CWEBP(1) - phpMan

CWEBP(1)                    General Commands Manual                   CWEBP(1)

NAME
       cwebp - compress an image file to a WebP file
SYNOPSIS
       cwebp [options] input_file -o output_file.webp
DESCRIPTION
       This manual page documents the cwebp command.
       cwebp  compresses  an image using the WebP format.  Input format can be
       either PNG, JPEG, TIFF or raw Y'CbCr samples.
OPTIONS
       The basic options are:
       -o string
              Specify the name of the output WebP file. If omitted, cwebp will
              perform compression but only report statistics.
       -h, -help
              A short usage summary.
       -H, -longhelp
              A summary of all the possible options.
       -version
              Print the version number (as major.minor.revision) and exit.
       -q float
              Specify  the  compression  factor for RGB channels between 0 and
              100. The default is 75.
              In case of lossy compression (default), a small factor  produces
              a  smaller  file with lower quality. Best quality is achieved by
              using a value of 100.
              In case of lossless  compression  (specified  by  the  -lossless
              option),  a  small  factor enables faster compression speed, but
              produces a larger file. Maximum compression is achieved by using
              a value of 100.
       -alpha_q int
              Specify  the  compression factor for alpha compression between 0
              and 100.  Lossless compression of  alpha  is  achieved  using  a
              value  of 100, while the lower values result in a lossy compres-
              sion. The default is 100.
       -f int Specify the strength of the deblocking  filter,  between  0  (no
              filtering)  and  100 (maximum filtering). A value of 0 will turn
              off any filtering.  Higher value will increase the  strength  of
              the  filtering  process  applied after decoding the picture. The
              higher the value the smoother the picture will  appear.  Typical
              values are usually in the range of 20 to 50.
       -preset string
              Specify  a  set  of  pre-defined parameters to suit a particular
              type of source material. Possible values are:   default,  photo,
              picture, drawing, icon, text. Since -preset overwrites the other
              parameters' values (except  the  -q  one),  this  option  should
              preferably appear first in the order of the arguments.
       -sns int
              Specify  the  amplitude  of  the  spatial noise shaping. Spatial
              noise shaping (or sns for short) refers to a general  collection
              of  built-in algorithms used to decide which area of the picture
              should use relatively less bits, and where else to better trans-
              fer  these  bits.  The  possible range goes from 0 (algorithm is
              off) to 100 (the maximal effect). The default value is 80.
       -m int Specify the compression method to use. This  parameter  controls
              the  trade  off  between  encoding speed and the compressed file
              size and quality.  Possible values range from 0  to  6.  Default
              value is 4.  When higher values are used, the encoder will spend
              more  time  inspecting  additional  encoding  possibilities  and
              decide  on  the  quality gain.  Lower value can result is faster
              processing time at the expense of larger  file  size  and  lower
              compression quality.
       -jpeg_like
              Change  the  internal  parameter  mapping  to  better  match the
              expected size of JPEG compression. This flag will generally pro-
              duce  an output file of similar size to its JPEG equivalent (for
              the same -q setting), but with less visual distortion.
       -mt    Use multi-threading for encoding, if possible.  This  option  is
              only  effective  when using lossy compression on a source with a
              transparency channel.
       -low_memory
              Reduce memory usage of lossy encoding by saving four  times  the
              compressed  size (typically). This will make the encoding slower
              and the output slightly different in size and  distortion.  This
              flag  is  only  effective  for  methods  3 and up, and is off by
              default. Note that leaving this flag off  will  have  some  side
              effects  on  the bitstream: it forces certain bitstream features
              like number of partitions  (forced  to  1).  Note  that  a  more
              detailed report of bitstream size is printed by cwebp when using
              this option.
       -af    Turns auto-filter on. This algorithm will spend additional  time
              optimizing the filtering strength to reach a well-balanced qual-
              ity.

ADDITIONAL OPTIONS
       More advanced options are:
       -sharpness int
              Specify the sharpness of the filtering (if used).   Range  is  0
              (sharpest) to 7 (least sharp). Default is 0.
       -strong
              Use  strong  filtering (if filtering is being used thanks to the
              -f option). Strong filtering is on by default.
       -nostrong
              Disable strong filtering (if filtering is being used  thanks  to
              the -f option) and use simple filtering instead.
       -segments int
              Change  the  number of partitions to use during the segmentation
              of the sns algorithm. Segments  should  be  in  range  1  to  4.
              Default value is 4.  This option has no effect for methods 3 and
              up, unless -low_memory is used.
       -partition_limit int
              Degrade quality by limiting the number of bits used by some mac-
              roblocks.  Range is 0 (no degradation, the default) to 100 (full
              degradation).  Useful values are usually around 30-70 for moder-
              ately  large  images.   In the VP8 format, the so-called control
              partition has a limit of 512k and is used to store the following
              information: whether the macroblock is skipped, which segment it
              belongs to, whether it is coded as  intra  4x4  or  intra  16x16
              mode,  and  finally  the prediction modes to use for each of the
              sub-blocks.  For a very large image, 512k only  leaves  room  to
              few  bits  per 16x16 macroblock.  The absolute minimum is 4 bits
              per macroblock. Skip, segment, and mode information can  use  up
              almost  all  these 4 bits (although the case is unlikely), which
              is problematic for very large images. The partition_limit factor
              controls  how  frequently  the  most bit-costly mode (intra 4x4)
              will be used. This is useful in case the 512k limit  is  reached
              and  the  following  message is displayed: Error code: 6 (PARTI-
              TION0_OVERFLOW: Partition #0 is too big to fit 512k).  If  using
              -partition_limit  is not enough to meet the 512k constraint, one
              should use less segments in order to save more header  bits  per
              macroblock.  See the -segments option.
       -size int
              Specify  a  target size (in bytes) to try and reach for the com-
              pressed output.  Compressor will make several  pass  of  partial
              encoding in order to get as close as possible to this target.
       -psnr float
              Specify  a  target  PSNR  (in  dB) to try and reach for the com-
              pressed output.  Compressor will make several  pass  of  partial
              encoding in order to get as close as possible to this target.
       -pass int
              Set  a maximum number of passes to use during the dichotomy used
              by options -size or -psnr. Maximum value is 10.
       -crop x_position y_position width height
              Crop the source to a rectangle with top-left corner  at  coordi-
              nates  (x_position,  y_position)  and size width x height.  This
              cropping area must be fully contained within the source  rectan-
              gle.
       -s width height
              Specify that the input file actually consists of raw Y'CbCr sam-
              ples following the ITU-R BT.601 recommendation, in 4:2:0  linear
              format.  The luma plane has size width x height.
       -map int
              Output  additional  ASCII-map  of encoding information. Possible
              map values range from 1 to 6. This is only meant to help  debug-
              ging.
       -pre int
              Specify  a  pre-processing  filter. This option is a placeholder
              and has currently no effect.
       -alpha_filter string
              Specify the predictive filtering method for the alpha plane. One
              of  'none', 'fast' or 'best', in increasing complexity and slow-
              ness order. Default is 'fast'. Internally,  alpha  filtering  is
              performed  using  four  possible  predictions (none, horizontal,
              vertical, gradient). The 'best' mode will try each mode in  turn
              and  pick  the one which gives the smaller size. The 'fast' mode
              will just try to form an  a-priori  guess  without  testing  all
              modes.
       -alpha_method int
              Specify  the algorithm used for alpha compression: 0 or 1. Algo-
              rithm 0 denotes no compression, 1 uses WebP lossless format  for
              compression. The default is 1.
       -alpha_cleanup
              Modify  unseen  RGB values under fully transparent area, to help
              compressibility.  The default is off.
       -noalpha
              Using this option will discard the alpha channel.
       -lossless
              Encode the image without any loss.
       -hint string
              Specify the hint about input image type.  Possible  values  are:
              photo, picture or graph.
       -metadata string
              A comma separated list of metadata to copy from the input to the
              output if present.  Valid values: all,  none,  exif,  icc,  xmp.
              The default is none.
              Note: each input format may not support all combinations.
       -noasm Disable all assembly optimizations.
       -v     Print extra information (encoding time in particular).
       -print_psnr
              Compute and report average PSNR (Peak-Signal-To-Noise ratio).
       -print_ssim
              Compute  and  report average SSIM (structural similarity metric,
              see http://en.wikipedia.org/wiki/SSIM for additional details).
       -print_lsim
              Compute and report local similarity metric (sum of lowest  error
              amongst the collocated pixel neighbors).
       -progress
              Report encoding progress in percent.
       -quiet Do not print anything.
       -short Only  print  brief  information  (output file size and PSNR) for
              testing purpose.

BUGS
       Please    report     all     bugs     to     our     issue     tracker:
       http://code.google.com/p/webp/issues
       Patches  welcome!  See  this  page  to get started: http://www.webmpro-
       ject.org/code/contribute/submitting-patches/

EXAMPLES
       cwebp -q 50 -lossless picture.png -o picture_lossless.webp
       cwebp -q 70 picture_with_alpha.png -o picture_with_alpha.webp
       cwebp -sns 70 -f 50 -size 60000 picture.png -o picture.webp

AUTHORS
       cwebp was written by the WebP team.
       The latest source tree is available at http://www.webmproject.org/code
       This  manual  page  was  written  by  Pascal   Massimino   <pascal.mas-
       simino AT gmail.com>, for the Debian project (and may be used by others).

SEE ALSO
       dwebp(1), gif2webp(1).
       Please refer to http://developers.google.com/speed/webp/ for additional
       information.

                                March 13, 2013                        CWEBP(1)