Tribblix: manual page: cwebp.1

CWEBP(1) User Commands 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, WebP or raw Y'CbCr samples. Note: Animated
PNG and WebP files are not supported.

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. Using
"-" as output name will direct output to 'stdout'.

-- string
Explicitly specify the input file. This option is useful if
the input file starts with a '-' for instance. This option
must appear last. Any other options afterward will be
ignored.

-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.

-lossless
Encode the image without any loss. For images with fully
transparent area, the invisible pixel values (R/G/B or Y/U/V)
will be preserved only if the -exact option is used.

-near_lossless int
Specify the level of near-lossless image preprocessing. This
option adjusts pixel values to help compressibility, but has
minimal impact on the visual quality. It triggers lossless
compression mode automatically. The range is 0 (maximum
preprocessing) to 100 (no preprocessing, the default). The
typical value is around 60. Note that lossy with -q 100 can at
times yield better results.

-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.

-z int Switch on lossless compression mode with the specified level
between 0 and 9, with level 0 being the fastest, 9 being the
slowest. Fast mode produces larger file size than slower ones.
A good default is -z 6. This option is actually a shortcut
for some predefined settings for quality and method. If
options -q or -m are subsequently used, they will invalidate
the effect of this option.

-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.

-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.

-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.

-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. Note: the cropping is applied before any scaling.

-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.
Note: scaling is applied after cropping.

-mt Use multi-threading for encoding, if possible.

-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.

LOSSY OPTIONS

These options are only effective when doing lossy encoding (the
default, with or without alpha).

-size int
Specify a target size (in bytes) to try and reach for the
compressed output. The compressor will make several passes of
partial encoding in order to get as close as possible to this
target. If both -size and -psnr are used, -size value will
prevail.

-psnr float
Specify a target PSNR (in dB) to try and reach for the
compressed output. The compressor will make several passes of
partial encoding in order to get as close as possible to this
target. If both -size and -psnr are used, -size value will
prevail.

-pass int
Set a maximum number of passes to use during the dichotomy
used by options -size or -psnr. Maximum value is 10, default
is 1. If options -size or -psnr were used, but -pass wasn't
specified, a default value of '6' passes will be used. If
-pass is specified, but neither -size nor -psnr are, a target
PSNR of 40dB will be used.

-qrange int int
Specifies the permissible interval for the quality factor.
This is particularly useful when using multi-pass (-size or
-psnr options). Default is 0 100. If the quality factor is
outside this range, it will be clamped. If the minimum value
must be less or equal to the maximum one.

-af Turns auto-filter on. This algorithm will spend additional
time optimizing the filtering strength to reach a
well-balanced 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.

Advanced options:

-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.

-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.

-sharp_yuv
Use more accurate and sharper RGB->YUV conversion if needed.
Note that this process is slower than the default 'fast'
RGB->YUV conversion.

-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 50.

-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 for a 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. Note the -m and -q options also influence
the encoder's decisions and ability to hit this limit.

LOGGING OPTIONS

These options control the level of output:

-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 https://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 purposes.

-map int
Output additional ASCII-map of encoding information. Possible
map values range from 1 to 6. This is only meant to help
debugging.

ADDITIONAL OPTIONS

More advanced options are:

-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.

-pre int
Specify some preprocessing 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.

-exact Preserve RGB values in transparent area. The default is off,
to help compressibility.

-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.

-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.

BUGS

Please report all bugs to the issue tracker:
https://bugs.chromium.org/p/webp
Patches welcome! See this page to get started:
https://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 is a part of libwebp and was written by the WebP team.
The latest source tree is available at
https://chromium.googlesource.com/webm/libwebp

This manual page was written by Pascal Massimino
<pascal.massimino@gmail.com>, for the Debian project (and may be used
by others).