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Pamarith User Manual(0) 						  Pamarith User Manual(0)

NAME
       pamarith - perform arithmetic on two Netpbm images

SYNOPSIS
       pamarith  -add  |  -subtract  |	-multiply | -divide | -difference | -minimum | -maximum |
       -mean | -compare | -and | -or | -nand | -nor | -xor | -shiftleft  |  -shiftright  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  PBM,  PGM,  PPM,  or PAM images as input.	It performs the specified
       binary arithmetic operation on their sample values 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  located
       tuples to generate the identically located tuple of the output.

       For  the  purpose  of  the calculation, it assumes any PBM, PGM, or PPM input 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  argu-
       ment 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.  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 contains the tuple (t1,t2,...tN), and  the  func-
       tion 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  tuple  (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)).

   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 corresponds to some maximum value (in the case of a visual  image,
       it  corresponds	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, -sub-
       tract,  -multiply,  -divide,  -difference,  -minimum,  -maximum, -mean, 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 argu-
       ment 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 maximum of the two input
       maxvals,  except with -compare, where pamarith uses an output maxval of 2.  (Before Netpbm
       10.14 (February 2003), there was no exception for -compare; 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 being 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 oper-
       ation as taking place between the sample values directly, 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 meaning.  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, repre-
       sents  a bit string, not a number.  The maxval tells how wide the bit string is.  The max-
       val 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 image 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.

   The Operations
       Most of the operations are obvious from the option name.  The following	paragraphs  cover
       those that aren't.

       -subtract subtracts a value in the right input image from a value in the left input image.

       -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 left 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 maxval.	This option was new in Netpbm 10.30 (October 2005).

       -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, due to the precision with which it does the arithmetic.
       However, 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, and -xor 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 and -shiftright 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 opposed 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.

   Notes
       If you want to apply a unary function, e.g. "halve", to a single image, use pamfunc.

SEE ALSO
       pamfunc(1) , pamsummcol(1) , pamsumm(1) , pnminvert(1) , ppmbrighten(1) , ppmdim(1) , pnm-
       convol(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 compatible 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 removed  by
       mistake).

       But starting with Netpbm 10.9 (September 2002), pamarith allows the images to have differ-
       ent 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).

netpbm documentation			  08 April 2007 		  Pamarith User Manual(0)
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