Pnmconvol User Manual(0) | Pnmconvol User Manual(0) |

# NAME

pnmconvol - general MxN convolution on a Netpbm image

# SYNOPSIS

**pnmconvol** { **-matrix=***convolution_matrix* |
**-matrixfile=***filename*[**,***filename*[**,** ...]] }
[*netpbmfile*]

**pnmconvol** *convolution_matrix_file*
[**-normalize**] [**-nooffset**] [*netpbmfile*]

Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen to denote options. You may use white space in place of the equals sign to separate an option name from its value.

# 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.

Convolution means replacing each pixel with a weighted average of
the nearby pixels. The weights and the area to average are determined by the
convolution matrix (sometimes called a convolution kernel), which you supply
in one of several ways. See

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.

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.

## 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.

The PNM file option is the hardest, and exists only because until Netpbm 10.49 (December 2009), it was the only way.

The regular convolution matrix file is slightly easier to read
than the **-matrix** option value, and makes your command line less
messy, but requires you to manage a separate file. With the file, you can
have separate convolution matrices for the individual color components,
which you can't do with **-matrix**.

In any case, the convolution matrix **pnmconvol** uses is a
matrix of real numbers. They can be whole or fractional, positive, negative,
or zero.

The convolution matrix always has an odd width and height, so that
there is a center element. **pnmconvol** figuratively places that center
element over a pixel of the input image and weights that pixel and its
neighbors as indicated by the convolution matrix to produce the pixel in the
same location of the output image.

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 numbers in the matrix add up to 1. If they
don't, **pnmconvol** warns you.

The elements of the matrix are actually tuples, one for each sample in the input. (I.e. if the input is an RGB image, each element of the convolution matrix has one weight for red, one for green, and one for blue.

**Directly On the Command Line**

An example of a **-matrix** option is

```
-matrix=0,.2,0;.2,.2,.2;0,.2,0
```

The 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.

**Regular Matrix File**

Specify the name of the matrix file with a **-matrixfile**
option. Or specify a list of them, one for each plane of the image.

Examples:

-matrixfile=mymatrix -matrixfile=myred,mygreen,myblue

Each 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.

**PNM File**

Before Netpbm 10.49 (December 2009), this was the only way to
specify a convolution matrix. **pnmconvol** used this method in an
attempt to exploit the generic matrix processing capabilities of Netpbm, but
as the Netpbm formats don't directly allow matrices with the kinds of
numbers you need in a convolution matrix, it is quite cumbersome. In fact,
there simply is no way to specify some convolution matrices with a legal
Netpbm image, so make it work, **pnmconvol** has to relax the Netpbm file
requirement that sample values be no greater than the image's maxval. I.e.
it isn't even a real PNM file.

The way you select this method of supplying the convolution matrix
is to *not* specify **-matrix** or **-matrixfile**. When you do
that, you must specify a second non-option argument -- that is the name of
the PNM file (or PNM equivalent PAM file).

There are two ways **pnmconvol** interprets the PNM convolution
matrix image pixels as weights: with offsets, and without offsets.

The simpler of the two is without offsets. That is what happens
when you specify the **-nooffset** option. In that case, **pnmconvol**
simply normalizes the sample values in the PNM image by dividing by the
maxval.

For example, here is a sample convolution file that causes an output pixel to be a simple average of its corresponding input pixel and its 8 neighbors, resulting in a smoothed image:

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**(5)** ).

**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.

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 10

To 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

**-matrix=***convolution_matrix*- The value of the convolution matrix. See
Convolution Matrix .
You may not specify both this and

**-matrixfile**.This option was new in Netpbm 10.49 (December 2009). Before that, use a PNM file for the convolution matrix.

**-matrixfile=***filename*- This specifies that you are supplying the convolution matrix in a file and
names that file. See Convolution
Matrix .
You may not specify both this and

**-matrix**.This option was new in Netpbm 10.49 (December 2009). Before that, use a PNM file for the convolution matrix.

**-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 has no effect when you specify the convolution matrix via pseudo-PNM file.

This option was new in Netpbm 10.50 (March 2010).

**-nooffset=**- This is part of the obsolete PNM image method of specifying the convolution matrix. See Convolution Matrix .

# 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) , pnm(5)

# AUTHORS

Copyright (C) 1989, 1991 by Jef Poskanzer. Modified 26 November
1994 by Mike Burns, *burns@chem.psu.edu*

03 January 2010 | netpbm documentation |