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MiguelBridge/WebPConverter/libwebp-0.4.1-mac-10.8/doc/cwebp.txt
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CWEBP(1) 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 YCbCr 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 compres
sion. 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 predefined parameters to suit a particular
type of source material. Possible values are: default, photo,
picture, drawing, icon, text. Sincepreset 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 builtin algorithms used to decide which area of the picture
should use relatively less bits, and where else to better trans
fer 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 pro
duce an output file of similar size to its JPEG equivalent (for
the sameq setting), but with less visual distortion.
mt Use multithreading 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 autofilter on. This algorithm will spend additional time
optimizing the filtering strength to reach a wellbalanced qual
ity.
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
thef 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, unlesslow_memory is used.
partition_limit int
Degrade quality by limiting the number of bits used by some mac
roblocks. Range is 0 (no degradation, the default) to 100 (full
degradation). Useful values are usually around 3070 for moder
ately large images. In the VP8 format, the socalled 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
subblocks. 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 bitcostly 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 (PARTI_
TION0_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 thesegments option.
size int
Specify a target size (in bytes) to try and reach for the com
pressed 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 com
pressed 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 optionssize orpsnr. 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 aspectratio.
crop x_position y_position width height
Crop the source to a rectangle with topleft corner at coordi
nates (x_position, y_position) and size width x height. This
cropping area must be fully contained within the source rectan
gle.
s width height
Specify that the input file actually consists of raw YCbCr sam
ples following the ITUR BT.601 recommendation, in 4:2:0 linear
format. The luma plane has size width x height.
map int
Output additional ASCIImap of encoding information. Possible
map values range from 1 to 6. This is only meant to help debug
ging.
pre int
Specify some preprocessing steps. Using a value of 2 will
trigger qualitydependent pseudorandom 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 slow
ness 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 apriori guess without testing all
modes.
alpha_method int
Specify the algorithm used for alpha compression: 0 or 1. Algo
rithm 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 (PeakSignalToNoise 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.webmpro
ject.org/code/contribute/submittingpatches/
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.mas
simino@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.
July 22, 2014 CWEBP(1)
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