Cut circle out of image with RMagick

require 'rmagick'

im = Magick::Image.read('walter.jpg').first

circle = Magick::Image.new 200, 200
gc = Magick::Draw.new
gc.fill 'black'
gc.circle 100, 100, 100, 1
gc.draw circle

mask = circle.blur_image(0,1).negate

mask.matte = false
im.matte = true
im.composite!(mask, Magick::CenterGravity, Magick::CopyOpacityCompositeOp)

im.write 'walter_circle.png'