[R] displaying intensity through opacity on an image

[Ranjan Maitra]

Dear colleagues,

I have an image which I can display in the greyscale using image. On this image, for some pixels, which I know, I want to display their activity based on a third measure. One way to do that would be to color these differently, and use an opacity measure to display the third measure. An example of what I am trying to do is at:

http://www.public.iastate.edu/~maitra/papers/mrm02.pdf

page 26, for instance. There are two different kinds of voxels, given by greens and red. At the low end, there is transparency on the red scale and at the upper end there is opacity in the red and the green. 

A simpler example involving only one kind of voxels is on page 24 of the same paper. Either way, that figure was done using Matlab, but I was wondering how do i do this using R.

Any suggestions, please?

Many thanks and best wishes,
Ranjan