Pamarith User Manual(1) General Commands Manual Pamarith User Manual(1)
NAME
pamarith - perform arithmetic on two Netpbm images
SYNOPSIS
pamarith -add | -subtract | -multiply | -divide | -difference | -mini-
mum | -maximum | -mean | -equal | -compare | -and | -or | -nand | -nor
| -xor | -shiftleft | -shiftright [-closeness=N] pamfile1 pamfile2 ...
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one. You may separate an option name
and its value with white space instead of an equals sign.
DESCRIPTION
This program is part of Netpbm(1).
pamarith reads two or more PBM, PGM, PPM, or PAM images as input. It
performs the specified binary arithmetic operation on their sample val-
ues and produces an output of a format which is the more general of the
two input formats. The two input images must be of the same width and
height. The arithmetic is performed on each pair of identically lo-
cated tuples to generate the identically located tuple of the output.
For functions that are commutative and associative, pamarith applies
the binary function repetitively on as many input images as you sup-
ply. For
example, for -add , the output is the sum of all the inputs. For
other functions (e.g. -subtract), the program fails if you supply
more than two input images. (Before Netpbm 10.93 (December 2020),
the
program always failed with more than two input images).
For some other functions, pamarith could theoretically compute a
meaningful result for multiple arguments, but it fails nonetheless if
you
give more than two input images. -mean and -equal are in that
category.
Most of what pamarith does is not meaningful for visual images. It
works toward Netpbm's secondary purpose of just manipulating arbi-
trary
matrices of numbers.
For the purpose of the calculation, it assumes any PBM, PGM, or PPM in-
put image is the equivalent PAM image of tuple type BLACKANDWHITE,
GRAYSCALE, or RGB, respectively, and if it produces a PBM, PGM, or PPM
output, produces the equivalent of the PAM image which is the result of
the calculation.
The first pamfile argument identifies the "left" argument image; the
second pamfile argument identifies the "right" one.
If the output is PAM, the tuple type is the same as the tuple type of
the left input image.
pamarith performs the arithmetic on each pair of identically located
tuples in the two input images.
The arithmetic operation is in all cases fundamentally a function from
two integers to an integer (but see below - the functions are defined
in ways that you can effectively e.g. add real numbers). The operation
is performed on two tuples as follows. The two input images must have
the same depth, or one of them must have depth one. pamarith fails if
one of these is not the case.
If they have the same depth, pamarith simply carries out the arithmetic
one sample at a time. I.e. if at a particular position the left input
image contains the tuple (s1,s2,...,sN) and the right input image con-
tains the tuple (t1,t2,...tN), and the function is f, then the output
image contains the tuple (f(s1,t1),f(s2,t2),...,f(sN,tN)).
If one of the images has depth 1, the arithmetic is performed between
the one sample in that image and each of the samples in the other.
I.e. if at a particular position the left input image contains the tu-
ple (s) and the right input image contains the tuple (t1,t2,...tN), and
the function is f, then the output image contains the tuple
(f(s,t1),f(s,t2),...,f(s,tN)).
PBM Oddness
If you're familiar with the PBM format, you may find pamarith's opera-
tion on PBM images to be nonintuitive. Because in PBM black is repre-
sented as 1 and white as 0, you might be expecting black minus black to
be white.
But the PBM format is irrelevant, because pamarith operates on the num-
bers found in the PAM equivalent (see above). In a PAM black and white
image, black is 0 and white is 1. So black minus black is black.
Maxval
The meanings of the samples with respect to the maxval varies according
to the function you select.
In PAM images in general, the most usual meaning of a sample (the one
that applies when a PAM image represents a visual image), is that it
represents a fraction of some maximum. The maxval of the image corre-
sponds to some maximum value (in the case of a visual image, it corre-
sponds to "full intensity."), and a sample value divided by the maxval
gives the fraction.
For pamarith, this interpretation applies to the regular arithmetic
functions: -add, -subtract, -multiply, -divide, -difference, -minimum,
-maximum, -mean, -equal, and -compare. For those, you should think of
the arguments and result as numbers in the range [0,1). For example,
if the maxval of the left argument image is 100 and the maxval of the
right argument image is 200 and the maxval of the output image is 200,
and the left sample value in an -add calculation is 50 and the right
sample is 60, the actual calculation is 50/100 + 60/200 = 160/200, and
the output sample value is 160.
For these functions, pamarith makes the output image's maxval the maxi-
mum of the two input maxvals, except with -equal and -compare. For
-equal, the output maxval is always 1. For -compare, it is always 2.
(Before Netpbm 10.14 (February 2003), there was no exception for -com-
pare; in 10.14, the exception was just that the maxval was at least 2,
and sometime between 10.18 and 10.26 (January 2005), it changed to be-
ing exactly 2).
If the result of a calculation falls outside the range [0, 1), pamarith
clips it -- i.e. considers it to be zero or 1-.
In many cases, where both your input maxvals are the same, you can just
think of the operation as taking place between the sample values di-
rectly, with no consideration of the maxval except for the clipping.
E.g. an -add of sample value 5 to sample value 8 yields sample value
13.
But with -multiply, this doesn't work. Say your two input images have
maxval 255, which means the output image also has maxval 255. Consider
a location in the image where the input sample values are 5 and 10.
You might think the multiplicative product of those would yield 50 in
the output. But pamarith carries out the arithmetic on the fractions
5/255 and 10/255. It multiplies those together and then rescales to
the output maxval, giving a sample value in the output PAM of 50/255
rounded to the nearest integer: 0.
With the bit string operations, the maxval has a whole different mean-
ing. The operations in question are: -and, -or, -nand, -nor, -xor, and
-shiftleft, -shiftright.
With these, each sample value in one or both input images, and in the
output image, represents a bit string, not a number. The maxval tells
how wide the bit string is. The maxval must be a full binary count (a
power of two minus one, such as 0xff) and the number of ones in it is
the width of the bit string. For the dyadic bit string operations
(that's everything but the shift functions), the maxvals of the input
images must be the same and pamarith makes the maxval of the output im-
age the same.
For the bit shift operations, the output maxval is the same as the left
input maxval. The right input image (which contains the shift counts)
can have any maxval and the maxval is irrelevant to the interpretation
of the samples. The sample value is the actual shift count. But it's
still required that no sample value exceed the maxval.
NOTE: UNARY FUNCTIONS
pamarith applies only binary functions. If you want to apply a unary
function, e.g. "halve", to a single image, use pamfunc.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pamarith recognizes the
following command line options:
The Function
These options select the function that pamarith applies.
You must specify one of these, and cannot specify more than one.
-add Adds the two values. If the result is larger than maxval, it is
clipped.
-subtract
Subtracts a value in the right input image from a value in the
left input image.
-minimum
Chooses the smaller value of the two.
-maximum
Chooses the larger value of the two.
-difference
Calculates the absolute value of the difference.
-multiply
Does an ordinary arithmetic multiplication, but tends to produce
nonobvious results because of the way pamarith interprets sample
values. See Maxval ⟨#maxval⟩ .
-divide
Divides a value in the left input image by the value in the
right input image. But like -multiply, it tends to produce
nonobvious results. Note that pamarith clipping behavior makes
this of little use when the left argument (dividend) is greater
than the right argument (divisor) -- the result in that case is
always the maxval. If the divisor is 0, the result is the max-
val.
-divide was new in Netpbm 10.30 (October 2005).
-equal Produces maxval when the values in the two images are equal and
zero when they are not. Note that the output maxval is always 1
for -equal.
If the maxvals of the input images are not identical, pamarith
may claim two values are not equal when in fact they are, be-
cause of the precision with which it does the arithmetic. How-
ever, it will never say A is greater than B if A is less than B.
You can make the equality test approximate with the -closeness
option. This gives the percentage of maxval by which the sam-
ples can differ and still be considered equal.
-equal was new in Netpbm 10.93 (December 2020).
-compare
Produces the value 0 when the value in the left input image is
less than the value in the right input image, 1 when the values
are equal, and 2 when the left is greater than the right.
If the maxvals of the input images are not identical, pamarith
may claim two values are not equal when in fact they are, be-
cause of the precision with which it does the arithmetic. How-
ever, it will never say A is greater than B if A is less than B.
-compare was new in Netpbm 10.13 (December 2002).
-and, -nand, -or, -nor, -xor
These consider the input and output images to contain bit
strings; they compute bitwise logic operations. Note that if
the maxval is 1, you can also look at these as logic operations
on boolean input values. See section Maxval ⟨#maxval⟩ for the
special meaning of maxval with respect to bit string operations
such as these.
-shiftleft, -shiftright
These consider the left input image and output image to contain
bit strings. They compute a bit shift operation, with bits
falling off the left or right end and zeroes shifting in, as op-
posed to bits off one end to the other. The right input image
sample value is the number of bit positions to shift.
Note that the maxval (see Maxval ⟨#maxval⟩ ) determines the
width of the frame within which you are shifting.
Other
-closeness
This changes the meaning of -equal. It is not valid with any
other function. See the description of -equal.
SEE ALSO
pamfunc(1), pamsummcol(1), pamsumm(1), pnminvert(1), pambrighten(1),
ppmdim(1), pnmconvol(1), pamdepth(1), pnmpsnr(1), pnm(1), pam(1)
HISTORY
pamarith replaced pnmarith in Netpbm 10.3 (June 2002).
In Netpbm 10.3 through 10.8, though, pamarith was not backward compati-
ble because it required the input images to be of the same depth, so
you could not multiply a PBM by a PPM as is often done for masking.
(It was not intended at the time that pnmarith would be removed from
Netpbm -- the plan was just to rewrite it to use pamarith; it was re-
moved by mistake).
But starting with Netpbm 10.9 (September 2002), pamarith allows the im-
ages to have different depths as long as one of them has depth 1, and
that made it backward compatible with pnmarith.
The original pnmarith did not have the -mean option.
The -compare option was added in Netpbm 10.13 (December 2002).
The bit string operations were added in Netpbm 10.27 (March 2005).
The -divide option was added in Netpbm 10.30 (October 2005).
The ability to have more than one input (operand) was added in Netpbm
10.93 (December 2020).
The -equal option was added in Netpbm 10.93 (December 2020).
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML
source. The master documentation is at
http://netpbm.sourceforge.net/doc/pamarith.html
netpbm documentation 24 October 2020 Pamarith User Manual(1)
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