> I use rmagick to create a new image.
> How can I make this image with 50% transparency?

Take a look at the tutorials on the rmagick website ( 
http://rmagick.rubyforge.org/ ). This one features transparency/opacity 
:

http://rmagick.rubyforge.org/Polaroid/polaroid.html

I don't use rmagick very much, but I found these by googling "rmagick 
transparent".

Hope this helps,

Chris
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