how to get the 0-th component for vector image

[dchen (Dave Chen)]

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:

add alpha channel to an image in simpleitk

You can use the VectorIndexSelectionCast function to split an RGB image into separate components. Then create an alpha image of matching dimensions and use the Compose function to combine all four channels into an RGBA image. Here’s an example rgb = sitk.Image(100,100,sitk.sitkVectorUInt8) r = sitk.VectorIndexSelectionCast(rgb, 0) g = sitk.VectorIndexSelectionCast(rgb, 1) b = sitk.VectorIndexSelectionCast(rgb, 2) a = sitk.Image(100,100,sitk.sitkUInt8) rgba = sitk.Compose(r, g, b, a)