interactor;
interactor->SetRenderWindow(renderWindow.GetPointer());
renderer->ResetCamera();
renderer->ResetCameraClippingRange();
renderWindow->Render();
interactor->Start();
}
```
here is the result when SetInterpolateScalarsBeforeMapping(1)
but it should be like this SetInterpolateScalarsBeforeMapping(0)
So is this a bug or something wrong with my code?
Need help and thanks.
---
### Author:  [@SkyFishers](https://discourse.vtk.org/u/SkyFishers)
#### Post date: [March 14, 2024, 2:39am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/2 "2024-03-14T02:39:50Z")
Do you have solve this? if do, how to solve this? Thanks!
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [December 23, 2024, 9:51am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/3 "2024-12-23T09:51:51Z")
Hi,
Did you have any luck with this? I am facing a similar error.
This is what I get before using InterpolateScalarsBeforeMappingOn()
and this is what happens after
Below is the Python code I am using
> #Extract vorticity
> mesh = gradient\_filter.GetOutput()
>
> #Create a lookup table
> lookup\_table = vtkLookupTable()
> lookup\_table.SetRange(-0.5, 0.5)
> lookup\_table.SetHueRange(0.6667,0.0)
> lookup\_table.Build()
> print(f"{lookup\_table}")
>
> #Create a mapper and actor
> mapper = vtkDataSetMapper()
> mapper.SetLookupTable(lookup\_table);
> mapper.SetInputData(mesh)
> mapper.ColorByArrayComponent(“Vorticity”,2)
> mapper.SetScalarRange([-0.5,0.5])
> mapper.SetScalarModeToUsePointFieldData()
> #mapper.SetInterpolateScalarsBeforeMapping(1)
> mapper.InterpolateScalarsBeforeMappingOn()
> mapper.Update()
>
> #Check the scalar range of “Vorticity”
> point\_data = mesh.GetPointData()
> vorticity\_array = point\_data.GetArray(“Vorticity”)
> print(f"{point\_data.GetArray(‘Vorticity’)}“)
> min\_value = vorticity\_array.GetRange(2)[0]
> max\_value = vorticity\_array.GetRange(2)[1]
> print(f"Vorticity Scalar Range: {min\_value} to {max\_value}”)
> actor = vtkActor()
> actor.SetMapper(mapper)
> actor.GetProperty().SetRepresentationToSurface()
>
> #Create a renderer and set up a background color
> renderer = vtkRenderer()
> renderer.AddActor(actor)
> renderer.SetBackground(1.0, 1.0, 1.0) # Set background to white
>
> #Create a render window
> render\_window = vtkRenderWindow()
> #render\_window.SetOffScreenRendering(1)
> render\_window.AddRenderer(renderer)
> render\_window.SetSize(1080, 1080)
> camera = renderer.GetActiveCamera()
> camera.SetPosition(0, 0, 30) # Position of the camera
> camera.SetFocalPoint(0, 0, 0) # Focus on the center of the object
>
> #Render the scene
> render\_window.Render()
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [December 23, 2024, 3:15pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/4 "2024-12-23T15:15:33Z")
You almost got it right. Ask the mapper to use the scalar range already given to the lookup table instead of using the mapper’s default `ScalarRange` which is [0,1]. This is why everything is navy blue in your screenshot.
```cpp
mapper->SetUseLookupTableScalarRange(true);
```
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [December 26, 2024, 6:03am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/5 "2024-12-26T06:03:16Z")
I am facing the issue even with using SetUseLookupTableScalarRange(true) flag.
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [December 26, 2024, 1:55pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/6 "2024-12-26T13:55:36Z")
Can you please share dataset corresponding to the `mesh` in your code?
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [December 27, 2024, 7:18am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/7 "2024-12-27T07:18:07Z")
[MESH.DAT](https://discourse.vtk.org/uploads/short-url/sk3KzHeQlFaPOeVORuToi4wNtjv.DAT) (3.5 MB)
[4475000.B](https://discourse.vtk.org/uploads/short-url/ha2pWGB6F7Ps5PFy1QynKyaA8t4.B) (1.4 MB)
[postprocess\_2D.so](https://discourse.vtk.org/uploads/short-url/iQbl0NFxBD75vUR3ew8Zs7tpaI8.so) (47.0 KB)
[postprocess\_2D.py](https://discourse.vtk.org/uploads/short-url/rsnPAZfIp0Zywnb2fj26R6PenP.py) (3.8 KB)
I have a MESH.DAT file which I read using Tecplot reader. My dataset (.B file) contain arrays which are read through a Fortran code wrapped in Python library. I add those arrays to my mesh block and then create a rendered scene. I have also attached the code and library I am using.
Thank you for your time 🙂
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [December 27, 2024, 3:04pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/8 "2024-12-27T15:04:10Z")
Could you instead send a .vtu file please? `.so` shared libs are not easily redistributable, it won’t work on my machine.
Instead, can you modify your postprocess\_2D.py to generate a `.vtu` file? add these lines after you get output of gradient filter and share the `mesh.vtu` file here.
```py
# Extract the gradients of u and v from the gradient filter output
mesh = gradient_filter.GetOutput()
writer = vtkXMLUnstructuredGridWriter()
writer.SetInputData(mesh)
writer.SetFileName("mesh.vtu")
writer.Update()
```
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [December 28, 2024, 9:57am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/9 "2024-12-28T09:57:34Z")
[output.vtu](https://discourse.vtk.org/uploads/short-url/hdMXyI7ORsrEqrZP47kI401nocN.vtu) (2.7 MB)
This is the vtu file. Although, the interpolation works alright when I load this in ParaView.
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [January 2, 2025, 7:41am UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/10 "2025-01-02T07:41:58Z")
Hi!
Any luck with the data?
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [January 2, 2025, 2:50pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/11 "2025-01-02T14:50:26Z")
Yes, you have to additionally set the vector component on the lookup table so that it generates texture coordinates correctyl.
Call `lookup_table.SetVectorComponent(2)` anytime before rendering. It doesn’t **need** to be done before the lookup table is built.
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [January 2, 2025, 2:58pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/12 "2025-01-02T14:58:11Z")
Complete code that uses vtk \>= 9.4.0
```py
import vtk
reader = vtk.vtkXMLUnstructuredGridReader(file_name="./output.vtu")
#Create a lookup table
lut = vtk.vtkLookupTable(range=(-0.5, 0.5), hue_range=(0.6667, 0.0))
lut.Build()
mapper = vtk.vtkDataSetMapper(
interpolate_scalars_before_mapping=True,
lookup_table=lut,
scalar_mode=vtk.VTK_SCALAR_MODE_USE_POINT_FIELD_DATA,
use_lookup_table_scalar_range=True,)
mapper.ColorByArrayComponent("Vorticity", 2)
mapper.SetScalarModeToUsePointFieldData()
# create a connection from reader's output to mapper's input
reader >> mapper
actor = vtk.vtkActor(mapper=mapper)
#Create a renderer and set up a background color
renderer = vtk.vtkRenderer(background=(1, 1, 1))
renderer.AddActor(actor)
camera = renderer.GetActiveCamera()
camera.SetPosition(0, 0, 30) # Position of the camera
camera.SetFocalPoint(0, 0, 0) # Focus on the center of the object
#Create a render window
window = vtk.vtkRenderWindow(size=(1080, 1080))
window.AddRenderer(renderer)
# Ensure lookup table uses the correct vector component of "Vorticity"
lut.SetVectorComponent(2)
interactor = vtk.vtkRenderWindowInteractor(render_window=window)
interactor.Start()
```
---
### Author:  [@Hamza\_Ali](https://discourse.vtk.org/u/Hamza_Ali)
#### Post date: [January 5, 2025, 1:02pm UTC](https://discourse.vtk.org/t/vtkmapper-interpolate-scalars-before-mapping-does-not-work-as-expected/9601/13 "2025-01-05T13:02:52Z")
It worked. Thank you very much!