#!/bin/sh

# make-png-google-logo 0.1.0 (2004-Feb-12-Thu)
# http://www.nicemice.net/amc/utils/#ppmuncomposite
# Adam M. Costello
# http://www.nicemice.net/amc/

# This script constructs an unofficial Google logo sticker as a PNG
# file with a full alpha channel (so that it can be composited against
# any background).  The constructed image is derived algorithmically
# from two of the official GIF image files that are pre-composited
# against black and white backgrounds.  Those files, Logo_60blk.gif and
# Logo_60wht.gif, are fetched from the web if they don't already exist
# in the current directory.  The PNG stream is written to Logo_60any.png
# in the current working directory, which must not already exist.
#
# WARNING: While Google has given everyone permission to use their
# official logo sticker images in certain ways, they have *not* given
# permission to display or distribute modified images, such as the image
# produced by this script.  See http://www.google.com/stickers.html.
# This script is not blessed by Google in any way.
#
# The primary purpose of this script is not to generate a Google logo
# image per se, but to demonstrate the technique of reverse-engineering
# an uncomposited transparent image from two pre-composited images.
#
# NOTE: The PNG file produced by this script does a very good job of
# approximating the official logo images when properly composited
# against white and black backgrounds.  Mozilla Firefox 0.8 (and
# earlier) does the compositing improperly--it neglects to undo
# the gamma encoding before compositing and redo it afterward, as
# recommended and demonstrated in the PNG spec, which states that the
# alpha channel indicates how to mix intensity samples, not how to
# mix gamma-encoded samples.  I don't know about other browsers.  The
# ImageMagick 5.5.7.9 command "composite -compose over" has the same
# bug.
#
# This script depends on another script, ppmuncomposite, which must
# either be in $PATH or be in the same directory as this script.  This
# script also depends on wget, some of the netpbm utilities, pngcrush,
# and some customary Unix commands.

tmpdir=/tmp/make-png-google-logo-$$

onblack_gif=Logo_60blk.gif
onwhite_gif=Logo_60wht.gif
gif_web_dir=http://www.google.com/logos/
width=176
height=77
outfile=Logo_60any.png

cleanup() {
  cd /
  rm -rf $tmpdir
}

warn() {
  echo "$*" >&2
}

fail() {
  warn "$*"
  cleanup
  exit 1
}

# Make sure the output file doesn't already exist.
[ ! -f "$outfile" ] || fail "$outfile already exists"

# Create a temporary directory and copy the source GIF images into it.

mkdir $tmpdir || fail "cannot mkdir $tmpdir"
[ ! -f "$onblack_gif" ] || cp "$onblack_gif" $tmpdir
[ ! -f "$onwhite_gif" ] || cp "$onwhite_gif" $tmpdir
onblack_uri=$gif_web_dir$onblack_gif
onwhite_uri=$gif_web_dir$onwhite_gif

( cd $tmpdir || fail "cannot cd $tmpdir"

[ -f "$onblack_gif" ] || wget "$onblack_uri" ||
  fail "cannot wget $onblack_uri"
[ -f "$onwhite_gif" ] || wget "$onwhite_uri" ||
  fail "cannot wget $onwhite_uri"

read black_sum black_length junk << EOF
`cksum "$onblack_gif"`
EOF

[ "$black_sum $black_length" = "3336891779 3455" ] ||
  fail "unexpected checksum/length for $onblack_gif"

read white_sum white_length junk << EOF
`cksum "$onwhite_gif"`
EOF

[ "$white_sum $white_length" = "3221466317 5614" ] ||
  fail "unexpected checksum/length for $onwhite_gif"

# The GIF images contain some (completely) transparent pixels.  Replace
# them with black or white pixels (as appropriate) so that we have
# simple opaque images to start from.
#
# In this case it's okay to use pnmcomp on gamma-encoded samples, even
# though it does no gamma decoding before compositing, because all the
# alpha values are 0 or 1 (GIF supports only binary transparency), and
# therefore there is no mixing of pixels.

giftopnm -alphaout=black-mask.pgm < "$onblack_gif" > black-fore.ppm
giftopnm -alphaout=white-mask.pgm < "$onwhite_gif" > white-fore.ppm
pbmmake -black $width $height |
  pnmcomp -alpha=black-mask.pgm black-fore.ppm > onblack.ppm
pbmmake -white $width $height |
  pnmcomp -alpha=white-mask.pgm white-fore.ppm > onwhite.ppm

# Align the images.  Oddly, the black and white logos are not quite
# aligned with each other.  We need to shift the black image 1 pixel
# to the left.  (Actually, I think it wants to be shifted a little
# more than 1 pixel to the left, and a little more than 0 pixels
# upward.  That could be accomplished by scaling up, then shifting,
# then subsampling, but I doubt that the benefit of sub-pixel alignment
# would outweigh the distortion introduced by resampling, and I doubt my
# ability to determine the proper alignment so precisely, because the
# two images are simply different in various ways when you look closely.
# If we did do resampling, we'd first need to do gamma decoding, and
# we'd want to convert to 16-bit samples before doing that, to avoid
# losing precision.)

mv onblack.ppm onblack-unshifted.ppm
pnmcut -pad -left 1 -width $width < onblack-unshifted.ppm > onblack.ppm

  # pnmcut's -pad option conveniently pads with black pixels.  If we had
  # been shifting the white image, we would have needed to use pnmpad.

# Pop back out of the temporary directory so we can find ppmuncomposite.
)

# Uncomposite.

if type ppmuncomposite > /dev/null 2>&1; then
  ppmuncomposite=ppmuncomposite
else
  ppmuncomposite=`dirname "$0"`/ppmuncomposite
  [ -x "$ppmuncomposite" ] || fail 'cannot find ppmuncomposite'
fi

"$ppmuncomposite" $tmpdir/onblack.ppm $tmpdir/onwhite.ppm \
                  $tmpdir/uncomp.ppm  $tmpdir/alpha.pgm

# Convert to PNG.
pnmtopng -alpha $tmpdir/alpha.pgm < $tmpdir/uncomp.ppm > $tmpdir/tmp.png

# Add sRGB and tEXt chunks to the PNG image (and compress it better).

pngcrush -srgb 1 \
  -text b Title 'unofficial Google logo sticker, 60% scale, any background' \
  -text b Description '
Full-alpha image derived (without permission) algorithmically from two
Google-supplied images pre-composited onto black and white backgrounds:

http://www.google.com/logos/Logo_60blk.gif
http://www.google.com/logos/Logo_60wht.gif

The conversion script was written on 2004-Jan-21-Wed by
Adam M. Costello
http://www.nicemice.net/amc/
' $tmpdir/tmp.png "$outfile"

# Clean up.
cleanup