#!/bin/sh

# pnminterp 1.0.1 (2000-Sep-03-Sun)
# Adam M. Costello <amc@cs.berkeley.edu>

# pnminterp <factor> [ <infile> ]
#
# Scales up an image by an integer factor using linear interpolation.
# Note that the depth of the input file affects the round-off error,
# so you may wish to deepen the image using pnmdepth before calling
# pnminterp.

factor=$1
shift

fail() {
  echo "$*" >&2
  exit 1
}

[ $# -le 1 ] || fail 'at most two arguments are allowed'
case "$factor" in *[!0-9]* ) fail 'factor must be an integer' ;; esac
[ $factor -gt 0 ] || fail 'factor must be positive'
doublefactor=`expr $factor \* 2`
filterwidth=`expr $doublefactor - 1`
maxval=`expr $filterwidth \* 2`

tmpprefix=/tmp/pnminterp$$
xfilter=$tmpprefix-x.pgm
yfilter=$tmpprefix-y.pgm

{ echo P2
  echo 1 1
  echo $maxval
  echo $doublefactor
} | pnmscale -width $filterwidth -height 1 > $xfilter

pnmflip -xy $xfilter > $yfilter

pnmscale $doublefactor ${1+"$@"} | pnmconvol $xfilter | pnmconvol $yfilter |
  pnmscale 0.5

rm -f $xfilter $yfilter