cwebp − compress an image file to a WebP file
cwebp [options] input_file −o output_file.webp
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.
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. A small factor produces a smaller file with lower quality. Best quality is achieved using a value of 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.
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−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.
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−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 is faster processing time at the expense of larger file size and lower compression quality. | ||
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−af |
Turns auto-filter on. This algorithm will spend additional time optimizing the filtering strength to reach a well-balanced quality. |
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 a stronger filtering than the default one (if filtering is being used thanks to the −f option). Strong filtering is off by default.
−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.
−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.
−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 a pre-processing filter. This option is a placeholder and has currently no effect.
−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.
−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.
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−noasm |
Disable all assembly optimizations. | ||
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−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)
−progress
Report encoding progress in percent.
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−quiet |
Do not print anything. | ||
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−short |
Only print brief information (output file size and PSNR) for testing purpose. |
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/
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 −strong −af
−size 60000 picture.png −o picture.webp
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).
dwebp(1).
Please refer to http://developers.google.com/speed/webp/ for
additional information.