OPTIPNG(1) User Commands 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 minimum, without losing semantic information. In
addition, this program shall perform a suite of auxiliary functions
like integrity checks, metadata recovery and pixmap-to-PNG
conversion.

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 version. 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 composed 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 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

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 parameters 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.

Not all of the above steps need to be executed. The behavior depends
on the actual input files and user options.

Step 1 may be customized via the no-reduce options -nb, -nc, -np and
-nx. Step 2 may be customized via the -o option, and may be fine-
tuned via the options -zc, -zm, -zs and -zw. Step 3 is always
executed. Step 4 is executed only if a new IDAT is being created, and
may be fine-tuned via the option -zw.

Extremely exhaustive searches are not generally expected to yield
significant improvements in compression ratio, and are recommended to
advanced users only.

EXAMPLES

optipng file.png # default speed
optipng -o5 file.png # slow
optipng -o7 file.png # very slow

BUGS

Lossless image reductions are not completely implemented. (This does
not affect the integrity of the output files.) Here are the missing
pieces:

- The color palette reductions are implemented only partially.
- The bit depth reductions below 8, for grayscale images, are
not implemented yet.

Encoding of images whose total IDAT size exceeds 2GB is not
supported.

TIFF support is limited to uncompressed, PNG-compatible (grayscale,
RGB and RGBA) images.

Metadata is not imported from the external image formats.

There is no support for pipes, streams, extended file attributes or
access control lists.

SEE ALSO

png(5), libpng(3), zlib(3), pngcrush(1), pngrewrite(1).

STANDARDS

The files produced by OptiPNG are compliant with PNG-2003:
Glenn Randers-Pehrson et al. Portable Network Graphics (PNG)
Specification, Second Edition.
W3C Recommendation 10 November 2003; ISO/IEC IS 15948:2003 (E).
http://www.w3.org/TR/PNG/

AUTHOR

OptiPNG is written and maintained by Cosmin Truta.

This manual page was originally written by Nelson A. de Oliveira for
the Debian Project. It was later updated by Cosmin Truta, and is now
part of the OptiPNG distribution.

OptiPNG version 0.7.8 2023-Nov-03 OPTIPNG(1)