A ZSH command file to produce the various PNG files required for iOS and macOS app Icons using `sips` (scriptable image processing system) available on any macOS. The resulting PNGs in the "AppIcons" folder can then be dragged into Xcode's `Assets.xcassets` onto a placeholder for an AppIcon.

AppIcons.command

#!/bin/zsh
# Shell Script to create all relevant icons from an high resolution artwork

if [ "x$1" != "x" -a -f "$1" ] ; then
    INPUT=$1
else 
    INPUT="Artwork.png"
fi

if [ ! -f "$INPUT" ]; then
    echo "Input file not specified"
    exit 1
fi
mkdir -p AppIcons

while IFS= read -r line ; do
  FILENAME=${line% *}
  SIZE=${line#* }
  if [ "x$FILENAME" != "x" ] ; then
    COMMAND="sips -s format png \"$INPUT\" --resampleHeightWidth $SIZE $SIZE --out \"AppIcons/$FILENAME\""
    echo "$COMMAND"
    eval "$COMMAND"
  else
    echo ""
  fi
done <<EOF
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EOF

Ah, I thought the pixels where shifting from he resampling algorithm used in your script. I checked my 1024pt icon that I ran the script on and the text is actually off center. Thanks.