#!/bin/sh

# ppmtogray 0.1.0 (2004-Jan-19-Mon)
# Adam M. Costello
# http://www.nicemice.net/amc/

# ppmtogray [file]
#
# Converts a PPM file to a PGM file.  Computes luminance, unlike
# ppmtopgm, which computes luma.  We assume that the input is sRGB, and
# we produce sRGB output.  The output maxval is always 65535 (16-bit
# samples); pipe through pnmdepth if you want something else.

# Formula for luminance:
#
# 0.21264 R + 0.71517 G + 0.07219 B
#
# Formula for luma (used by ppmtopgm):
#
# 0.299 R' + 0.587 G' + 0.114 B'
#
# (The primes indicate gamma-encoded components.)


# Create a filter to be used later:

filter=/tmp/ppmtogray-$$.ppm
cat > $filter << EOF
P3
1
1
65534
51894
65534
49798
EOF

# Get 16-bit linear samples:
pnmdepth 65535 $1 | pnmgamma -srgbramp -ungamma |

# Use preprocessing and postprocessing to effectively alter the
# coefficients of ppmtopgm:

pnmconvol $filter 2>&- | ppmtopgm |
#pgmnorm -bvalue 0 -wvalue 53790 |
# As of this writing a bug in pgmnorm prevents it from handling 16-bit
# values, but 15-bit is okay.  Once the bug is fixed we can remove this
# workaround and restore the line above.
pnmdepth 32767 | pgmnorm -bvalue 0 -wvalue 26895 | pnmdepth 65535 |

# Convert back to nonlinear samples:
pnmgamma -srgbramp

# Clean up the temporary file:
rm $filter