[magick-users] "-channel A -fx intensity" using -evaluate?

[David Vanderson]

Hello,

A few weeks ago we were doing cloud composition, which is close to 
something that I've been doing - setting the alpha channel of each pixel 
to the intensity of each pixel.  Anthony asked why we were using -fx, 
which is slow, instead of -evaluate.

I confess that I've tried and failed a few times to figure out how to do 
this with -evaluate.  Can anyone enlighten me?

Thanks,
Dave