[vtkusers] crop an image

[francesco caruso]

Hello

I need to set alpha value for a rectangular
region of a vtkImageActor
this actor has been generated from an array of unsigned char
in the RGBA form, with alpha value initially set to 255.

How can I set, for example, alpha = 0 for the region 	(xmin,xmax,ymin,ymax)?

Here is the snippet of code which I use to get the imageActor

def UnsignedCharArrayToImageActor( array, xsize, ysize, bpp ): #return
an imageActor from an array
	imageImporter = vtk.vtkImageImport()
  	imageImporter.SetDataScalarTypeToUnsignedChar()
  	imageImporter.SetNumberOfScalarComponents(bpp+1)
  	imageImporter.SetDataSpacing( 1, 1, 1 )
  	imageImporter.SetWholeExtent( 0, xsize-1, 0, ysize-1, 0, 0 )
  	imageImporter.SetImportVoidPointer( array.GetVoidPointer(0) )
  	imageImporter.SetDataOrigin( -(xsize-1)/2, -(ysize-1)/2,0)
  	imageActor = vtk.vtkImageActor()
  	imageActor.SetInput( imageImporter.GetOutput() )
  	return imageActor

any help would be appreciated
thanks
francesco