
I was
so impressed
with Maxim Stepin's
hq3x and related
algorithms for
scaling up sprite art that I ported his code to Linux
and wrote a
pnmhqxscale wrapper around it to use
with the
netpbm tools.
The port is a hack, but it works OK. You can
download it for
yourself.
pngtopnm s.png | pnmhqxscale -3 | pnmtopng > 3x.png