# invert a lookup table
**URL:** https://discourse.vtk.org/t/invert-a-lookup-table/1210
**Category:** Support
**Created:** [June 25, 2019, 2:07pm UTC](https://discourse.vtk.org/t/invert-a-lookup-table/1210 "2019-06-25T14:07:44Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@PBrockmann](https://discourse.vtk.org/u/PBrockmann)
#### Post date: [June 25, 2019, 2:07pm UTC](https://discourse.vtk.org/t/invert-a-lookup-table/1210/1 "2019-06-25T14:07:44Z")
Is there a function to invert a lookup table from vtk (vtk.js) ?
This is a button to do that from paraview:
---
### Author:  [@PBrockmann](https://discourse.vtk.org/u/PBrockmann)
#### Post date: [June 25, 2019, 3:26pm UTC](https://discourse.vtk.org/t/invert-a-lookup-table/1210/2 "2019-06-25T15:26:26Z")
```auto
var colorMap = $("#selectColorMap").val();
var lookup = vtk.Rendering.Core.vtkColorTransferFunction.newInstance();
var preset = vtk.Rendering.Core.vtkColorTransferFunction.vtkColorMaps.getPresetByName(colorMap);
var table = preset.RGBPoints;
var tableInvert = [];
for (var i = table.length/4-1; i >= 0; i--) {
tableInvert.push(-1*table[i*4], table[i*4+1], table[i*4+2], table[i*4+3]);
}
preset.RGBPoints = tableInvert;
lookup.applyColorMap(preset);
lookup.setNanColor(1.0, 1.0, 1.0, 1.0);
lookup.build();
```
will do it.
Is there a better way ?
---
### Author:  [@Aron\_Helser](https://discourse.vtk.org/u/Aron_Helser)
#### Post date: [June 15, 2023, 10:54pm UTC](https://discourse.vtk.org/t/invert-a-lookup-table/1210/3 "2023-06-15T22:54:44Z")
I was looking into this, too, and there still doesn’t seem to be a way to invert a colormap in VTK proper. The ParaView button ends up calling `vtkSMTransferFunctionProxy::InvertTransferFunction()` which does a loop like this:
```auto
for (size_t cc = 0; cc < points.size(); cc++)
{
double& x = points[cc].GetData()[0];
x = (1.0 - x);
}
```
and then sets those points on the LUT proxy. So pretty close to your solution, except `1 - x` instead of `-x`.
---
### Author:  [@marlon.ma](https://discourse.vtk.org/u/marlon.ma)
#### Post date: [June 16, 2023, 1:17am UTC](https://discourse.vtk.org/t/invert-a-lookup-table/1210/4 "2023-06-16T01:17:06Z")
if (lut)
{
int index = lut-\>GetNumberOfTableValues();
unsigned char swap[4];
size\_t num = 4 \* sizeof(unsigned char);
vtkUnsignedCharArray\* table = lut-\>GetTable();
for (int count = 0; count \< --index; count++)
{
unsigned char\* rgba1 = table-\>GetPointer(4 \* count);
unsigned char\* rgba2 = table-\>GetPointer(4 \* index);
memcpy(swap, rgba1, num);
memcpy(rgba1, rgba2, num);
memcpy(rgba2, swap, num);
}
```
// force the lookuptable to update its InsertTime to avoid
// rebuilding the array
double temp[4];
lut->GetTableValue(0, temp);
lut->SetTableValue(0, temp);
} have a try
```