1Pamarith User Manual(0) Pamarith User Manual(0)
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6 pamarith - perform arithmetic on two Netpbm images
7
10 pamarith -add | -subtract | -multiply | -divide | -difference | -mini‐
11 mum | -maximum | -mean | -equal | -compare | -and | -or | -nand | -nor
12 | -xor | -shiftleft | -shiftright [-closeness=N] pamfile1 pamfile2 ...
13 All options can be abbreviated to their shortest unique prefix. You
15 may use two hyphens instead of one. You may separate an option name
16 and its value with white space instead of an equals sign.
17
20 This program is part of Netpbm(1).
21 pamarith reads two or more PBM, PGM, PPM, or PAM images as input. It
23 performs the specified binary arithmetic operation on their sample val‐
24 ues and produces an output of a format which is the more general of the
25 two input formats. The two input images must be of the same width and
26 height. The arithmetic is performed on each pair of identically lo‐
27 cated tuples to generate the identically located tuple of the output.
28 For functions that are commutative and associative, pamarith applies
30 the binary function repetitively on as many input images as you sup‐
31 ply. For
32 example, for -add , the output is the sum of all the inputs. For
33 other functions (e.g. -subtract), the program fails if you supply
34 more than two input images. (Before Netpbm 10.93 (December 2020),
35 the
36 program always failed with more than two input images).
37 For some other functions, pamarith could theoretically compute a
39 meaningful result for multiple arguments, but it fails nonetheless if
40 you
41 give more than two input images. -mean and -equal are in that
42 category.
43 Most of what pamarith does is not meaningful for visual images. It
46 works toward Netpbm's secondary purpose of just manipulating arbi‐
47 trary
48 matrices of numbers.
49 For the purpose of the calculation, it assumes any PBM, PGM, or PPM in‐
52 put image is the equivalent PAM image of tuple type BLACKANDWHITE,
53 GRAYSCALE, or RGB, respectively, and if it produces a PBM, PGM, or PPM
54 output, produces the equivalent of the PAM image which is the result of
55 the calculation.
56 The first pamfile argument identifies the "left" argument image; the
58 second pamfile argument identifies the "right" one.
59 If the output is PAM, the tuple type is the same as the tuple type of
61 the left input image.
62 pamarith performs the arithmetic on each pair of identically located
64 tuples in the two input images.
65 The arithmetic operation is in all cases fundamentally a function from
67 two integers to an integer (but see below - the functions are defined
68 in ways that you can effectively e.g. add real numbers). The operation
69 is performed on two tuples as follows. The two input images must have
70 the same depth, or one of them must have depth one. pamarith fails if
71 one of these is not the case.
72 If they have the same depth, pamarith simply carries out the arithmetic
74 one sample at a time. I.e. if at a particular position the left input
75 image contains the tuple (s1,s2,...,sN) and the right input image con‐
76 tains the tuple (t1,t2,...tN), and the function is f, then the output
77 image contains the tuple (f(s1,t1),f(s2,t2),...,f(sN,tN)).
78 If one of the images has depth 1, the arithmetic is performed between
80 the one sample in that image and each of the samples in the other.
81 I.e. if at a particular position the left input image contains the tu‐
82 ple (s) and the right input image contains the tuple (t1,t2,...tN), and
83 the function is f, then the output image contains the tuple
84 (f(s,t1),f(s,t2),...,f(s,tN)).
85 PBM Oddness
89 If you're familiar with the PBM format, you may find pamarith's opera‐
90 tion on PBM images to be nonintuitive. Because in PBM black is repre‐
91 sented as 1 and white as 0, you might be expecting black minus black to
92 be white.
93 But the PBM format is irrelevant, because pamarith operates on the num‐
95 bers found in the PAM equivalent (see above). In a PAM black and white
96 image, black is 0 and white is 1. So black minus black is black.
97 Maxval
101 The meanings of the samples with respect to the maxval varies according
102 to the function you select.
103 In PAM images in general, the most usual meaning of a sample (the one
105 that applies when a PAM image represents a visual image), is that it
106 represents a fraction of some maximum. The maxval of the image corre‐
107 sponds to some maximum value (in the case of a visual image, it corre‐
108 sponds to "full intensity."), and a sample value divided by the maxval
109 gives the fraction.
110 For pamarith, this interpretation applies to the regular arithmetic
112 functions: -add, -subtract, -multiply, -divide, -difference, -minimum,
113 -maximum, -mean, -equal, and -compare. For those, you should think of
114 the arguments and result as numbers in the range [0,1). For example,
115 if the maxval of the left argument image is 100 and the maxval of the
116 right argument image is 200 and the maxval of the output image is 200,
117 and the left sample value in an -add calculation is 50 and the right
118 sample is 60, the actual calculation is 50/100 + 60/200 = 160/200, and
119 the output sample value is 160.
120 For these functions, pamarith makes the output image's maxval the maxi‐
122 mum of the two input maxvals, except with -equal and -compare. For
123 -equal, the output maxval is always 1. For -compare, it is always 2.
124 (Before Netpbm 10.14 (February 2003), there was no exception for -com‐
125 pare; in 10.14, the exception was just that the maxval was at least 2,
126 and sometime between 10.18 and 10.26 (January 2005), it changed to be‐
127 ing exactly 2).
128 If the result of a calculation falls outside the range [0, 1), pamarith
130 clips it -- i.e. considers it to be zero or 1-.
131 In many cases, where both your input maxvals are the same, you can just
133 think of the operation as taking place between the sample values di‐
134 rectly, with no consideration of the maxval except for the clipping.
135 E.g. an -add of sample value 5 to sample value 8 yields sample value
136 13.
137 But with -multiply, this doesn't work. Say your two input images have
139 maxval 255, which means the output image also has maxval 255. Consider
140 a location in the image where the input sample values are 5 and 10.
141 You might think the multiplicative product of those would yield 50 in
142 the output. But pamarith carries out the arithmetic on the fractions
143 5/255 and 10/255. It multiplies those together and then rescales to
144 the output maxval, giving a sample value in the output PAM of 50/255
145 rounded to the nearest integer: 0.
146 With the bit string operations, the maxval has a whole different mean‐
148 ing. The operations in question are: -and, -or, -nand, -nor, -xor, and
149 -shiftleft, -shiftright.
150 With these, each sample value in one or both input images, and in the
152 output image, represents a bit string, not a number. The maxval tells
153 how wide the bit string is. The maxval must be a full binary count (a
154 power of two minus one, such as 0xff) and the number of ones in it is
155 the width of the bit string. For the dyadic bit string operations
156 (that's everything but the shift functions), the maxvals of the input
157 images must be the same and pamarith makes the maxval of the output im‐
158 age the same.
159 For the bit shift operations, the output maxval is the same as the left
161 input maxval. The right input image (which contains the shift counts)
162 can have any maxval and the maxval is irrelevant to the interpretation
163 of the samples. The sample value is the actual shift count. But it's
164 still required that no sample value exceed the maxval.
165
169 pamarith applies only binary functions. If you want to apply a unary
170 function, e.g. "halve", to a single image, use pamfunc.
171
175 In addition to the options common to all programs based on libnetpbm
176 (most notably -quiet, see
177 Common Options ⟨index.html#commonoptions⟩ ), pamarith recognizes the
178 following command line options:
179 The Function
182 These options select the function that pamarith applies.
183 You must specify one of these, and cannot specify more than one.
185 -add Adds the two values. If the result is larger than maxval, it is
190 clipped.
191 -subtract
194 Subtracts a value in the right input image from a value in the
195 left input image.
196 -minimum
199 Chooses the smaller value of the two.
200 -maximum
203 Chooses the larger value of the two.
204 -difference
207 Calculates the absolute value of the difference.
208 -multiply
211 Does an ordinary arithmetic multiplication, but tends to produce
212 nonobvious results because of the way pamarith interprets sample
213 values. See Maxval ⟨#maxval⟩ .
214 -divide
217 Divides a value in the left input image by the value in the
218 right input image. But like -multiply, it tends to produce
219 nonobvious results. Note that pamarith clipping behavior makes
220 this of little use when the left argument (dividend) is greater
221 than the right argument (divisor) -- the result in that case is
222 always the maxval. If the divisor is 0, the result is the max‐
223 val.
224 -divide was new in Netpbm 10.30 (October 2005).
226 -equal Produces maxval when the values in the two images are equal and
229 zero when they are not. Note that the output maxval is always 1
230 for -equal.
231 If the maxvals of the input images are not identical, pamarith
233 may claim two values are not equal when in fact they are, be‐
234 cause of the precision with which it does the arithmetic.
235 You can make the equality test approximate with the -closeness
237 option. This gives the percentage of maxval by which the sam‐
238 ples can differ and still be considered equal.
239 -equal was new in Netpbm 10.93 (December 2020).
241 -compare
244 Produces the value 0 when the value in the left input image is
245 less than the value in the right input image, 1 when the values
246 are equal, and 2 when the left is greater than the right.
247 If the maxvals of the input images are not identical, pamarith
249 may claim two values are not equal when in fact they are, be‐
250 cause of the precision with which it does the arithmetic. How‐
251 ever, it will never say A is greater than B if A is less than B.
252 -compare was new in Netpbm 10.13 (December 2002).
254 -and, -nand, -or, -nor, -xor
257 These consider the input and output images to contain bit
258 strings; they compute bitwise logic operations. Note that if
259 the maxval is 1, you can also look at these as logic operations
260 on boolean input values. See section Maxval ⟨#maxval⟩ for the
261 special meaning of maxval with respect to bit string operations
262 such as these.
263 -shiftleft, -shiftright
266 These consider the left input image and output image to contain
267 bit strings. They compute a bit shift operation, with bits
268 falling off the left or right end and zeroes shifting in, as op‐
269 posed to bits off one end to the other. The right input image
270 sample value is the number of bit positions to shift.
271 Note that the maxval (see Maxval ⟨#maxval⟩ ) determines the
273 width of the frame within which you are shifting.
274 Other
279 -closeness
280 This changes the meaning of -equal. It is not valid with any
281 other function. See the description of -equal.
282
287 pamfunc(1), pamsummcol(1), pamsumm(1), pnminvert(1), pambrighten(1),
288 ppmdim(1), pnmconvol(1), pamdepth(1), pnmpsnr(1), pnm(1), pam(1)
289
293 pamarith replaced pnmarith in Netpbm 10.3 (June 2002).
294 In Netpbm 10.3 through 10.8, though, pamarith was not backward compati‐
296 ble because it required the input images to be of the same depth, so
297 you could not multiply a PBM by a PPM as is often done for masking.
298 (It was not intended at the time that pnmarith would be removed from
299 Netpbm -- the plan was just to rewrite it to use pamarith; it was re‐
300 moved by mistake).
301 But starting with Netpbm 10.9 (September 2002), pamarith allows the im‐
303 ages to have different depths as long as one of them has depth 1, and
304 that made it backward compatible with pnmarith.
305 The original pnmarith did not have the -mean option.
307 The -compare option was added in Netpbm 10.13 (December 2002).
309 The bit string operations were added in Netpbm 10.27 (March 2005).
311 The -divide option was added in Netpbm 10.30 (October 2005).
313 The ability to have more than one input (operand) was added in Netpbm
315 10.93 (December 2020).
316 The -equal option was added in Netpbm 10.93 (December 2020).
318
320 This manual page was generated by the Netpbm tool 'makeman' from HTML
321 source. The master documentation is at
322 http://netpbm.sourceforge.net/doc/pamarith.html
324netpbm documentation 24 October 2020 Pamarith User Manual(0)