img_bg_rm

#!/usr/bin/env bash
set -euo pipefail

#
# clear black from an afni anatomical underlay image (-simple)
# or any background (top left pixel color) from any image
#
# will overwrite output image 
#
_img_bg_rm(){
   if [ $# -lt 2 ]; then
       cat  >&2 <<HEREDOC 
USAGE: $0 input.png output.png [-simple] [-white2red] [-bg=black] [-fuzz=8] [-palette]; use '-x' as output to autoname" 
  $0 input.ppm -x         # makes input_alpha.png
  $0 input.ppm out.png    # makes out.png
  $0 mask.png -x -palette # smaller file size w/pngnq to reduce color pallete 
  $0 input.png -x -simple # usues 8% fuzz, does not remove inner black
  $0 mask.png -x -simple -white2red # changes white to red
  $0 mask.png -x -simple -bg=white # make white the alpha color
HEREDOC
       return 1
   fi
   [ ! -r "$1" ] && 
       echo "input image $1 does not exist" >&2 && return 1

   # if you want to use the inputname as output
   local in="$1"
   local out="$2"

   # use for creating out if '-x' and for warning message
   local in_name=$(basename $in) # remove path
   in_name=${in_name%.*} # remove ext

   if [ "$2" == "-x" ]; then
      out=$(dirname $in)/${in_name}_alpha.png
   fi
   extraCmd=""
   simpleFuzz=8
   #change white to red
   if [ $# -ge 3 ] && [[ "$@" =~ -white2red ]]; then
      simpleFuzz=13
      extraCmd="-fuzz 100% -fill red -opaque white"
   fi

   # set color
   bgcolor="black"
   if [ $# -ge 3 ] && [[ "$@" =~ -bg=([^ ]+) ]]; then
      bgcolor=${BASH_REMATCH[1]}
   fi
   # fuzz, default to 8, 13 if white2red
   if [ $# -ge 3 ] && [[ "$@" =~ -fuzz=([^ ]+) ]]; then
      simpleFuzz=${BASH_REMATCH[1]}
   fi
 
   #echo "settings: fuzz=$simpleFuzz extra=$extraCmd bg=$bgcolor"

   # use simple and low fuzz (BTC derived 8% for afni exported images)
   # or complicated w/ nice smoothing and no within-shell alpha
   if [ $# -ge 3 ] && [[ "$@" =~ -simple ]]; then
      convert ${in} \
         -fuzz $simpleFuzz% -transparent $bgcolor \
         $extraCmd \
         ${out}
   else
      # https://stackoverflow.com/questions/9155377/set-transparent-background-using-imagemagick-and-commandline-prompt
      local color=$( convert "$in" -format "%[pixel:p{0,0}]" info:- )
      [ -z "$color" ] && echo "$in top pixel is no color!" >&2 && return 1

      # check size of input
      if ! identify "$in" | awk -F'[ x]' '($3*$4 > 1000^2){exit(1)}'; then
         echo "WARNING:  image is kind of big. convert may take a while!" >&2
         identify $in |sed 's/^/  /' >&2
         echo "consider: " >&2
         echo "  $(basename $0) $in $out -simple -bg=$color -fuzz=30" >&2
         echo " OR " >&2
         echo "  convert '$in' -scale 50% ${in_name}_small.png" >&2
         echo "  $(basename $0) ${in_name}_small.png ${@:2}" >&2
         echo >&2
      fi 

      set -x
      convert "$in"\
         -alpha off -bordercolor $color -border 1 \
         \( +clone -fuzz 30% -fill none -floodfill +0+0 $color \
         -alpha extract -geometry 200% -blur 0x0.5 \
         -morphology erode square:1 -geometry 50% \) \
         -compose CopyOpacity -composite -shave 1 \
         $extraCmd \
         "$out"
      set +x
   fi

   if [ $# -ge 3 ] && [[ "$@" =~ -palette ]]; then
      ! [[ $out =~ (.*).png$ ]] && echo "palette only works for png!" >&2 && return
      pngnq $out
      mv ${BASH_REMATCH[1]}-nq8.png $out
   fi
}

# run if script name is bg_rm
[ "$(basename $0)" == "img_bg_rm" ] && _img_bg_rm $@