Create a vertical stack of images, each horizontally centered

Vertical_Stacker.groovy

#@ ImageJ ij
#@ File[] imageFiles
#@ File(style = "save") outFile
#@ boolean showStack

import java.util.Arrays
import net.imglib2.FinalInterval
import net.imglib2.type.numeric.integer.UnsignedByteType
import net.imglib2.util.Intervals

whiteValue = new UnsignedByteType(255)

centerHorizontal = { image, newWidth ->
	dims = Intervals.dimensionsAsLongArray(image)
	dims[0] = newWidth
	extended = ij.op().transform().extendValueView(image, whiteValue)
	return ij.op().transform().intervalView(extended, new FinalInterval(dims))
}

images = Arrays.stream(imageFiles).map{file -> ij.io().open(file.getAbsolutePath())}.collect()
maxWidth = images.stream().map{image -> image.dimension(0)}.collect().max()
centeredImages = images.stream().map{image -> centerHorizontal(image, maxWidth)}.collect()
stacked = ij.op().transform().concatenateView(centeredImages, 1)
if (showStack) ij.ui().show(stacked)
ij.io().save(stacked, outFile.getAbsolutePath())