Convert a PNM file into an X11 window dump file. More information: https://netpbm.sourceforge.net/doc/pnmtoxwd.html.
pnmtoxwd path/to/input_file.pnm > path/to/output_file.xwd
pnmtoxwd -directcolor path/to/input_file.pnm > path/to/output_file.xwd
pnmtoxwd -pseudodepth b path/to/input_file.pnm > path/to/output_file.xwd