# how to get the 0-th component for vector image

Let’s I have a vector image:

``````using VectorPixelType = itk::Vector< float, 3>;
using DisplacementFieldType = itk::Image< VectorPixelType, 3>;
DisplacementFieldType ::Pointer displacementField = DisplacementFieldType ::New();
``````

The value displacementField is a 3-dimension image, which has 3 component. Then, I want to multiply the x-component by 2, multiply the y-component by 3, and multiply the z-component by 4.

I know the image iterator is a helpful class for this function, just like:

``````using ImageIterator = itk::ImageRegionIterator<DisplacementFieldType>;
ImageIterator it(displacementField, displacementField->GetRequestedRegion());
while(!it.IsAtEnd())
{
it.Set(it.Get()*2);
}
``````

However, this code would multiply x-, y-, and z-component by 2. I want to seperately multiply the x-, y- and z- component by a factor. How can I do this?

You could use VectorIndexSelectionCastImageFilter to split the multicomponent image into separate images. Then you could multiply each component image by whatever factor you’d like. Finally you’d use the ComposeImageFilter to combine the separated component images back together into one vector image.

Here are the docs of the two filters:
https://itk.org/Doxygen/html/classitk_1_1VectorIndexSelectionCastImageFilter.html
https://itk.org/Doxygen/html/classitk_1_1ComposeImageFilter.html

A similar question came up earlier (except using SimpleITK and Python). You can see my example reply here:

You should be able to modify the pixels of a vector image using subscript operator `[]`, e.g. `pixel[0]*=2; // x`. Or to modify your iterator fragment:

``````using ImageIterator = itk::ImageRegionIterator<DisplacementFieldType>;
ImageIterator it(displacementField, displacementField->GetRequestedRegion());
while(!it.IsAtEnd())
{
auto p=it.Get();
p[0]*=2;
p[1]*=3;
p[2]*=4;
it.Set(p);
}
``````
2 Likes

Thank you very much. It is a very concise method.

In addition, I have another question. The displacementField is a displacement field from a 3D image registration. The 0-th component is x-component, 1-th component is y-component, and 2-th component is z-component? Am I right?

Or 0-th component is z-component, 1-th component is x-component, 2-th component is z-component?

0 should be `x`.

no ++it in the while loop above, probably typo

1 Like

Thank you. I have added it.