| .\" Hey, EMACS: -*- nroff -*- |
| .TH CWEBP 1 "July 19, 2012" |
| .SH NAME |
| cwebp \- compress an image file to a WebP file |
| .SH SYNOPSIS |
| .B cwebp |
| .RI [ options ] " input_file \-o output_file.webp |
| .br |
| .SH DESCRIPTION |
| This manual page documents the |
| .B cwebp |
| command. |
| .PP |
| \fBcwebp\fP compresses an image using the WebP format. |
| Input format can be either PNG, JPEG, TIFF or raw Y'CbCr samples. |
| .SH OPTIONS |
| The basic options are: |
| .TP |
| .B \-o string |
| Specify the name of the output WebP file. If omitted, \fBcwebp\fP will |
| perform compression but only report statistics. |
| .TP |
| .B \-h, \-help |
| A short usage summary. |
| .TP |
| .B \-H, \-longhelp |
| A summary of all the possible options. |
| .TP |
| .B \-version |
| Print the version number (as major.minor.revision) and exit. |
| .TP |
| .B \-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. |
| .TP |
| .B \-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. |
| .TP |
| .B \-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. |
| .TP |
| .B \-preset string |
| Specify a set of pre-defined parameters to suit a particular type of |
| source material. Possible values are: \fBdefault\fP, \fBphoto\fP, |
| \fBpicture\fP, \fBdrawing\fP, \fBicon\fP, \fBtext\fP. Since |
| \fB\-preset\fP overwrites the other parameters' values (except the |
| \fB\-q\fP one), this option should preferably appear first in the |
| order of the arguments. |
| .TP |
| .B \-sns int |
| Specify the amplitude of the spatial noise shaping. Spatial noise shaping |
| (or \fBsns\fP 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. |
| .TP |
| .B \-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. |
| .TP |
| .B \-af |
| Turns auto-filter on. This algorithm will spend additional time optimizing |
| the filtering strength to reach a well-balanced quality. |
| |
| .SH ADDITIONAL OPTIONS |
| More advanced options are: |
| .TP |
| .B \-sharpness int |
| Specify the sharpness of the filtering (if used). |
| Range is 0 (sharpest) to 7 (least sharp). Default is 0. |
| .TP |
| .B \-strong |
| Use a stronger filtering than the default one (if filtering is being |
| used thanks to the \fB\-f\fP option). Strong filtering is off by default. |
| .TP |
| .B \-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. |
| .TP |
| .B \-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: |
| \fIError code: 6 (PARTITION0_OVERFLOW: Partition #0 is too big to fit 512k)\fP. |
| If using \fB-partition_limit\fP is not enough to meet the 512k constraint, one |
| should use less segments in order to save more header bits per macroblock. |
| See the \fB-segments\fP option. |
| .TP |
| .B \-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. |
| .TP |
| .B \-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. |
| .TP |
| .B \-pass int |
| Set a maximum number of passes to use during the dichotomy used by |
| options \fB\-size\fP or \fB\-psnr\fP. Maximum value is 10. |
| .TP |
| .B \-crop x_position y_position width height |
| Crop the source to a rectangle with top-left corner at coordinates |
| (\fBx_position\fP, \fBy_position\fP) and size \fBwidth\fP x \fBheight\fP. |
| This cropping area must be fully contained within the source rectangle. |
| .TP |
| .B \-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 \fBwidth\fP x \fBheight\fP. |
| .TP |
| .B \-map int |
| Output additional ASCII-map of encoding information. Possible map values |
| range from 1 to 6. This is only meant to help debugging. |
| .TP |
| .B \-pre int |
| Specify a pre-processing filter. This option is a placeholder |
| and has currently no effect. |
| .TP |
| .B \-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. |
| .TP |
| .B \-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. |
| .TP |
| .B \-alpha_cleanup |
| Modify unseen RGB values under fully transparent area, to help compressibility. |
| The default is off. |
| .TP |
| .B \-noalpha |
| Using this option will discard the alpha channel. |
| .TP |
| .B \-lossless |
| Encode the image without any loss. |
| .TP |
| .B \-hint string |
| Specify the hint about input image type. Possible values are: |
| \fBphoto\fP, and \fBpicture\fP. |
| .TP |
| .B \-noasm |
| Disable all assembly optimizations. |
| .TP |
| .B \-v |
| Print extra information (encoding time in particular). |
| .TP |
| .B \-print_psnr |
| Compute and report average PSNR (Peak-Signal-To-Noise ratio). |
| .TP |
| .B \-print_ssim |
| Compute and report average SSIM (structural similarity metric) |
| .TP |
| .B \-progress |
| Report encoding progress in percent. |
| .TP |
| .B \-quiet |
| Do not print anything. |
| .TP |
| .B \-short |
| Only print brief information (output file size and PSNR) for testing purpose. |
| |
| .SH BUGS |
| Please report all bugs to our issue tracker: |
| http://code.google.com/p/webp/issues |
| .br |
| Patches welcome! See this page to get started: |
| http://www.webmproject.org/code/contribute/submitting-patches/ |
| |
| .SH EXAMPLES |
| cwebp \-q 50 -lossless picture.png \-o picture_lossless.webp |
| .br |
| cwebp \-q 70 picture_with_alpha.png \-o picture_with_alpha.webp |
| .br |
| cwebp \-sns 70 \-f 50 \-strong \-af \-size 60000 picture.png \-o picture.webp |
| |
| .SH AUTHORS |
| \fBcwebp\fP was written by the WebP team. |
| .br |
| The latest source tree is available at http://www.webmproject.org/code |
| .PP |
| This manual page was written by Pascal Massimino <pascal.massimino@gmail.com>, |
| for the Debian project (and may be used by others). |
| |
| .SH SEE ALSO |
| .BR dwebp (1). |
| .br |
| Please refer to http://developers.google.com/speed/webp/ for additional |
| information. |