-O Optimize; remove unused color-table entries. Normally these
trigger an error message and disable -d (but conversion is
completed anyway). Also, use zlib compression level 9 (best
compression) instead of the default level. The recovery
algorithm enabled by -r is as follows: Unused color table
entries will not trigger an error message as they normally
do, but will still be preserved unless -O is also on, in
which case they will be discarded. Missing color tables will
be patched with a default that puts black at index 0, white at
index 1, and supplies red, green, blue, yellow, purple and
cyan as the remaining color values. Missing image pixels will
be set to 0. Unrecognized or corrupted extensions will be
discarded.