OPTIPNG(1) General Commands Manual OPTIPNG(1)
NAME
OptiPNG — Optimize Portable Network Graphics files
SYNOPSIS
optipng [-? | -h | -help]
optipng [options…] files…
DESCRIPTION
The OptiPNG program shall attempt to optimize PNG files, i.e. reduce their size to a mini‐
mum, without losing semantic information. In addition, this program shall perform a suite
of auxiliary functions like integrity checks, metadata recovery and pixmap-to-PNG conver‐
sion.
The optimization attempts are not guaranteed to succeed. Valid PNG files that cannot be optimized by this program are normally left intact; their size will not grow. The user may request to override this default behavior.
FILES
The input files are raster image files encoded either in PNG format (the native format),
or in an external format. The currently supported external formats are GIF, BMP, PNM and
TIFF.
OptiPNG processes each image file given in the command line as follows:
- If the image is in PNG format:
Attempts to optimize the given file in-place. If optimization is successful, or if
the option -force is enabled, replaces the original file with its optimized ver‐
sion. The original file is backed up if the option -keep is enabled.
- If the image is in an external format:
Creates an optimized PNG version of the given file. The output file name is com‐
posed from the original file name and the .png extension.
Existing files are not overwritten, unless the option -clobber is enabled.
OPTIONS
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 gener‐
ally guaranteed. The program may even increase the file size, e.g., by reconstruct‐
ing 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 min‐
imal 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 pro‐
gram 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 fil‐
ters 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 con‐
catenation) 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 lev‐
els 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 lev‐
els 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
Options may come in any order (except for —), before, after, or alternating with file
names. Option names are case-insensitive and may be abbreviated to their shortest unique
prefix.
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
EXTENDED DESCRIPTION
The PNG optimization algorithm consists of the following steps:
1. Reduce the bit depth, the color type and the color palette of the image. This step
may reduce the size of the uncompressed image, which, indirectly, may reduce the size
of the compressed image (i.e. the size of the output PNG file).
2. Run a suite of compression methods and strategies and select the compression parame‐
ters that yield the smallest output file.
3. Store all IDAT contents into a single chunk, eliminating the overhead incurred by
repeated IDAT headers and CRCs.
4. Set the zlib window size inside IDAT to a mininum that does not affect the compression
ratio, reducing the memory requirements of PNG decoders.
-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
Options may come in any order (except for —), before, after, or alternating with file
names. Option names are case-insensitive and may be abbreviated to their shortest unique
prefix.
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
EXTENDED DESCRIPTION
The PNG optimization algorithm consists of the following steps:
1. Reduce the bit depth, the color type and the color palette of the image. This step
may reduce the size of the uncompressed image, which, indirectly, may reduce the size
of the compressed image (i.e. the size of the output PNG file).
2. Run a suite of compression methods and strategies and select the compression parame‐
ters that yield the smallest output file.