# Need Help with Image Arithmetic (vtkImageMathematics, subtraction, division by a scalar)
**URL:** https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716
**Category:** Support
**Created:** [September 6, 2019, 10:22pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716 "2019-09-06T22:22:10Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@MichelleKline](https://discourse.vtk.org/u/MichelleKline)
#### Post date: [September 6, 2019, 10:22pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/1 "2019-09-06T22:22:10Z")
Hi!
I have a Python script which reads two DICOM images. I want to subtract one image from the other, and then divide the resulting difference by a scalar. In other words:
output\_image = (input\_image2 - input\_image1) / some\_float
I am trying to use vtkImageMathematics to accomplish this, but I am running in to problems.
First, some cells in input\_image1 contain values that are larger than input\_image2. When I use vtkImageMathematics.SetOperationToSubtract(), the values of these cells in the output\_image appears to have overflowed the maximum allowable value the cell’s data type. I think the cell data type must be unsigned short, because, for example, 30 - 40 results in 65525. Is there a way to control the behavior of vtkImageMathematics.SetOperationToSubtract() such that negative values become 0?
Second, how to I apply another arithmetic operation to the output\_image?
Here is the subtraction:
imageMath = vtk.vtkImageMathematics()
imageMath .SetOperationToSubtract()
imageMath .SetInput1Data(input\_image2)
imageMath.SetInput2Data(input\_image1.GetOutput())
imageMath.Update()
How do I apply division by a scalar? the vtkImageMathematics object doesn’t seem to operate on itself, it seems to require an input from some other object. Also, there is no “SetOperationToDivisionByK” or anything similar. Here is what I unsuccessfully tried:
imageMath2= vtk.vtkImageMathematics()
imageMath2.SetInput1Data(imageMath.GetOutput())
K = -0.89 # for example
imageMath2.SetConstantK(1/K)
imageMath2.SetOperationToMultiplyByK()
imageMath2.Update()
Thanks in advance for any feedback.
Michelle Kline
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [September 7, 2019, 1:13am UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/2 "2019-09-07T01:13:53Z")
In Python, I would perform these operations using numpy. See description of the API here: [https://blog.kitware.com/improved-vtk-numpy-integration/](https://blog.kitware.com/improved-vtk-numpy-integration/)
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### Author:  [@MichelleKline](https://discourse.vtk.org/u/MichelleKline)
#### Post date: [September 8, 2019, 11:43pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/3 "2019-09-08T23:43:37Z")
Thank you, Andras. I will try to do my image arithmetic using numpy instead.
- Michelle
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### Author:  [@MichelleKline](https://discourse.vtk.org/u/MichelleKline)
#### Post date: [September 10, 2019, 1:00pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/4 "2019-09-10T13:00:41Z")
Hello again, Andras. I have tried using numpy to subtract one vtkImageData from another. But I am having trouble. In order to use numpy, I need to first convert the vtkImageData to a numpy array, then perform the arithmetic, then finally convert the results back to a vtkImageData.
The converting is not straightforward. Does it sound to you like I am trying to go about this correctly?
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### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [September 10, 2019, 4:48pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/5 "2019-09-10T16:48:40Z")
You can use this function to get read-write access to voxels of a vtkImageData object as a numpy array:
```python
def vtk_image_to_numpy_array(vimage):
import vtk.util.numpy_support
narray_shape = tuple(reversed(vimage.GetDimensions()))
narray = vtk.util.numpy_support.vtk_to_numpy(vimage.GetPointData().GetScalars()).reshape(narray_shape)
return narray
```
There is no need to “convert back” from numpy array to vtkImageData, just use this function to get read-write access to all the input and output vtkImageData.
Notes:
- Voxel indices in the numpy array are reversed (k,j,i vs. i,j,k).
- When you set values in the output image, you don’t want numpy to reallocate the array but instead set values in the existing array, so use the `[:]` operator. Example: `narrayOutput[:] = narrayInput*2+15`
- If the output image is used in a pipeline: call `vimageOutput.GetPointData().GetScalars().Modified()` when all your modifications are completed.
---
### Author:  [@MichelleKline](https://discourse.vtk.org/u/MichelleKline)
#### Post date: [September 10, 2019, 7:26pm UTC](https://discourse.vtk.org/t/need-help-with-image-arithmetic-vtkimagemathematics-subtraction-division-by-a-scalar/1716/6 "2019-09-10T19:26:04Z")
Thanks, Andras! I understand, and this is very helpful.
- Michelle