opacity

This is probably a FAQ, but I haven't found a solution. Essentially I
want to do

  gm convert -mask mask.eps skill.eps out.png

but try as I may, out.png is not transparent. I tried converting the eps to
png first, so mask.png is an anti-aliased black ellipse on a white background.
I then want to say, "make the opacity chanel of out.png the same as the
(any of R, G or B) channel of mask.png". I can see how you copy say Red to
Red, or Opacity to Opacity, but not from one to the other. (I have a feeling
that -mask should do exactly what I want, but it doesn't seem to..)

Thoughts?

Cheers,

Patrick


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