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, 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.

-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 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.

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).