3):
color[j] = int(255.0 * dcolor[j])
self.colors_init.InsertTuple(index_id, color)
mesh.GetPointData().SetScalars(self.colors_init)
self.Recolor_Actor(mesh)
```
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [February 7, 2023, 2:00pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/4 "2023-02-07T14:00:25Z")
You can apply the lookup table with the mapper instead of changing the mesh dataset:
```python
mapper.SetLookupTable(lut)
mapper.SetScalarRange(self.min, self.max)
```
Changing the appearance of the dataset via the mapper and/or actor is always more efficient than modifying the dataset itself. Modifying the dataset always requires that downstream filters have to re-execute.
To be more clear, what I’m saying is that the mesh scalars should be the `efieldnorms`, not the colors. The coloring can be applied (very efficiently) by the mapper.
---
### Author:  [@sotodela](https://discourse.vtk.org/u/sotodela)
#### Post date: [February 7, 2023, 2:16pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/5 "2023-02-07T14:16:06Z")
Hi,
Ok thank you! I will look into it. I just found examples using the LUT this way, since I am just coloring a circle that moves interactively on the surface of the mesh.
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [February 7, 2023, 2:24pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/6 "2023-02-07T14:24:55Z")
The mapper applies the LUT to the whole mesh, not just part of the mesh. So if you’re only recoloring part of the mesh, then what I suggested will not work.
Instead, you can generate the normals _before_ recoloring the mesh, instead of after. The `vtkPolyDataNormals` filter can be used on meshes that have no scalars.
---
### Author:  [@sotodela](https://discourse.vtk.org/u/sotodela)
#### Post date: [February 7, 2023, 2:28pm UTC](https://discourse.vtk.org/t/efficient-use-of-vtkpolydatanormals-to-show-smooth-surfaces/10635/7 "2023-02-07T14:28:41Z")
Ok, thank you very much!