Do feedback loops on an image with transparenty

feedback.sh

#!/bin/bash

# applies a black and white feedback effect to still images.
# usage: ./feedback.sh image.png 18
# this would apply the feedback effect 18 times to the image.png image and output the result as feedback.png

cp $1 feedback.png

loop=0

while [ $loop -le $2 ]

do convert \( feedback.png -distort SRT  1.03,0 -threshold 50% \) \( feedback.png -negate \) -background none -composite feedback.png

loop=$(($loop + 1))

done