[R-sig-Geo] semi-transparent layer with image()

[Ingo Holz]

Hi,

 I have two objects of "class image" (a list of x, y, z coordinates, where z is a 
matrix and x and y are row names / col names). 

 I would like to overlay one of this images on top of the other. As a semi-
transparent overlay. Is this possible using function image()?

 I suppose I could set some of the z-values as NA (every second?). Could 
this be a way to get a semi-transparent overlay?

  I know I could do this with a GIS or something similar...

Thank you for your help,
Ingo