[magick-users] compose with a mask with blurred edges

[Günter Bachelier]

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-examples/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