#!/bin/sh
#
# isblack - are all of the files supplied as arguments black images?
#
# $Id$

# thank you, http://www.imagemagick.org/discourse-server/viewtopic.php?t=17111

IsBlack()
{
   convert "$1" -format '%[mean]' info: 2>/dev/null |
   grep -q '^0$'
}

for f in "$@"
do
  if IsBlack "$f"
  then
    :
  else
    exit 1  # no
  fi
done

exit 0  # yes