OptiPNG - Optimize Portable Network Graphics files
optipng [
-? |
-h |
-help]
optipng [
options...]
files...
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.
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.
-
-?, -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.
-
-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.
- -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.
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
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 minimum 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.
optipng file.png # default speed
optipng -o5 file.png # slow
optipng -o7 file.png # very slow
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.
png(5),
libpng(3),
zlib(3),
pngcrush(1),
pngrewrite(1).
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/
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.