1Pnmconvol User Manual(0) Pnmconvol User Manual(0)
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6 pnmconvol - general MxN convolution on a Netpbm image
7
10 pnmconvol { -matrix=convolution_matrix | -matrixfile=filename[,file‐
11 name[, ...]] } [netpbmfile]
12 pnmconvol convolution_matrix_file [-nooffset] [netpbmfile]
14 Minimum unique abbreviation of option is acceptable. You may use dou‐
16 ble hyphens instead of single hyphen to denote options. You may use
17 white space in place of the equals sign to separate an option name from
18 its value.
19
23 This program is part of Netpbm(1).
24 pnmconvol reads a Netpbm images as input, convolves it with a specified
26 convolution matrix, and writes a Netpbm image as output.
27 Convolution means replacing each pixel with a weighted average of the
29 nearby pixels. The weights and the area to average are determined by
30 the convolution matrix (sometimes called a convolution kernel), which
31 you supply in one of several ways. See
32 Convolution Matrix ⟨#convolutionmatrix⟩ .
33 At the edges of the convolved image, where the convolution matrix would
35 extend over the edge of the image, pnmconvol just copies the input pix‐
36 els directly to the output.
37 The convolution computation can result in a value which is outside the
39 range representable in the output. When that happens, pnmconvol just
40 clips the output, which means brightness is not conserved.
41 Convolution Matrix
45 There are three ways to specify the convolution matrix:
46 · directly with a -matrix option.
50 · In a file (or set of them) named by a -matrixfile option, whose
53 contents are similar to a -matrix option value.
54 · With a special PNM file.
57 The PNM file option is the hardest, and exists only because until
60 Netpbm 10.49 (December 2009), it was the only way.
61 The regular convolution matrix file is slightly easier to read than the
63 -matrix option value, and makes your command line less messy, but
64 requires you to manage a separate file. With the file, you can have
65 separate convolution matrices for the individual color components,
66 which you can't do with -matrix.
67 In any case, the convolution matrix pnmconvol uses is a matrix of real
69 numbers. They can be whole or fractional, positive, negative, or zero.
70 The convolution matrix always has an odd width and height, so that
72 there is a center element. pnmconvol figuratively places that center
73 element over a pixel of the input image and weights that pixel and its
74 neighbors as indicated by the convolution matrix to produce the pixel
75 in the same location of the output image.
76 For a normal convolution, where you're neither adding nor subtracting
78 total value from the image, but merely moving it around, you'll want to
79 make sure that all the numbers in the matrix add up to 1. If they
80 don't, pnmconvol warns you.
81 The elements of the matrix are actually tuples, one for each sample in
83 the input. (I.e. if the input is an RGB image, each element of the
84 convolution matrix has one weight for red, one for green, and one for
85 blue.
86 Directly On the Command Line
89 An example of a -matrixopt option is
91 -matrixopt=.1,.1,.1;.1,0,.1;.1,.1,.1
93 The value consists of each row of the matrix from top to bottom, sepa‐
95 rated by semicolons. Each row consists of the elements of the row from
96 left to right, separated by commas. You must of course have the same
97 number of elements in each row. Each element is a decimal floating
98 point number and is the weight to give to each component of a pixel
99 that corresponds to that matrix location.
100 There is no way with this method to have different weights for differ‐
102 ent components of a pixel.
103 Regular Matrix File
106 Specify the name of the matrix file with a -matrixfile option. Or
108 specify a list of them, one for each plane of the image.
109 Examples:
111 -matrixfile=mymatrix
113 -matrixfile=myred,mygreen,myblue
115 Each file applies to one plane of the image (e.g. red, green, or blue),
117 in order. The matrix in each file must have the same dimensions. If
118 the input image has more planes than the number of files you specify,
119 the first file applies to the extra planes as well.
120 pnmconvol interprets the file as text, with lines delimited by Unix
122 newline characters (line feeds).
123 Each line of the file is one row of the matrix, in order from top to
125 bottom.
126 For each row, the file contains a floating point decimal number for
128 each element in the row, from left to right, separated by spaces. This
129 is not just any old white space -- it is exactly one space. Two spaces
130 in a row mean you've specified a null string for an element (which is
131 invalid). If you want to line up your matrix visually, use leading and
132 trailing zeroes in the floating point numbers to do it.
133 There is no way to put comments in the file. There is no signature or
135 any other metadata in the file.
136 PNM File
139 Before Netpbm 10.49 (December 2009), this was the only way to specify a
141 convolution matrix. pnmconvol used this method in an attempt to
142 exploit the generic matrix processing capabilities of Netpbm, but as
143 the Netpbm formats don't directly allow matrices with the kinds of num‐
144 bers you need in a convolution matrix, it is quite cumbersome. In
145 fact, there simply is no way to specify some convolution matrices with
146 a legal Netpbm image, so make it work, pnmconvol has to relax the
147 Netpbm file requirement that sample values be no greater than the
148 image's maxval. I.e. it isn't even a real PNM file.
149 The way you select this method of supplying the convolution matrix is
151 to not specify -matrix or -matrixfile. When you do that, you must
152 specify a second non-option argument -- that is the name of the PNM
153 file (or PNM equivalent PAM file).
154 There are two ways pnmconvol interprets the PNM convolution matrix
156 image pixels as weights: with offsets, and without offsets.
157 The simpler of the two is without offsets. That is what happens when
159 you specify the -nooffset option. In that case, pnmconvol simply nor‐
160 malizes the sample values in the PNM image by dividing by the maxval.
161 For example, here is a sample convolution file that causes an output
163 pixel to be a simple average of its corresponding input pixel and its 8
164 neighbors, resulting in a smoothed image:
165 P2
167 3 3
168 18
169 2 2 2
170 2 2 2
171 2 2 2
172 (Note that the above text is an actual PGM file -- you can cut and
174 paste it. If you're not familiar with the plain PGM format, see theP‐
175 GMformatspecification[1m(1)).
176 pnmconvol divides each of the sample values (2) by the maxval (18) so
178 the weight of each of the 9 input pixels gets is 1/9, which is exactly
179 what you want to keep the overall brightness of the image the same.
180 pnmconvol creates an output pixel by multiplying the values of each of
181 9 pixels by 1/9 and adding.
182 Note that with maxval 18, the range of possible values is 0 to 18.
184 After scaling, the range is 0 to 1.
185 For a normal convolution, where you're neither adding nor subtracting
187 total value from the image, but merely moving it around, you'll want to
188 make sure that all the scaled values in (each plane of) your convolu‐
189 tion PNM add up to 1, which means all the actual sample values add up
190 to the maxval.
191 When you don't specify -nooffset, pnmconvol applies an offset, the pur‐
193 pose of which is to allow you to indicate negative weights even though
194 PNM sample values are never negative. In this case, pnmconvol sub‐
195 tracts half the maxval from each sample and then normalizes by dividing
196 by half the maxval. So to get the same result as we did above with
197 -nooffset, the convolution matrix PNM image would have to look like
198 this:
199 P2
201 3 3
202 18
203 10 10 10
204 10 10 10
205 10 10 10
206 To see how this works, do the above-mentioned offset: 10 - 18/2 gives
208 1. The normalization step divides by 18/2 = 9, which makes it 1/9 -
209 exactly what you want. The equivalent matrix for 5x5 smoothing would
210 have maxval 50 and be filled with 26.
211 Note that with maxval 18, the range of possible values is 0 to 18.
213 After offset, that's -9 to 9, and after normalizing, the range is -1 to
214 1.
215 The convolution file will usually be a PGM, so that the same convolu‐
217 tion gets applied to each color component. However, if you want to use
218 a PPM and do a different convolution to different colors, you can cer‐
219 tainly do that.
220
223 -matrix=convolution_matrix
224 The value of the convolution matrix. See Convolution Matrix
225 ⟨#matrixopt⟩ .
226 You may not specify both this and -matrixfile.
228 This option was new in Netpbm 10.49 (December 2009). Before
230 that, use a PNM file for the convolution matrix.
231 -matrixfile=filename
234 This specifies that you are supplying the convolution matrix in
235 a file and names that file. See Convolution Matrix
236 ⟨#matrixfile⟩ .
237 You may not specify both this and -matrix.
239 This option was new in Netpbm 10.49 (December 2009). Before
241 that, use a PNM file for the convolution matrix.
242 -nooffset=
246 This is part of the obsolete PNM image method of specifying the
247 convolution matrix. See Convolution Matrix ⟨#matrixpnm⟩ .
248
253 The -nooffset option was new in Netpbm 10.23 (July 2004), making it
254 substantially easier to specify a convolution matrix, but still hard.
255 In Netpbm 10.49 (December 2009), the PNM convolution matrix tyranny was
256 finally ended with the -matrix and -matrixfile options. In between,
257 pnmconvol was broken for a while because the Netpbm library started
258 enforcing the requirement that a sample value not exceed the maxval of
259 the image. pnmconvol used the Netpbm library to read the PNM convolu‐
260 tion matrix file, but in the pseudo-PNM format that pnmconvol uses, a
261 sample value sometimes has to exceed the maxval.
262
266 pnmsmooth(1), pgmmorphconv(1), pnmnlfilt(1), pgmkernel(1), pamgauss(1),
267 pammasksharpen(1), pnm(1)
268
271 Copyright (C) 1989, 1991 by Jef Poskanzer. Modified 26 November 1994
272 by Mike Burns, burns@chem.psu.edu
273netpbm documentation 07 December 2009 Pnmconvol User Manual(0)