vtkImageResliceMapper has weird interpolation artefacts

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1

I am trying to add colors to a volume. It works for me using the vtkImageMapper, but the vtkImageResliceMapper gives weird interpolation artefacts (see images).

code for the initialisation of the actor/mapper:

    this.mapper = vtkImageResliceMapper.newInstance();
    this.slicePlane = vtkPlane.newInstance();
    this.slicePlane.setNormal(0, 0, 1);
    this.mapper.setSlicePlane(this.slicePlane);

    this.actor = vtkImageSlice.newInstance();
    this.actor.setMapper(this.mapper as any);

Code for the colors:

      const cfun = vtkColorTransferFunction.newInstance();
      const ofun = vtkPiecewiseFunction.newInstance();

      cfun.addRGBPoint(0, 0.0,0.0,0.0);
      ofun.addPoint(0, 0.0);
      cfun.addRGBPoint(1,1.0,1.0,1.0);
      ofun.addPoint(0, 0.0);

      this.actor.getProperty().setRGBTransferFunction(0, cfun);
      this.actor.getProperty().setPiecewiseFunction(0, ofun);
      this.actor.getProperty().setInterpolationTypeToNearest();

How can I ensure that there is no interpolation between the labels?

artefact
artefact752×700 201 KB

artefact_zoom
artefact_zoom726×682 121 KB

artefact_zoom_zoom
artefact_zoom_zoom680×676 63.1 KB

2

Could you please share what was the expected result and how it is different with the reslice mapper?

3

Hi Sankhesh,

This is the expected result:

result
result750×720 157 KB

The difference is that the border is a hard edge, and that the colored lines are not present

4

The CTF code looks strange, since “0” is added to the opacity function twice.

  cfun.addRGBPoint(0, 0.0,0.0,0.0);
  ofun.addPoint(0, 0.0);
  cfun.addRGBPoint(1,1.0,1.0,1.0);
  ofun.addPoint(0, 0.0);

Is this the CTF that is is applied to the labels? And is there just one label (binary image) or multiple labels? When using a binary image, it can be useful to vary only the opacity, while keeping the color constant:

  cfun.addRGBPoint(0, 0.0,1.0,0.0);
  ofun.addPoint(0, 0.0);
  cfun.addRGBPoint(1, 0.0,1.0,0.0);
  ofun.addPoint(1, 1.0);

In any case, I agree that in your example it looks like setInterpolationTypeToNearest() is ignored by the mapper.

5

Hi,

The code looked strange because I edited out some things wrong :slight_smile: We are trying to show multiple labels. In the picture below you can see the result of you function (I made all the labels green, and gave some an opacity of 0, and some 0.5):

Screenshot 2023-05-05 at 15.19.32
Screenshot 2023-05-05 at 15.19.32736×662 124 KB

Screenshot 2023-05-05 at 15.19.46
Screenshot 2023-05-05 at 15.19.46884×614 7.59 KB

you can see that the edges of the labels get rounded off, and when zooming in you can see that some type of interpolation is happening between the opaque and translucent parts. Do you have an idea what causes this?

6

Can you re-edit the code to fix the mistakes? That would help to clear up some of my confusion.

7 
    Object.keys(colormap).forEach((label) => {
        const l = Number(label);
        cfun.addRGBPoint(l, 0.0,1.0,0.0);
        if (l < 10) {
            ofun.addPoint(0, 0.0);
        } else {
            ofun.addPoint(1, 1.0);
        }

=> I generated the picture with the green labels like this.
We have a working version using vtkImageMapper, and I can confirm that we get the results of the first pictures that I sent when I change the interpolation method to ‘LINEAR’. I believe the vtkImageResliceMapper indeed ignores the interpolation settings. Do you know if we can patch it, or if there is an alternative way to get the same results as the vtkImageResliceMapper?

8

Hi, I think I found the problem. In ‘vtk.js/Sources/Rendering/OpenGL/ImageResliceMapper/index.js’, the import of of InterpolationType is done like

import InterpolationType from 'vtk.js/Sources/Rendering/Core/ImageProperty/Constants';

while it should be done like

import { InterpolationType } from 'vtk.js/Sources/Rendering/Core/ImageProperty/Constants';

I patched it in my version and it seems to work as expected!

9

Excellent @Tim_De_Bauw. Would you be able to submit a PR with your change?