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.
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 an '-' 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.
- -q float
- Specify the compression factor for RGB channels between 0 and 100. The
default is 75.
- -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).
SEE ALSO¶
dwebp(1),
gif2webp(1)
Please refer to
http://developers.google.com/speed/webp/ for additional
information.