NAME
pnmconvol - general MxN convolution on a Netpbm imageSYNOPSIS
pnmconvol { -matrix=convolution_matrix | -matrixfile= filename[,filename[, ...]] } [ -normalize] [ -bias=n]DESCRIPTION
This program is part of Netpbm(1). pnmconvol reads a Netpbm image as input, convolves it with a specified convolution matrix, and writes a Netpbm image as output. A command use for convolution is blurring. See examples in the pamgauss(1) manual.Convolution Matrix . At the edges of the convolved image, where the convolution matrix would extend over the edge of the image, pnmconvol just copies the input pixels directly to the output. It's often better to deal with the pixels near an edge by assuming some blank or background color beyond the edge. To do that, use pnmpad to add a margin all around whose size is half that of your convolution matrix size, not counting its center, in the same dimension. (E.g. if your convolution matrix is 5 wide by 3 high, use pnmpad -left=2 -right=2 -top=1 -bottom=1). Feed that enlarged image to pnmconvol, then use pamcut to chop the edges off the convolved output, getting back to your original image dimensions. (E.g. pamcut -left=2 -right=-2 -top=1 -bottom=-1). The convolution computation can result in a value which is outside the range representable in the output. When that happens, pnmconvol just clips the output, which means brightness is not conserved. To avoid clipping, you may want to scale your input values. For example, if your convolution matrix might produce an output value as much as double the maximum value in the input, then make sure the maxval of the input (which is also the maxval of the output) is at least twice the actual maximum value in the input. Clipping negative numbers deserves special consideration. If your convolution matrix includes negative numbers, it is possible for pnmconvol to calculate an output pixel as a negative value, which pnmconvol would of course clip to zero, since Netpbm formats cannot represent negative numbers.
Convolution Matrix
There are three ways to specify the convolution matrix:- •
- directly with a -matrix option.
- •
- In a file (or set of them) named by a -matrixfile option, whose contents are similar to a -matrix option value.
- •
- With a special PNM file.
-matrix=0,.2,0;.2,.2,.2;0,.2,0
-matrix=-1,3,-1The value consists of each row of the matrix from top to bottom, separated by semicolons. Each row consists of the elements of the row from left to right, separated by commas. You must of course have the same number of elements in each row. Each element is a decimal floating point number and is the weight to give to each component of a pixel that corresponds to that matrix location. Note that when you supply this option via a shell, semicolon (";") probably means something to the shell, so use quotation marks. There is no way with this method to have different weights for different components of a pixel. The -normalize option is often quite handy with -matrix because it lets you quickly throw together the command without working out the math to make sure the matrix isn't biased. Note that if you use the -normalize option, the weights in the matrix aren't actually the numbers you specify in the -matrix option.
-matrixfile=mymatrix -matrixfile=myred,mygreen,myblueEach file applies to one plane of the image (e.g. red, green, or blue), in order. The matrix in each file must have the same dimensions. If the input image has more planes than the number of files you specify, the first file applies to the extra planes as well. pnmconvol interprets the file as text, with lines delimited by Unix newline characters (line feeds). Each line of the file is one row of the matrix, in order from top to bottom. For each row, the file contains a floating point decimal number for each element in the row, from left to right, separated by spaces. This is not just any old white space -- it is exactly one space. Two spaces in a row mean you've specified a null string for an element (which is invalid). If you want to line up your matrix visually, use leading and trailing zeroes in the floating point numbers to do it. There is no way to put comments in the file. There is no signature or any other metadata in the file. Note that if you use the -normalize option, the weights in the matrix aren't actually what is in the file.
P2 3 3 18 2 2 2 2 2 2 2 2 2(Note that the above text is an actual PGM file -- you can cut and paste it. If you're not familiar with the plain PGM format, see the PGM format specification(1)). pnmconvol divides each of the sample values (2) by the maxval (18) so the weight of each of the 9 input pixels gets is 1/9, which is exactly what you want to keep the overall brightness of the image the same. pnmconvol creates an output pixel by multiplying the values of each of 9 pixels by 1/9 and adding. Note that with maxval 18, the range of possible values is 0 to 18. After scaling, the range is 0 to 1. For a normal convolution, where you're neither adding nor subtracting total value from the image, but merely moving it around, you'll want to make sure that all the scaled values in (each plane of) your convolution PNM add up to 1, which means all the actual sample values add up to the maxval. Alternatively, you can use the -normalize option to scale the scaled values further to make them all add up to 1 automatically. When you don't specify -nooffset, pnmconvol applies an offset, the purpose of which is to allow you to indicate negative weights even though PNM sample values are never negative. In this case, pnmconvol subtracts half the maxval from each sample and then normalizes by dividing by half the maxval. So to get the same result as we did above with -nooffset, the convolution matrix PNM image would have to look like this:
P2 3 3 18 10 10 10 10 10 10 10 10 10To see how this works, do the above-mentioned offset: 10 - 18/2 gives 1. The normalization step divides by 18/2 = 9, which makes it 1/9 - exactly what you want. The equivalent matrix for 5x5 smoothing would have maxval 50 and be filled with 26. Note that with maxval 18, the range of possible values is 0 to 18. After offset, that's -9 to 9, and after normalizing, the range is -1 to 1. The convolution file will usually be a PGM, so that the same convolution gets applied to each color component. However, if you want to use a PPM and do a different convolution to different colors, you can certainly do that.
Other Forms of Convolution
pnmconvol does only arithmetic, linear combination convolution. There are other forms of convolution that are especially useful in image processing. pgmmedian does median filtering, which is a form of convolution where the output pixel value, rather than being a linear combination of the pixels in the window, is the median of a certain subset of them. pgmmorphconv does dilation and erosion, which is like the median filter but the output value is the minimum or maximum of the values in the window.OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, seeCommon Options ), pnmconvol recognizes the following command line options:
- -matrix=convolution_matrix
- The value of the convolution matrix. See
- -matrixfile=filename
- This specifies that you are supplying the convolution matrix in a file and names that file. See
- -normalize
- This option says to adjust the weights in your convolution
matrix so they all add up to one. You usually want them to add up to one
so that the convolved result tends to have the same overall brightness as
the input. With -normalize, pnmconvol scales all the weights
by the same factor to make the sum one. It does this for each plane.
This can be quite convenient because you can just throw numbers into the
matrix that have roughly the right relationship to each other and let
pnmconvol do the work of normalizing them. And you can adjust a
matrix by raising or lowering certain weights without having to modify all
the other weights to maintain normalcy. And you can use friendly integers.
Example:
$ pnmconvol myimage.ppm -normalize -matrix=1,1,1;1,1,1;1,1,1
This is of course a basic 3x3 average, but without you having to specify 1/9 (.1111111) for each weight. This option was new in Netpbm 10.50 (March 2010). But before Netpbm 10.79 (June 2017), it has no effect when you specify the convolution matrix via pseudo-PNM file.
- -bias=n
- This specifies an amount to add to the convolved value for each sample. The purpose of this addition is normally to handle negative convolution results. Because the convolution matrix can contain negative numbers, the convolved value for a pixel could be negative. But Netpbm formats cannot contain negative sample values, so without any bias, such samples would get clipped to zero. The bias allows the output image to retain the information, and a program that pocesses that output, knowing the bias value, could reconstruct the real convolved values. For example, with bias=100, a sample whose convolved value is -5 appears as 95 in the output, whereas a sample whose convolved value is 5 appears as 105 in the output. A typical value for the bias is half the maxval, allowing the same range on either side of zero. If the sample value, after adding the bias, is still less than zero, pnmconvol clips it to zero. If it exceeds the maxval of the output image, it clips it to the maxval. The default is zero. This option was new in Netpbm 10.68 (September 2014).
- -nooffset=
- This is part of the obsolete PNM image method of specifying the convolution matrix. See
HISTORY
The -nooffset option was new in Netpbm 10.23 (July 2004), making it substantially easier to specify a convolution matrix, but still hard. In Netpbm 10.49 (December 2009), the PNM convolution matrix tyranny was finally ended with the -matrix and -matrixfile options. In between, pnmconvol was broken for a while because the Netpbm library started enforcing the requirement that a sample value not exceed the maxval of the image. pnmconvol used the Netpbm library to read the PNM convolution matrix file, but in the pseudo-PNM format that pnmconvol uses, a sample value sometimes has to exceed the maxval.SEE ALSO
pnmsmooth(1), pgmmorphconv(1), pgmmedian(1), pnmnlfilt(1), pgmkernel(1), pamgauss(1), pammasksharpen(1), pnmpad(1), pamcut(1), pnm(1)AUTHORS
Copyright (C) 1989, 1991 by Jef Poskanzer. Modified 26 November 1994 by Mike Burns, [email protected]DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at30 November 2018 | netpbm documentation |