$ optipng -h
OptiPNG: PNG optimizer version 0.4.3
Copyright (C) 2001-2003 Cosmin Truta

Usage:
    optipng [options] files.png ...
Basic options:
    -h, -help           show this help
    -v                  verbose mode / show copyright, version and build info
    -o  <level>         optimization level (0-7)                default 2
    -i  <type>          interlace type (0-1)                    default <input>
    -k, -keep           keep a backup of the input files
    -log                log messages to "optipng.log"
    -q, -quiet          quiet mode
Advanced options:
    -zc <levels>        zlib compression levels (1-9)           default 9
    -zm <levels>        zlib memory levels (1-9)                default 8
    -zs <strategies>    zlib compression strategies (0-2)       default 0-2
    -zw <window size>   zlib window size (32k,16k,8k,4k,2k,1k,512)
    -f  <filters>       PNG delta filters (0-5)                 default 0,5
    -fix                enable error recovery
    -force              write a new output even if it is larger than the input
    -nb                 no bit depth reduction
    -nc                 no color type reduction
    -no                 no output (simulation mode)
    -np                 no palette reduction
    -nx                 no IDAT recompression (disable all reductions)
    --                  stop option switch parsing
Optimization level presets:
    -o0  <=>  -nx
    -o1  <=>  [apply libpng heuristics]         (1 trial)
    -o2  <=>  -zc9 -zm8 -zs0-2 -f0,5            (6 trials)
    -o3  <=>  -zc9 -zm8-9 -zs0-2 -f0,5          (12 trials)
    -o4  <=>  -zc9 -zm8 -zs0-2 -f0-5            (18 trials)
    -o5  <=>  -zc9 -zm8-9 -zs0-2 -f0-5          (36 trials)
    -o6  <=>  -zc1-9 -zm8 -zs0-2 -f0-5          (114 trials)
    -o7  <=>  -zc1-9 -zm8-9 -zs0-2 -f0-5        (228 trials)
Examples:
    optipng -o5 file.png                        (moderately slow)
    optipng -o7 file.png                        (very slow)
    optipng -i1 -zc4,9 -zs0-2 -f0-2,4-5 file1.png file2.png
Notes:
  - The options are cummulative; e.g.
    -f0 -f5  <=>  -f0,5
    -zs0 -zs1 -zs2  <=>  -zs0,1,2  <=>  -zs0-2
  - The option letters are case-insensitive.
  - The libpng heuristics consist of:
    -o1  <=>  -zc9 -zm8 -zs0 -f0                (if PLTE is present)
    -o1  <=>  -zc9 -zm8 -zs1 -f5                (if PLTE is not present)
  - The zlib window size is set to a minimum that does not affect
    the compression ratio.
  - The output file will have all IDAT in a single chunk, even if
    no recompression is performed.
  - The most exhaustive search  -zc1-9 -zm1-9 -zs0-2 -f0-5  (1026 trials)
    is offered only as an advanced option, and it is not recommended.