Reads in a PBM or PGM format image from a file or standard input,
compresses it, and outputs the image as a
JBIG
bi-level image entity (BIE) file.
JBIG
is a highly effective lossless compression algorithm for bi-level
images (one bit per pixel), which is particularly suitable for scanned
document pages.
A
JBIG
encoded image can be stored in several resolutions (progressive mode).
These resolution layers can be stored all in one single BIE or they
can be stored in several separate BIE files. All resolution layers
except the lowest one are stored merely as differences to the next
lower resolution layer, because this requires less space than encoding
the full image completely every time. Each resolution layer has twice
the number of horizontal and vertical pixels than the next lower
layer.
JBIG
files can also store several bits per pixel as separate bitmap planes,
and
pnmtojbig
can read a PGM file and transform it into a multi-bitplane BIE.
OPTIONS
-
A single hyphen instead of an input file name will cause
pnmtojbg
to read the data from standard input instead from a file.
-q
Encode the image in one single resolution layer (sequential mode). This
is usually the most efficient compression method. By default, the number
of resolution layers is chosen automatically such that the lowest layer
image is not larger than 640 × 480 pixels.
-x number
Specify the maximal horizontal size of the lowest resolution layer.
The default is 640 pixels.
-y number
Specify the maximal vertical size of the lowest resolution layer.
The default is 480 pixels.
-l number
Select the lowest resolution layer that will be written to the
BIE. It is possible to store the various resolution layers of a
JBIG
image in progressive mode into different BIEs. Options
-l
and
-h
allow to select the resolution-layer interval that will appear
in the created BIE. The lowest resolution layer has number 0 and
this is also the default value. By default all layers will be written.
-h number
Select the highest resolution layer that will be written to the
BIE. By default all layers will be written. See also option
-l.
-b
Use binary values instead of Gray code words in order to encode pixel
values in multiple bitplanes. This option has only an effect if the
input is a PGM file and if more than one bitplane is produced. Note
that the decoder has to make the same selection but cannot determine
from the BIE, whether Gray or binary code words were used by the
encoder.
-d number
Specify the total number of differential resolution layers into which the
input image will be split in addition to the lowest layer. Each additional
layer reduces the size of layer 0 by 50 %. This option overrides options
-x
and
-y
which are usually a more comfortable way of selecting the number of
resolution layers.
-s number
The
JBIG
algorithm splits each image into a number of horizontal stripes. This
option specifies that each stripe shall have
number
lines in layer 0. The default value is selected so that approximately
35 stripes will be used for the whole image.
-m number
Select the maximum horizontal offset of the adaptive template pixel.
The
JBIG
encoder uses a number of neighbour pixels in order to get statistical
a priori knowledge of the probability, whether the next pixel will be black
or white. One single pixel out of this template of context neighbor
pixels can be moved around. Especially for dithered images it can be a
significant advantage to have one neighbor pixel which has a distance large
enough to cover the period of a dither function. By default, the
adaptive template pixel can be moved up to 8 pixels away. This encoder
supports up to 23 pixels, however as decoders are only required to
support at least a distance of 16 pixels by the standard, no higher value
than 16 for
number
is recommended in order to maintain interoperability with other
JBIG
implementations. The maximal vertical offset of the adaptive
template pixel is always zero.
-t number
Encode only the specified number of most significant bit planes. This
option allows to reduce the depth of an input PGM file if not all
bits per pixel are needed in the output.
-o number
JBIG
separates an image into several horizontal stripes, resolution layers
and planes, were each plane contains one bit per pixel. One single
stripe in one plane and layer is encoded as a data unit called stripe
data entity (SDE) inside the BIE. There are 12 different possible
orders in which the SDEs can be stored inside the BIE and
number
selects which one shall be used. The order of the SDEs is only relevant
for applications that want to decode a
JBIG
file which has not yet completely arrived from e.g. a slow network connection.
For instance some applications prefer that the outermost of the three loops
(stripes, layers, planes) is over all layers so that all data of the lowest
resolution layer is transmitted first.
The following values for
number
select these loop arrangements for writing the SDEs (outermost
loop first):
0 planes, layers, stripes
2 layers, planes, stripes
3 layers, stripes, planes
4 stripes, planes, layers
5 planes, stripes, layers
6 stripes, layers, planes
All loops count starting with zero, however by adding 8 to the above
order code, the layer loop can be reversed so that it counts down to zero
and then higher resolution layers will be stored before lower layers.
Default order is 3 which writes at first all planes of the first
stripe and then completes layer 0 before continuing with the next
layer and so on.
-p number
This option allows to activate or deactivate various optional algorithms
defined in the
JBIG
standard. Just add the numbers of the following options which you want to
activate in
order to get the
number
value:
4 deterministic prediction (DPON)
8 layer 0 typical prediction (TPBON)
16 diff. layer typ. pred. (TPDON)
64 layer 0 two-line template (LRLTWO)
Except for special applications (like communication with
JBIG
subset implementations) and for debugging purposes you will normally
not want to change anything here. The default is 28, which provides
the best compression result.
-c
The adaptive template pixel movement is determined as suggested in
annex C of the standard. By default the template change takes place
directly in the next line which is most effective. However a few conformance
test examples in the standard require the adaptive template change to be
delayed until the first line of the next stripe. This option selects
this special behavior, which is normally not required except in order
to pass some conformance test suite.
-v
After the BIE has been created, a few technical details of the created
file will be listed (verbose mode).
FORMATS
Most of the format
pnmtojbig
creates is defined by the
JBIG
standard.
The standard, however, does not specify which values in the BIE mean
white and which mean black. It contains a recommendation that for a
single plane image zero mean background and one mean foreground, but
the Netpbm formats have no concept of foreground and background. And
the standard says nothing about values for multiple plane BIEs.
pnmtojbig
follows Markus Kuhn's implementation of the standard in the
pbmtojbg
program that comes with his
JBIG
library: If the BIE is a single plane BIE, zero means white and one
means black. If it is a multiple plane BIE, zero means black and the
maximal value is white.
STANDARDS
This program implements the
JBIG
image coding algorithm as specified in ISO/IEC 11544:1993 and
ITU-T T.82(1993).
AUTHOR
pnmtojbig
is based on the
JBIG
library by Markus Kuhn, part of his
JBIG-KIT
package.
The
pbmtojbg
program is part of the
JBIG-KIT
package. The most recent version of that library and tools set is
freely available on the Internet from anonymous ftp server
ftp.informatik.uni-erlangen.de in directory pub/doc/ISO/JBIG/.
pnmtojbig
is part of the Netpbm package of graphics tools.
If you use
pnmtojbig,
you are using various patents, particularly on its arithmetic encoding
method, and in all probability you do not have a license from the
patent owners to do so.