Some alpha-aware resizing script based on netpbm

scale.sh

#!/usr/bin/env bash

function usage() {
    echo "USAGE: $0 < input.png > output.png" 1>&2
    echo "USAGE: env gif=1 $0 < input.gif > output.png" 1>&2
    exit -1
}

if tty < /dev/stdin >&/dev/null; then
    usage
fi

if tty < /dev/stdout >&/dev/null; then
    usage
fi

set -e
image="$(mktemp /tmp/png_original_XXXXXX)"
alpha="$(mktemp /tmp/png_alpha_XXXXXX)"
alpha_2="$(mktemp /tmp/png_alpha_XXXXXX)"
bitmap="$(mktemp /tmp/png_bitmap_XXXXXX)"
trap "rm -f $image $alpha $alpha_2 $bitmap" EXIT
cat > $image

function intermediate {
    pamscale -linear -xysize 1500 1500
}

command="pngtopnm"

if [ ! "$gif" = "" ]; then
    set -x
    giftopnm -alpha=$alpha_2 < $image >/dev/null
    intermediate < $alpha_2 > $alpha
    command=giftopnm
else
    pngtopnm -alpha < $image | intermediate > $alpha
fi

$command < $image | intermediate > $bitmap

pnmtopng -compress 9 -filter 0 -force -alpha=$alpha < $bitmap