Section: User Commands (1)
Updated: April 2007
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potrace - transform bitmaps into vector graphics.  


potrace [options] [filename...]


potrace is a utility for tracing a bitmap, which means, transforming a bitmap into a smooth, scalable image. The input is a bitmap, which means, a pixel-based image composed of the two colors black and white only. The default output is an encapsulated PostScript file (EPS). A typical use is to create EPS files from scanned data, such as company or university logos, handwritten notes, etc. The resulting image is not "jaggy" like a bitmap, but smooth. It can then be rendered at any resolution.

potrace can read bitmaps in the following formats: PBM, PGM, PPM (collectively known as PNM, see pnm(5)), as well as BMP (Windows and OS/2 bitmap formats). The input image should only use the two colors black and white. If other pixel values appear in the input, they will be converted to black and white using a simple threshold method.

potrace can currently produce the following output formats: EPS, PostScript, PDF, SVG, PGM, and XFIG. An experimental gimppath backend is also available. Additional backends might be added in the future.  


The following options are supported. Dimensions (arguments of type dim) can have optional units, e.g. 6.5in, 15cm, 100pt. The default unit is inches (or centimeters, if this was configured at compile time, see COMPILE TIME CONFIGURATION below). For pixel-based output formats such as PGM and gimppath, the default unit is pixels.  

General options:

-h, --help
print help message and exit.
-v, --version
print version info and exit.
-l, --license
print license info and exit.
-V, --show-defaults
print the defaults which were compiled into this version of potrace, and exit.
display a progress bar for each bitmap that is processed. This is useful for interactive use, and requires a terminal. The default behavior is not to show any progress information.

Input/output options:

If filename arguments are given, then potrace will by default create one output file for each input filename given. The name of the output file is obtained from the input filename by changing its suffix according to the chosen backend. If changing the suffix is impossible because the names of the input and output files would be identical, then the output filename is created by adding the "-out" suffix to the name of the input file. If no filename arguments are given, then potrace acts as a filter, reading from standard input and writing to standard output. A filename of "-" may be given to specify reading from standard input.
-o filename, --output filename
write output to this file. All output is directed to the specified file. If this option is used, then multiple input filenames are allowed only for those backends which can handle multiple bitmaps; these are currently the PostScript and PGM backends. In this case, each input file may contain one or more bitmaps, and all the bitmaps from all the input files are processed and the output concatenated into a single file. A filename of "-" may be given to specify writing to standard output.

Backend selection:

-b name, --backend name
Select backend by name, where name is one of eps, postscript, ps, pdf, svg, pgm, gimppath, xfig. Backend names can be abbreviated by an unambiguous prefix.
-e, --eps
Encapsulated PostScript backend (default). The input is a single bitmap; the output is a stand-alone, encapsulated PostScript file which can be included in other documents.
-p, --postscript
PostScript backend. For this backend, the input may consist of multiple concatenated bitmaps, which will be turned into a multi-page PostScript document.
-b pdf, --backend pdf
PDF backend. For this backend, the input may consist of multiple concatenated bitmaps, which will be turned into a multi-page PDF document.
-s, --svg
SVG backend. This backend produces a scalable vector graphics (SVG) file. This backend can process one bitmap per output file.
-g, --pgm
PGM backend. This backend produces a portable greymap (PGM) file. It is a convenient backend for antialiasing a bitmap image. The input may consist of multiple concatenated bitmaps, in which case multiple greymaps will be written to the output.
-b gimppath, --backend gimppath
Experimental gimppath backend. This backend produces a path which can be imported by the GNU Image Manipulation Program (Gimp) (in the Layers, Channels & Paths dialog, select Paths, then right-click and select Import Path). Note that Gimp's paths have some peculiar features: they have no bounding box, they cannot be scaled, and all control points are forced to be integers. For these reasons, it is difficult to produce good Gimp paths at low resolutions. The gimppath backend ignores almost all scaling and placement options; instead, the --unit option must be used to set the desired magnification. The --rotate option is also recognized. Note that the imported path can lie outside the bounds of the current image in Gimp; in this case, one needs to move the path (alt-click) or work with a larger image.
-b xfig, --backend xfig
XFig backend. Note that XFig uses X-splines instead of Bezier curves, thus it is not possible to translate the output of potrace into the XFig format with absolute accuracy. This backend does a reasonable approximation using two control points for each Bezier curve segment. Note that the -u option has no effect for this backend, as control points are always rounded to the nearest 1/1200 of an inch in XFig. Rotation and color are implemented. Curve optimization is disabled.

Algorithm options:

For more detailed information on these options, see TECHNICAL DOCUMENTATION below.
-z policy, --turnpolicy policy
specify how to resolve ambiguities in path decomposition. Must be one of black, white, right, left, minority, majority, or random. Default is minority. Turn policies can be abbreviated by an unambigous prefix, e.g., one can specify min instead of minority.
-t n, --turdsize n
suppress speckles of up to this many pixels.
-a n, --alphamax n
set the corner threshold parameter. The default value is 1. The smaller this value, the more sharp corners will be produced. If this parameter is negative, then no smoothing will be performed and the output is a polygon. The largest useful value is 4/3 or 1.334, which suppresses all corners and leads to completely smooth output.
-n, --longcurve
turn off curve optimization. Normally potrace tries to join adjacent Bezier curve segments when this is possible. This option disables this behavior, resulting in a larger file size.
-O n, --opttolerance n
set the curve optimization tolerance. The default value is 0.2. Larger values allow more consecutive Bezier curve segments to be joined together in a single segment, at the expense of accuracy.
-u n, --unit n
set output quantization. Coordinates in the output are rounded to 1/unit pixels. The default of 10 usually gives good results. For some of the debug modes, a value of 100 gives more accurate output. In the case of the gimppath backend, the -u option is used to set the desired magnification. This option has no effect for the XFig backend, which always rasterizes to 1/1200 inch.
-d n, --debug n
produce debugging output of type n. This has different effects for different backends. For the PostScript/EPS backends, the values n=1,2,3 illustrate the intermediate stages of the potrace algorithm.

Scaling and placement options:

-W dim, --width dim
set the width of output image. If only one of width and height is specified, the other is adjusted accordingly so that the aspect ratio is preserved.
-H dim, --height dim
set the height of output image. See -W for details.
-r n[xn], --resolution n[xn]
set the resolution (in dpi). One inch in the output image corresponds to this many pixels in the input. Note that it follows that a larger value results in a smaller output image. It is possible to specify different resolutions in the x and y directions by giving an argument of the form nxn. This option has no effect for the PGM backend.
-x n[xn], --scale n[xn]
set the scaling factor (PGM backend only). A value greater than 1 enlarges the output, a value between 0 and 1 makes the output smaller. It is possible to specify different scaling factors in the x and y directions by giving an argument of the form nxn.
-S n, --stretch n
set the aspect ratio. A value greater than 1 means the image will be stretched in the y direction. A value between 0 and 1 means the image will be stretched in the x direction.
-A angle, --rotate angle
set the rotation angle (in degrees). The output will be rotated counterclockwise by this angle. This is useful for compensating for images that were scanned not quite upright.
-M dim, --margin dim
set all four margins. The effect of this depends on the backend. For variable-sized backends (EPS, PGM, SVG), the margins will simply be added around the output image (or subtracted, in case of negative margins). For fixed-size backends (PostScript, XFig), the margins settings can be used to control the placement of the image on the page. If only one of the left and right margin is given, the image will be placed this distance from the respective edge of the page, and similarly for top and bottom. If margins are given on opposite sides, the image is scaled to fit between these margins, but not if the scaling is already determined explicitly by one or more of the -W, -H, -r, or -x options.
-L dim, --leftmargin dim
set the left margin. See -M for details.
-R dim, --rightmargin dim
set the right margin. See -M for details.
-T dim, --topmargin dim
set the top margin. See -M for details.
-B dim, --bottommargin dim
set the bottom margin. See -M for details.

Output options:

These options are only supported by certain backends.
-C #rrggbb, --color #rrggbb
set the foreground color of the output image. The default is black. This option works for the PS/EPS, SVG, and XFig backends.
--fillcolor #rrggbb
set the fill color of the output image, i.e., the color of the "white" parts. The default is to leave these parts transparent. This option works for the PS/EPS, SVG, and XFig backends. Implies --opaque.
fill in the white parts of the image opaquely, instead of leaving them transparent. This only applies to interior white parts, i.e., those which are enclosed inside a black outline. This option works for the PS/EPS and SVG backends. Opaqueness is always in effect for the XFig backend.
try to group related paths together in the SVG output. Each path is grouped together with all paths that are contained inside it, so that they can be moved around as a unit with an SVG editor. This makes coloring individual components slightly more cumbersome, and thus it is not the default.

PostScript/EPS options:

-P format, --pagesize format
set page size. This is primarily used for the PostScript backend. In case of the EPS backend, it may influence the default image size. The following formats can be specified: a4, a3, a5, b5, letter, legal, tabloid, statement, executive, folio, quarto, 10x14. Also, an argument of the form dimxdim is accepted to specify arbitrary dimensions. The default page size is letter (or a4, if this was configured at compile time, see COMPILE TIME CONFIGURATION below). Page format names can be abbreviated by an unambigous prefix.
-c, --cleartext
do not compress the output. This option disables the use of compression filters in the PostScript output. If the -q option is also used, the resulting output can be easily read by other programs or even by humans.
-2, --level2
use PostScript level 2 compression (default). The resulting file size is ca. 40% smaller than if the -c option is used.
-3, --level3
use PostScript level 3 compression, if available. This gives slightly smaller files than using -2, but the resulting files may not print on older PostScript level 2 printers. If support for PostScript level 3 compression has been disabled at compile time, a warning message is printed and level 2 compression is used instead.
-q, --longcoding
turn off optimized numerical coding. Normally, potrace uses a very compact numerical format to represent Bezier curves in PostScript, taking advantage of some redundancy in the curve parameters. This option disables this behavior, resulting in longer, but more readable output (particularly if the -c option is also used).

PGM options:

-G n, --gamma n
set the gamma value for anti-aliasing (default is 2.2). Most computer displays do not render shades of grey linearly, i.e., a grey value of 0.5 is not displayed as being exactly half-way between black and white. The gamma parameter corrects for this, and therefore leads to nicer looking output. The default value of 2.2 is appropriate for most normal CRT displays.

Frontend options:

-k n, --blacklevel n
set the threshold level for converting input images to bitmaps. The potrace algorithm expects a bitmap, thus all pixels of the input images are converted to black or white before processing begins. Pixels whose brightness is less than n are converted to black, all other pixels to white. Here n is a number between 0 and 1. One case is treated specially: if the input is in an indexed color format with exactly 2 colors, then the blacklevel is ignored and the darker of the two colors is mapped to black.

Note: the method used by potrace for converting greymaps to bitmaps is very crude; much better results can be obtained if a separate program, such as mkbitmap(1), is used for this purpose. In particular, mkbitmap(1), which is distributed with potrace, has the ability to scale and interpolate the image before thresholding, which results in much better preservation of detail.

-i, --invert
invert the input bitmap before processing.


Certain aspects of the behavior of potrace can be configured at compile time by passing the following options to the ./configure script.

compile potrace without the zlib compression library. This means PostScript level 3 compression will not be available.
compile potrace with centimeters as the default unit instead of inches.
compile potrace with A4 as the default page size.


The exit status is 0 on successful completion, 1 if the command line was invalid, and 2 on any other error.  




Peter Selinger <selinger at users.sourceforge.net>  


For a detailed technical description of the potrace algorithm, see the file potrace.pdf, which is available from the potrace web site. For information on the Potrace library API, see potracelib.pdf.  


The latest version of potrace is available from http://potrace.sourceforge.net/. This site also contains a list of frequently asked questions, as well as information on how to obtain support.  




Copyright (C) 2001-2007 Peter Selinger

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA. See also http://www.gnu.org/.



General options:
Input/output options:
Backend selection:
Algorithm options:
Scaling and placement options:
Output options:
PostScript/EPS options:
PGM options:
Frontend options:

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