Getopts script advice using realpath

Hi, All

Hope this is the most relevant section. I am writing a script and learning bash while doing it and come at a standstill.

Using the script I want, when giving the -d option, to then give the directory as a $d to use in the script. Below is just a test.sh to see if I can get it working. I would like realpath to output as a full directory address so the script understands where to output to:

#! /bin/bash
while getopts :d:f: options; do
case $options in
d) # Will be to make $d desired directory.
d=realpath $OPTARG
;;
f) # Will be to list files for script to run on.
f=$OPTARG
;;
\?) # Wrong option.
echo "Error: Invalid option"
exit;;
esac
done
echo "$d/$f"

At the moment it outputs: from bash test.sh -d ~/Desktop -f file.png

test.sh: line 6: /home/aleroncw/Desktop: Is a directory
/file.png

I'm new to script writing to this extent because I got the main script to work but not this option thing and it behaves differently when using $d than if I just ran it from command line. So far tried basedir and truepath and not got it working yet.

Any advice? Thanks for reading and looking forward to your responses.