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

build/sanitize-png-files.sh

Index: build/sanitize-png-files.sh
diff --git a/build/sanitize-png-files.sh b/build/sanitize-png-files.sh
index 92ddeb898209f81fa06fbf0212b75e99a325e561..b90ec0dd049ae1affd340fb724fc6f2a47b2df9f 100755
--- a/build/sanitize-png-files.sh
+++ b/build/sanitize-png-files.sh
@@ -186,6 +186,15 @@ function final_compression {
   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 {
@@ -211,7 +220,19 @@ function optimize_size {
   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