Ubuntu Manpage: cwebp - compress an image file to a WebP file

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 compression. 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 transfer 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 in 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 produce 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 quality.

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 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: Error code: 6 (PARTITION0_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 compressed 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 compressed 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.

-resize width height
Resize the source to a rectangle with size width x height. If either (but not both) of the width
or height parameters is 0, the value will be calculated preserving the aspect-ratio.

-crop x_position y_position width height
Crop the source to a rectangle with top-left corner at coordinates (x_position, y_position) and
size width x height. This cropping area must be fully contained within the source rectangle.

-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 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 debugging.

-pre int
Specify some pre-processing steps. Using a value of '2' will trigger quality-dependent pseudo-
random dithering during RGBA->YUVA conversion (lossy compression only).

-alpha_filter string
Specify the predictive filtering method for the 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 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. Algorithm 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.

-blend_alpha int
This option blends the alpha channel (if present) with the source using the background color
specified in hexadecimal as 0xrrggbb. The alpha channel is afterward reset to the opaque value
255.

-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.webmproject.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
       cwebp -o picture.webp -- ---picture.png

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.massimino@gmail.com>, for the Debian project
       (and may be used by others).