Provided by: optipng_0.7.6-1ubuntu0.16.04.1_amd64 bug

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 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.

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.