# Unexpected background due to opacity in volume rendering

**URL:** https://discourse.vtk.org/t/unexpected-background-due-to-opacity-in-volume-rendering/13714
**Category:** Support
**Tags:** release-process
**Created:** [April 15, 2024, 5:53pm UTC](https://discourse.vtk.org/t/unexpected-background-due-to-opacity-in-volume-rendering/13714 "2024-04-15T17:53:52Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![huaweiping](https://discourse.vtk.org/user_avatar/discourse.vtk.org/huaweiping/32/8231_2.png) [@huaweiping](https://discourse.vtk.org/u/huaweiping)
#### Post date: [April 15, 2024, 5:53pm UTC](https://discourse.vtk.org/t/unexpected-background-due-to-opacity-in-volume-rendering/13714/1 "2024-04-15T17:53:52Z")

</div>

Hi all, I’m using the vtkVolume to render my oceanography dataset. The valid scalar value is originally in a NetCDF file accompanied by NaN, so I read it in C++ and convert it into a vtkImageData. It seems the NaN is set to zero based on my check, and I even manually set the value around zero to zero for the scalar data.

```auto
double s[1];
if(OW_vals_Vec[z][y][x]<0.0001 && OW_vals_Vec[z][y][x]>-0.0001)
    s[0] = 0;
else
    s[0] = OW_vals_Vec[z][y][x];
scalars->InsertTuple(omegaIndex,s);

```

However, when I do the volume rendering, I use the vtkPieceWiseFunction to set the opacity. The opacity function is coded as below:

```auto
vtkNew<vtkPiecewiseFunction> opacityFunc;
opacityFunc->AddPoint(-0.0001, 0.1);
opacityFunc->AddPoint(0, 0);

```

Since most of my data is between (-0.01, -0.001), I expect to see the volume itself with a clean background. However, the result is covered by a whole cube where it is supposed to be the NaN value in the original dataset.

Here are some examples of the rendering results covered by the NaN value.

 ![Screenshot from 2024-04-15 13-35-03](https://discourse.vtk.org/uploads/default/original/2X/0/0072a26ee34562c651f1aad953d58fe92da7b155.png)  
 ![Screenshot from 2024-04-15 13-35-27](https://discourse.vtk.org/uploads/default/original/2X/2/297e560c3a7d1420e4af6ee92389850693faff06.png)

The weirdest thing is that if I set the upper bound of PieceWiseFunction to -0.000095, then the problem disappeared, and I get what I expected.

```auto
vtkNew<vtkPiecewiseFunction> opacityFunc;
opacityFunc->AddPoint(-0.000096, 0.1);
opacityFunc->AddPoint(-0.000095, 0);

```

 ![Screenshot from 2024-04-15 13-43-09](https://discourse.vtk.org/uploads/default/original/2X/e/e4793559614ccaad58ad0cc5da30747f15d5a95c.png)

If I tweak the upper bound of the opacityFunc closer to zero (such as (-0.000095, -0.000094)), the problem will return. I believe that all the background values are between (-0.000095 and -0.000094), but I’m not sure what happened since there is not supposed to be any value (and I even manually set it to zero). Meanwhile, if I manually set the value to 0, then I will not see the black background or any data in the volume rendering.

Here are some of my guesses:  
(a) Something is wrong when converting the data to the vtkImageData. However, the value is manually checked and doesn’t have any value in that range.  
(b) A bug in the volume rendering function?  
(c) Precision of the data. (Such as some internal problem in the C++ or something else).

Sorry, I can’t create a minimal reproducible example as this is a huge project related to a bunch of data files but I’ll be really appreciate if someone can give me some ideas about this.