Ubuntu Manpage: mkbitmap - transform images into bitmaps with scaling and filtering

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 foreground features such as lines and text are preserved,
while at the same time compensating for uneven background. Optional blurring can be applied to smooth out
the image and remove visual noise. 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; the output for this argument will then be written to standard output.
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 background
gradients. 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.

-b n, --blur n Blur the image. The effect is to smooth out fine details and to reduce visual noise in the
image. The parameter n is the blurring radius, and should be chosen small (1 is a good
value to start with). This is implemented as an approximately Gaussian, non-directional
blur with standard deviation proportional to n. Blurring is applied after the highpass
filter, but before scaling and thresholding. If this option is not given, the default is
not to apply any blurring.

-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

       1.16

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.

COPYRIGHT

       Copyright (C) 2001-2019 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.   See
       also http://www.gnu.org/.

Version 1.16                                     September 2019                                      mkbitmap(1)