compose with a mask with blurred edges

Hello,

I want to transform a greyscale image (represented as RGB
png) in a mask
where white
is transparent, black is opaque and the greyscales are more
or less
transparent dependent
on their brightness. The example image is a shape that
should become a
mask with blurred
edges
http://www.vi-anec.de/Trance-Art/IM-example
s/IMCompose/SShape-002.png
The mask should be used in the compose operation to get a
continuos/soft
change.
How could this be achieved with IM/PerlMagick? Thank you!

regards
Günter


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