General options

-?, -h, -help

Show a complete summary of options.

-backup, -keep

Keep a backup of the modified files.

-clobber

Overwrite the existing output and backup files.
Under this option, if the option -backup is not enabled, the old backups of the overwritten files are deleted.

-dir directory

Write the output files to directory.

-fix

Enable error recovery. This option has no effect on valid input files.
The program will spend a reasonable amount of effort to recover as much data as possible, without increasing the output file size, but the success cannot be generally guaranteed. The program may even increase the file size, e.g., by reconstructing missing critical data. Under this option, integrity shall take precedence over file size.
When this option is not used, the invalid input files are left unprocessed.

-force

Enforce writing of a new output file.
This option overrides the program's decision not to write such file, e.g. when the PNG input is digitally signed (using dSIG), or when the PNG output becomes larger than the PNG input.

-log file

Log messages to file. For safety reasons, file must have the extension .log.
This option is deprecated and will be removed eventually. Use shell redirection.

-out file

Write output file to file. The command line must contain exactly one input file.

-preserve

Preserve file attributes (time stamps, file access rights, etc.) where applicable.

-quiet, -silent

Run in quiet mode.
The messages are still written to the log file if the option -log is enabled.

-simulate

Run in simulation mode: perform the trials, but do not create output files.

-v

Enable the options -verbose and -version.

-verbose

Run in verbose mode.

-version

Show copyright, version and build info.

--

Stop option switch parsing.

PNG encoding and optimization options

-o level

Select the optimization level.
The optimization level 0 enables a set of optimization operations that require minimal effort. There will be no changes to image attributes like bit depth or color type, and no recompression of existing IDAT datastreams.
The optimization level 1 enables a single IDAT compression trial. The trial chosen is what OptiPNG thinks it's probably the most effective.
The optimization levels 2 and higher enable multiple IDAT compression trials; the higher the level, the more trials.
The behavior and the default value of this option may change across different program versions. Use the option -h to see the details pertaining to your specific version.

-f filters

Select the PNG delta filters.
The filters argument is specified as a rangeset (e.g. -f0-5), and the default filters value depends on the optimization level set by the option -o.
The filter values 0, 1, 2, 3 and 4 indicate static filtering, and correspond to the standard PNG filter codes (None, Left, Up, Average and Paeth, respectively). The filter value 5 indicates adaptive filtering, whose effect is defined by the libpng(3) library used by OptiPNG.

-full

Produce a full report on IDAT. This option might slow down the trials.

-i type

Select the interlace type (0-1).
If the interlace type 0 is selected, the output image shall be non-interlaced (i.e. progressive-scanned). If the interlace type 1 is selected, the output image shall be interlaced using the Adam7 method.
By default, the output shall have the same interlace type as the input.

-nb

Do not apply bit depth reduction.

-nc

Do not apply color type reduction.

-np

Do not apply palette reduction.

-nx

Do not apply any lossless image reduction: enable the options -nb, -nc and -np.

-nz

Do not recode IDAT datastreams.
The IDAT optimization operations that do not require recoding (e.g. IDAT chunk concatenation) are still performed.
This option has effect on PNG input files only.

-zc levels

Select the zlib compression levels used in IDAT compression.
The levels argument is specified as a rangeset (e.g. -zc6-9), and the default levels value depends on the optimization level set by the option -o.
The effect of this option is defined by the zlib(3) library used by OptiPNG.

-zm levels

Select the zlib memory levels used in IDAT compression.
The levels argument is specified as a rangeset (e.g. -zm8-9), and the default levels value depends on the optimization level set by the option -o.
The effect of this option is defined by the zlib(3) library used by OptiPNG.

-zs strategies

Select the zlib compression strategies used in IDAT compression.
The strategies argument is specified as a rangeset (e.g. -zs0-3), and the default strategies value depends on the optimization level set by the option -o.
The effect of this option is defined by the zlib(3) library used by OptiPNG.

-zw size

Select the zlib window size (32k,16k,8k,4k,2k,1k,512,256) used in IDAT compression.
The size argument can be specified either in bytes (e.g. 16384) or kilobytes (e.g. 16k). The default size value is set to the lowest window size that yields an IDAT output as big as if yielded by the value 32768.
The effect of this option is defined by the zlib(3) library used by OptiPNG.

Editing options

-snip

Cut one image out of multi-image, animation or video files.
Depending on the input format, this may be either the first or the most relevant (e.g. the largest) image.

-strip objects

Strip metadata objects from a PNG file.
PNG metadata is the information stored in any ancillary chunk except tRNS. (tRNS represents the alpha channel, which, even if ignored in rendering, is still a proper image channel in the RGBA color space.)
The only option currently supported is -strip all.

Notes

Some options may have arguments that follow the option name, separated by whitespace or the equal sign ('='). If the option argument is a number or a rangeset, the separator may be omitted. For example:

-out newfile.png  <=>  -out=newfile.png
-o3  <=>  -o 3  <=>  -o=3
-f0,3-5  <=>  -f 0,3-5  <=>  -f=0,3-5

Rangeset arguments are cumulative; e.g.

-f0 -f3-5  <=>  -f0,3-5
-zs0 -zs1 -zs2-3  <=>  -zs0,1,2,3  <=>  -zs0-3