NAME
mkbitmap - transform images into bitmaps with scaling and filtering
SYNOPSIS
mkbitmap [options] [filename...]
DESCRIPTION
mkbitmap reads an image, and applies one or more of the following
operations to it, in this order: inversion, highpass filtering,
scaling, and thresholding. Each operation can be individually
controlled and turned on or off.
The principal use of mkbitmap is to convert color or greyscale images
into a format suitable as input for other programs, particularly the
tracing program potrace(1). It is particularly useful for converting
scanned line art, such as cartoons, handwritten text, etc., to high-
resolution bilevel images.
Highpass filtering can be used to ensure that features such as lines
and text are preserved, while at the same time compensating for uneven
background. Scaling is important because a scanned greyscale image
contains more visual detail than a bilevel image at the same
resolution. By scaling the image to a higher resolution (using
interpolation) before thresholding it, some of this detail is
preserved. Thresholding means converting a greyscale image to a bilevel
image using only black and white pixels. Pixels that are darker than a
certain threshold value are converted to black. Optional inversion is
useful if the input image shows bright features on dark background,
such as a picture of chalk drawings on a blackboard.
Supported input formats are PNM (PBM, PGM, PPM) and BMP. The output
formats are PBM for bitmaps, and PGM for greymaps.
OPTIONS
General options:
-h, --help print help message and exit.
-v, --version print version info and exit.
-l, --license print license info and exit.
Input/output options:
filename If filename arguments are given, then mkbitmap 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 to ".pbm" or
".pgm". If the name of the input file and output file
would be identical, then an additional suffix "-out" is
appended to the output filename. If no filename
arguments are given, then mkbitmap 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. Each input file may contain
one or more images.
-o filename, --output filename
write output to this file. All output is concatenated
and directed to the specified file. This overrides the
default behavior of creating one output file for each
input file. A filename of "-" may be given to specify
writing to standard output.
Image processing options:
-x, --nodefaults
Turn off default options. Normally, the following
options are preselected by default: -f 4 -s 2 -3 -t
0.45. The -x option disables these defaults; thus,
mkbitmap -x does nothing but copy a greyscale image from
the input to the output. Other processing options can
then be added one by one; e.g., mkbitmap -xf10 does only
highpass filtering, mkbitmap -xt0.5 does only
thresholding, etc.
-i, --invert Invert the input image. If this option is chosen, it is
applied to the image before any other operation. It is
used to deal with white-on-black images, such as
photographs of chalk drawings on a blackboard. Note that
the behavior of this option is not in general the same
as inverting the output bitmap, unless the thresholding
value is also inverted.
-f n, --filter n
Apply a highpass filter to the image. This filter is
approximately Gaussian and non-directional. The effect
is to preserve small detail while compensating for
unevenness in the background. The parameter n is a
radius (in pixels) which corresponds approximately to
the size of details which should be preserved. More
precisely, the filter is implemented by subtracting a
blurred version of the image from the original image.
The parameter n is equal to the standard deviation of
the blur. The output of the filtering step is a
normalized image whose average brightness is exactly
0.5. The default filter radius is 4.
-n, --nofilter Turn off highpass filtering.
-s n, --scale n
Scale the image by an integer factor n>0. Scaling is
done after highpass filtering, but before the
thresholding step. A scaling factor of 1 indicates that
no scaling is to be done. Otherwise, interpolation is
used to fill in the in-between pixels. If the output of
mkbitmap is to be used as input to a tracing program
such as potrace, a scaling factor of 2 is recommended.
This preserved the right amount of detail for the
tracing algorithm to work well. If a scaling factor of 1
is used, too much detail is lost. If a scaling factor of
3 or higher is used, the interpolation tends to "invent"
detail which was not present in the original image, thus
preventing potrace from doing a good job.
-1, --linear Use linear interpolation when scaling to a higher
resolution. This is slightly faster, but less nice, than
the default cubic interpolation.
-3, --cubic Use cubic interpolation when scaling to a higher
resolution. This is the default. It is slower than
linear interpolation, but leads to better results.
-t n, --threshold n
Set the threshold grey value for bilevel conversion. The
parameter n is a brightness value between 0 for black
and 1 for white. Any pixels below this brightness will
be converted to black (thus, smaller values of n will
lead to whiter output).
-g, --grey Disable bilevel conversion. If this option is given,
processing stops after the scaling step and a greymap is
output.
EXIT STATUS
The exit status is 0 on successful completion, 1 if the command line
was invalid, and 2 on any other error.
VERSION
1.8
AUTHOR
Peter Selinger <selinger at users.sourceforge.net>
WEB SITE AND SUPPORT
mkbitmap is distributed as part of the potrace package, and the latest
version is available from http://potrace.sourceforge.net/. This site
also contains documentation and information on how to obtain support.
SEE ALSO
potrace(1)
COPYRIGHT
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/.