Skip optipng if it reduced the color type from rgb to grayscale/grayscale+alpha.

build/sanitize-png-files.sh

 #!/bin/bash
 # Copyright (c) 2010 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # The optimization code is based on pngslim (http://goo.gl/a0XHg)
 # and executes a similar pipleline to optimize the png file size.
 # The steps that require pngoptimizercl/pngrewrite/deflopt are omitted,
 # but this runs all other processes, including:
 # 1) various color-dependent optimizations using optipng.
@@ skipping 168 matching lines @@
       optipng -q -nb -nc -zw$i -zc1-9 -zm1-9 -zs0-3 -f0-5 $file
     done
   fi
   for i in $(seq 1 4); do
     throbber
     advdef -q -z -$i $file
   done
   echo -ne "\r"
 }
 
+# Usage: get_color_type <file>
+# Returns the color type code of the png file.
+# See http://en.wikipedia.org/wiki/Portable_Network_Graphics#Color_depth
+# for details about the color type code.
+function get_color_type {
+  local file=$1
+  echo $(identify -verbose $file | awk '/IHDR.color_type/ {print $3}')
+}
+
 # Usage: optimize_size <file>
 # Performs png file optimization.
 function optimize_size {
   tput el
   local file=$1
   echo -n "$file "
 
   advdef -q -z -4 $file
 
   pngout -q -s4 -c0 -force $file $file.tmp.png
   if [ -f $file.tmp.png ]; then
     rm $file.tmp.png
     process_grayscale $file
     process_grayscale_alpha $file
   else
     pngout -q -s4 -c4 -force $file $file.tmp.png
     if [ -f $file.tmp.png ]; then
       rm $file.tmp.png
       process_grayscale_alpha $file
     else
       process_rgb $file
     fi
   fi
 
   echo -n "|filter"
-  optipng -q -zc9 -zm8 -zs0-3 -f0-5 $file
+  local old_color_type=$(get_color_type $file)
+  optipng -q -zc9 -zm8 -zs0-3 -f0-5 $file -out $file.tmp.png
+  local new_color_type=$(get_color_type $file.tmp.png)
+  # optipng may corrupt a png file when reducing the color type
+  # to grayscale/grayscale+alpha. Just skip such cases.
+  # crbug.com/174505, crbug.com/174084.

[Nico, 2013/02/06 21:53:30]

Should we fix this in optipng instead? Have you at least reported this to
upstream optipng?

[oshima, 2013/02/06 22:34:09]

Filed a bug and added the reference.

+  if [[ $old_color_type == 6 &&
+        ($new_color_type == 0 || $new_color_type == 4) ]] ; then
+    rm $file.tmp.png
+    echo -n "[skip opting]"
+  else
+    mv $file.tmp.png $file
+  fi
   pngout -q -k1 -s1 $file
 
   huffman_blocks $file
 
   # TODO(oshima): Experiment with strategy 1.
   echo -n "|strategy"
   if [ $OPTIMIZE_LEVEL == 2 ]; then
     for i in 3 2 0; do
       pngout -q -k1 -ks -s$i $file
     done
@@ skipping 145 matching lines @@
 done
 
 # Print the results.
 let diff=$TOTAL_OLD_BYTES-$TOTAL_NEW_BYTES
 let percent=$diff*100/$TOTAL_OLD_BYTES
 echo "Processed $PROCESSED_FILE files (out of $TOTAL_FILE files)" \
      "in $(date -u -d @$SECONDS +%T)s"
 echo "Result : $TOTAL_OLD_BYTES => $TOTAL_NEW_BYTES bytes" \
      "($diff bytes : $percent %)"