2011-03-02 01:01:47 +01:00
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.\" Hey, EMACS: -*- nroff -*-
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.TH CWEBP 1 "March 13, 2013"
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2011-03-02 01:01:47 +01:00
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.SH NAME
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cwebp \- compress an image file to a WebP file
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.SH SYNOPSIS
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.B cwebp
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2011-11-04 00:10:27 +01:00
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.RI [ options ] " input_file \-o output_file.webp
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2011-03-02 01:01:47 +01:00
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.br
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.SH DESCRIPTION
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This manual page documents the
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.B cwebp
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command.
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.PP
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2011-04-19 03:50:34 +02:00
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\fBcwebp\fP compresses an image using the WebP format.
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2012-07-04 09:21:14 +02:00
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Input format can be either PNG, JPEG, TIFF or raw Y'CbCr samples.
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.SH OPTIONS
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The basic options are:
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.TP
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.BI \-o " string
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Specify the name of the output WebP file. If omitted, \fBcwebp\fP will
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perform compression but only report statistics.
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.TP
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.B \-h, \-help
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A short usage summary.
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.TP
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.B \-H, \-longhelp
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A summary of all the possible options.
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.TP
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2011-03-28 20:12:39 +02:00
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.B \-version
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Print the version number (as major.minor.revision) and exit.
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.TP
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.BI \-q " float
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Specify the compression factor for RGB channels between 0 and 100. The default
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is 75.
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.br
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In case of lossy compression (default), a small factor produces a smaller file
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with lower quality. Best quality is achieved by using a value of 100.
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.br
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In case of lossless compression (specified by the \-lossless option), a small
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factor enables faster compression speed, but produces a larger file. Maximum
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compression is achieved by using a value of 100.
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.TP
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.BI \-alpha_q " int
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Specify the compression factor for alpha compression between 0 and 100.
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Lossless compression of alpha is achieved using a value of 100, while the lower
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values result in a lossy compression. The default is 100.
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.TP
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.BI \-f " int
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Specify the strength of the deblocking filter, between 0 (no filtering)
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and 100 (maximum filtering). A value of 0 will turn off any filtering.
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Higher value will increase the strength of the filtering process applied
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after decoding the picture. The higher the value the smoother the picture will
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appear. Typical values are usually in the range of 20 to 50.
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.TP
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.BI \-preset " string
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Specify a set of pre-defined parameters to suit a particular type of
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source material. Possible values are: \fBdefault\fP, \fBphoto\fP,
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\fBpicture\fP, \fBdrawing\fP, \fBicon\fP, \fBtext\fP. Since
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\fB\-preset\fP overwrites the other parameters' values (except the
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\fB\-q\fP one), this option should preferably appear first in the
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order of the arguments.
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.TP
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.BI \-sns " int
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Specify the amplitude of the spatial noise shaping. Spatial noise shaping
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(or \fBsns\fP for short) refers to a general collection of built-in algorithms
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used to decide which area of the picture should use relatively less bits,
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and where else to better transfer these bits. The possible range goes from
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0 (algorithm is off) to 100 (the maximal effect). The default value is 80.
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.TP
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.BI \-m " int
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Specify the compression method to use. This parameter controls the
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trade off between encoding speed and the compressed file size and quality.
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Possible values range from 0 to 6. Default value is 4.
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When higher values are used, the encoder will spend more time inspecting
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additional encoding possibilities and decide on the quality gain.
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Lower value can result is faster processing time at the expense of
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larger file size and lower compression quality.
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.TP
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.B \-jpeg_like
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Change the internal parameter mapping to better match the expected size
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of JPEG compression. This flag will generally produce an output file of
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similar size to its JPEG equivalent (for the same \fB\-q\fP setting), but
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with less visual distortion.
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.TP
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.B \-mt
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Use multi-threading for encoding, if possible. This option is only effective
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when using lossy compression on a source with a transparency channel.
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.TP
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.B \-low_memory
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Reduce memory usage of lossy encoding by saving four times the compressed
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size (typically). This will make the encoding slower and the output slightly
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different in size and distortion. This flag is only effective for methods
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3 and up, and is off by default. Note that leaving this flag off will have
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some side effects on the bitstream: it forces certain bitstream features
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like number of partitions (forced to 1). Note that a more detailed report
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of bitstream size is printed by \fBcwebp\fP when using this option.
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.TP
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.B \-af
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Turns auto-filter on. This algorithm will spend additional time optimizing
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the filtering strength to reach a well-balanced quality.
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.SH ADDITIONAL OPTIONS
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More advanced options are:
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.TP
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.BI \-sharpness " int
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Specify the sharpness of the filtering (if used).
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Range is 0 (sharpest) to 7 (least sharp). Default is 0.
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.TP
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.B \-strong
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Use strong filtering (if filtering is being used thanks to the
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\fB\-f\fP option). Strong filtering is on by default.
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.TP
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.B \-nostrong
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Disable strong filtering (if filtering is being used thanks to the
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\fB\-f\fP option) and use simple filtering instead.
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.TP
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.BI \-segments " int
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Change the number of partitions to use during the segmentation of the
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sns algorithm. Segments should be in range 1 to 4. Default value is 4.
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This option has no effect for methods 3 and up, unless \fB\-low_memory\fP
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is used.
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.TP
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.BI \-partition_limit " int
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Degrade quality by limiting the number of bits used by some macroblocks.
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Range is 0 (no degradation, the default) to 100 (full degradation).
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Useful values are usually around 30-70 for moderately large images.
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In the VP8 format, the so-called control partition has a limit of 512k and
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is used to store the following information: whether the macroblock is skipped,
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which segment it belongs to, whether it is coded as intra 4x4 or intra 16x16
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mode, and finally the prediction modes to use for each of the sub-blocks.
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For a very large image, 512k only leaves room to few bits per 16x16 macroblock.
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The absolute minimum is 4 bits per macroblock. Skip, segment, and mode
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information can use up almost all these 4 bits (although the case is unlikely),
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which is problematic for very large images. The partition_limit factor controls
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how frequently the most bit-costly mode (intra 4x4) will be used. This is
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useful in case the 512k limit is reached and the following message is displayed:
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\fIError code: 6 (PARTITION0_OVERFLOW: Partition #0 is too big to fit 512k)\fP.
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If using \fB-partition_limit\fP is not enough to meet the 512k constraint, one
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should use less segments in order to save more header bits per macroblock.
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See the \fB-segments\fP option.
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.TP
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.BI \-size " int
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Specify a target size (in bytes) to try and reach for the compressed output.
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Compressor will make several pass of partial encoding in order to get as
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close as possible to this target.
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.TP
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.BI \-psnr " float
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Specify a target PSNR (in dB) to try and reach for the compressed output.
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Compressor will make several pass of partial encoding in order to get as
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close as possible to this target.
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.TP
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.BI \-pass " int
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Set a maximum number of passes to use during the dichotomy used by
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options \fB\-size\fP or \fB\-psnr\fP. Maximum value is 10.
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.TP
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.BI \-crop " x_position y_position width height
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Crop the source to a rectangle with top-left corner at coordinates
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(\fBx_position\fP, \fBy_position\fP) and size \fBwidth\fP x \fBheight\fP.
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This cropping area must be fully contained within the source rectangle.
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.TP
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.BI \-s " width height
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Specify that the input file actually consists of raw Y'CbCr samples following
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the ITU-R BT.601 recommendation, in 4:2:0 linear format.
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The luma plane has size \fBwidth\fP x \fBheight\fP.
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.TP
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.BI \-map " int
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Output additional ASCII-map of encoding information. Possible map values
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range from 1 to 6. This is only meant to help debugging.
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.TP
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.BI \-pre " int
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Specify a pre-processing filter. This option is a placeholder
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and has currently no effect.
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.TP
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.BI \-alpha_filter " string
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Specify the predictive filtering method for the alpha plane. One of 'none',
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\&'fast' or 'best', in increasing complexity and slowness order. Default is
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\&'fast'. Internally, alpha filtering is performed using four possible
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predictions (none, horizontal, vertical, gradient). The 'best' mode will try
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each mode in turn and pick the one which gives the smaller size. The 'fast'
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mode will just try to form an a-priori guess without testing all modes.
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.TP
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.BI \-alpha_method " int
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Specify the algorithm used for alpha compression: 0 or 1. Algorithm 0 denotes
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no compression, 1 uses WebP lossless format for compression. The default is 1.
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.TP
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.B \-alpha_cleanup
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Modify unseen RGB values under fully transparent area, to help compressibility.
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The default is off.
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.TP
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.B \-noalpha
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Using this option will discard the alpha channel.
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.TP
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.B \-lossless
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Encode the image without any loss.
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.TP
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.BI \-hint " string
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Specify the hint about input image type. Possible values are:
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\fBphoto\fP, \fBpicture\fP or \fBgraph\fP.
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.TP
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.BI \-metadata " string
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A comma separated list of metadata to copy from the input to the output if
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present.
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Valid values: \fBall\fP, \fBnone\fP, \fBexif\fP, \fBicc\fP, \fBxmp\fP.
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The default is \fBnone\fP.
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Note: each input format may not support all combinations.
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.TP
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2011-04-22 21:14:45 +02:00
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.B \-noasm
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Disable all assembly optimizations.
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.TP
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2011-03-02 01:01:47 +01:00
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.B \-v
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Print extra information (encoding time in particular).
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.TP
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.B \-print_psnr
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Compute and report average PSNR (Peak-Signal-To-Noise ratio).
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.TP
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.B \-print_ssim
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Compute and report average SSIM (structural similarity
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metric, see http://en.wikipedia.org/wiki/SSIM for additional details).
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.TP
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.B \-print_lsim
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Compute and report local similarity metric (sum of lowest error amongst the
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collocated pixel neighbors).
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.TP
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2011-12-01 11:24:50 +01:00
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.B \-progress
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Report encoding progress in percent.
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.TP
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.B \-quiet
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Do not print anything.
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.TP
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.B \-short
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Only print brief information (output file size and PSNR) for testing purpose.
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2012-01-25 02:39:05 +01:00
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.SH BUGS
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Please report all bugs to our issue tracker:
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http://code.google.com/p/webp/issues
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.br
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Patches welcome! See this page to get started:
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http://www.webmproject.org/code/contribute/submitting-patches/
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2011-11-04 00:10:27 +01:00
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.SH EXAMPLES
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cwebp \-q 50 -lossless picture.png \-o picture_lossless.webp
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.br
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cwebp \-q 70 picture_with_alpha.png \-o picture_with_alpha.webp
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.br
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cwebp \-sns 70 \-f 50 \-size 60000 picture.png \-o picture.webp
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2011-11-04 00:10:27 +01:00
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.SH AUTHORS
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2011-03-02 01:01:47 +01:00
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\fBcwebp\fP was written by the WebP team.
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.br
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The latest source tree is available at http://www.webmproject.org/code
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.PP
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This manual page was written by Pascal Massimino <pascal.massimino@gmail.com>,
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for the Debian project (and may be used by others).
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.SH SEE ALSO
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.BR dwebp (1),
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.BR gif2webp (1).
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.br
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2012-07-21 01:19:12 +02:00
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Please refer to http://developers.google.com/speed/webp/ for additional
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information.
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