libwebp/man/cwebp.1

.\"                                      Hey, EMACS: -*- nroff -*-
.TH CWEBP 1 "November 3, 2011"
.SH NAME
cwebp \- compress an image file to a WebP file
.SH SYNOPSIS
.B cwebp
.RI [ options ] " input_file \-o output_file.webp
.br
.SH DESCRIPTION
This manual page documents the
.B cwebp
command.
.PP
\fBcwebp\fP compresses an image using the WebP format.
Input format can be either PNG, JPEG, or raw Y'CbCr samples.
When using PNG, the transparency information (alpha channel) is currently
discarded.
.SH OPTIONS
The basic options are:
.TP
.B \-o string
Specify the name of the output WebP file. If omitted, \fBcwebp\fP will
perform compression but only report statistics.
.TP
.B \-h, \-help
A short usage summary.
.TP
.B \-H, \-longhelp
A summary of all the possible options.
.TP
.B \-version
Print the version number (as major.minor.revision) and exit.
.TP
.B \-q float
Specify the compression factor for RGB channels between 0 and 100. A small
factor produces a smaller file with lower quality. Best quality is achieved
using a value of 100. The default is 75.
.TP
.B \-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 compression. The default is 100.
.TP
.B \-alpha_filter string
Specify the predictive filtering method for alpha plane. One of 'none', 'fast'
or 'best', in increasing complexity and slowness order. Default is 'fast'.
Internally, alpha filtering is performed using four possible predictions (none,
horizontal, vertical, gradient). The 'best' mode will try each modes 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.
.TP
.B \-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 smoother the picture will
appear. Typical values are usually in the range of 20 to 50.
.TP
.B \-preset string
Specify a set of pre-defined parameters to suit a particular type of
source material. Possible values are:  \fBdefault\fP, \fBphoto\fP,
\fBpicture\fP, \fBdrawing\fP, \fBicon\fP, \fBtext\fP. Since
\fB\-preset\fP overwrites the other parameters' values (except the
\fB\-q\fP one), this option should preferably appear first in the
order of the arguments.
.TP
.B \-sns int
Specify the amplitude of the spatial noise shaping. Spatial noise shaping
(or \fBsns\fP 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 transfer these bits. The possible range goes from
0 (algorithm is off) to 100 (the maximal effect). The default value is 80.
.TP
.B \-m int
Specify the compression method to use. This parameter controls the
tradeoff 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 filesize and lower compression quality.
.TP
.B \-af
Turns auto-filter on. This algorithm will spend additional time optimizing
the filtering strength to reach a well-balanced quality.

.SH ADDITIONAL OPTIONS
More advanced options are:
.TP
.B \-sharpness int
Specify the sharpness of the filtering (if used).
Range is 0 (sharpest) to 7 (least sharp). Default is 0.
.TP
.B \-strong
Use a stronger filtering than the default one (if filtering is being
used thanks to the \fB\-f\fP option). Strong filtering is off by default.
.TP
.B \-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.
.TP
.B \-partition_limit int
Degrade quality by limiting the number of bits used by some macroblocks.
Range is 0 (no degradation, the default) to 100 (full degradation).
Useful values are usually around 30-70 for moderately 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:
\fIError code: 6 (PARTITION0_OVERFLOW: Partition #0 is too big to fit 512k)\fP.
If using \fB-partition_limit\fP is not enough to meet the 512k constraint, one
should use less segments in order to save more header bits per macroblock.
See the \fB-segments\fP option.
.TP
.B \-size int
Specify a target size (in bytes) to try and reach for the compressed output.
Compressor will make several pass of partial encoding in order to get as
close as possible to this target.
.TP
.B \-psnr float
Specify a target PSNR (in dB) to try and reach for the compressed output.
Compressor will make several pass of partial encoding in order to get as
close as possible to this target.
.TP
.B \-pass int
Set a maximum number of pass to use during the dichotomy used by
options \fB\-size\fP or \fB\-psnr\fP. Maximum value is 10.
.TP
.B \-crop x_position y_position width height
Crop the source to a rectangle with top-left corner at coordinates
(\fBx_position\fP, \fBy_position\fP) and size \fBwidth\fP x \fBheight\fP.
This cropping area must be fully contained within the source rectangle.
.TP
.B \-s width height
Specify that the input file actually consists of raw Y'CbCr samples following
the ITU-R BT.601 recommendation, in 4:2:0 linear format.
The luma plane has size \fBwidth\fP x \fBheight\fP.
.TP
.B \-map int
Output additional ASCII-map of encoding information. Possible map values
range from 1 to 6. This is only meant to help debugging.
.TP
.B \-pre int
Specify a pre-processing filter. This option is a placeholder
and has currently no effect.
.TP
.B \-alpha_method int
Specify the algorithm used for alpha compression: 0 or 1. Algorithm 0 denotes
no compression, 1 uses uses backward reference counts encoded with arithmetic
encoder. The default is 1.
.TP
.B \-alpha_cleanup
Modify unseen RGB values under fully transparent area, to help compressibility.
The default is off.
.TP
.B \-noalpha
Using this option will discard the alpha channel.
.TP
.B \-noasm
Disable all assembly optimizations.
.TP
.B \-v
Print extra information (encoding time in particular).
.TP
.B \-print_psnr
Compute and report average PSNR (Peak-Signal-To-Noise ratio).
.TP
.B \-print_ssim
Compute and report average SSIM (structural similarity metric)
.TP
.B \-progress
Report encoding progress in percent.
.TP
.B \-quiet
Do not print anything.
.TP
.B \-short
Only print brief information (output file size and PSNR) for testing purpose.

.SH EXAMPLES
cwebp \-q 70 picture.png \-o picture.webp
.br
cwebp \-sns 70 \-f 50 \-strong \-af \-size 60000 picture.png \-o picture.webp

.SH SEE ALSO
.BR dwebp (1).
.br
Please refer to http://code.google.com/speed/webp/ for additional
information.
.SH AUTHORS
\fBcwebp\fP was written by the WebP team.
.br
The latest source tree is available at http://www.webmproject.org/code
.PP
This manual page was written by Pascal Massimino <pascal.massimino@gmail.com>,
for the Debian project (and may be used by others).