# Change Pixel values of Images where expression
**URL:** https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465
**Category:** Beginner Questions
**Tags:** simpleitk
**Created:** [September 16, 2020, 7:45am UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465 "2020-09-16T07:45:08Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@reox](https://discourse.itk.org/u/reox)
#### Post date: [September 16, 2020, 7:45am UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465/1 "2020-09-16T07:45:08Z")
For example, I want to set all pixel values of an image to 1 where the current pixel value is \>1.
In numpy, this could be done in the following way:
```
x[x > 1] = 1
```
However, for Images that does not work:
```
# x -> SimpleITK.Image
x[x > 1] = 1
```
results in an `IndexError: too many indices for image`
But there are the `Mask` and `MaskNegated` functions. However, they are a little bit awkward to use.
Because if I want to specify x \> 1 = 1, then I would need to use the MaskNegated, which throws me an error:
```
sitk.MaskNegated(x, x > 1, 1))
RuntimeError: Exception thrown in SimpleITK MaskNegated: /tmp/SimpleITK/Code/Common/include/sitkProcessObject.h:354:
sitk::ERROR: Failure to convert SimpleITK image of dimension: 2 and pixel type: "8-bit unsigned integer" to ITK image of dimension: 2 and pixel type: "64-bit float"!
```
And when using Mask, you need to use the opposite for searching:
```
sitk.Mask(x, sitk.Not(x > 1), 1))
```
Is there no other, more convenient way?
---
### Author:  [@zivy](https://discourse.itk.org/u/zivy)
#### Post date: [September 16, 2020, 1:30pm UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465/2 "2020-09-16T13:30:41Z")
Hello @reox,
Below is a short function that will allow you to set an arbitrary value to the set of pixels defined by a mask. The mask is an image with pixel type sitkUint8, with values of zero and one. This is the returned type from all the logical SimpleITK operators (==,\<,\>,\<=,\>=), or any arbitrary mask you define.
```auto
import SimpleITK as sitk
def set_mask_value(image, mask, value):
msk32 = sitk.Cast(mask, sitk.sitkFloat32)
return sitk.Cast(sitk.Cast(image, sitk.sitkFloat32) *
sitk.InvertIntensity(msk32, maximum=1.0) +
msk32*value, image.GetPixelID())
img = sitk.ReadImage('patient.mha')
#set all values under -500 to 200
sitk.Show(set_mask_value(img, img<-500, 200))
```
---
### Author:  [@blowekamp](https://discourse.itk.org/u/blowekamp)
#### Post date: [September 16, 2020, 1:45pm UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465/3 "2020-09-16T13:45:21Z")
> [@reox](#):
>
> For example, I want to set all pixel values of an image to 1 where the current pixel value is \>1.
> In numpy, this could be done in the following way:
>
> ```auto
> x[x > 1] = 1
>
> ```
For this particular case you could do:
```auto
sitk.Clamp(x,upperBound=1)
```
Also `x <= 1` should be equivalent to `sitk.Not(x > 1)`
---
### Author:  [@blowekamp](https://discourse.itk.org/u/blowekamp)
#### Post date: [September 16, 2020, 3:30pm UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465/4 "2020-09-16T15:30:55Z")
> [@reox](#):
>
> Because if I want to specify x \> 1 = 1, then I would need to use the MaskNegated, which throws me an error:
>
> ```auto
> sitk.MaskNegated(x, x > 1, 1))
>
> RuntimeError: Exception thrown in SimpleITK MaskNegated: /tmp/SimpleITK/Code/Common/include/sitkProcessObject.h:354:
> sitk::ERROR: Failure to convert SimpleITK image of dimension: 2 and pixel type: "8-bit unsigned integer" to ITK image of dimension: 2 and pixel type: "64-bit float"!
>
> ```
Hrmm… Looking at this error further it looks like `MaskNegated` is expecting both images to be the same type. However, this is contraction ( like your expectations ) [common conventions for masks](https://simpleitk.readthedocs.io/en/master/conventions.html#mask-image-types-and-default-values) in SimpleITK. So that is a bug, that should be addressed.
I frequently use just the `Mask` filter because it is a dual dispatch type for filter which can accept any integer image for the mask argument making more flexible.
---
### Author:  [@reox](https://discourse.itk.org/u/reox)
#### Post date: [September 17, 2020, 12:43pm UTC](https://discourse.itk.org/t/change-pixel-values-of-images-where-expression/3465/5 "2020-09-17T12:43:13Z")
I suspected that there would be functions for certain tasks 🙂 I just have to read more of the API I guess 😉
`Clamp` seems to be very useful!
> [@blowekamp](#):
>
> Also `x <= 1` should be equivalent to `sitk.Not(x > 1)`
yep, that works well with the `Mask` too.