Manual: pnmnlfilt
Pnmnlfilt User Manual(1) General Commands Manual Pnmnlfilt User Manual(1)
NAME
pnmnlfilt - non-linear filters: smooth, alpha trim mean, optimal estimation smoothing, edge enhancement.
SYNOPSIS
pnmnlfilt alpha radius [pnmfile]
DESCRIPTION
This program is part of Netpbm(1).
pnmnlfilt produces an output image where the pixels are a summary of multiple pixels near the corresponding location
in an input image.
This program works on multi-image streams.
This is something of a swiss army knife filter. It has 3 distinct operating modes. In all of the modes pnmnlfilt
examines each pixel in the image and processes it according to the values of it and its surrounding pixels. Rather
than using a square block of surrounding pixels (e.g. the subject pixel and its 8 immediate neighbors, in a 3x3
square), pnmnlfilt uses 7 hexagonal areas. You choose the size of the hexagons with the radius parameter. A radius
value of 1/3 means that the 7 hexagons essentially fit into the subject pixel (ie. there will be no filtering ef‐
fect). A radius value of 1.0 means that the 7 hexagons essentially cover the 3x3 immediate neighbor square.
Your choice of "alpha" parameter selects among the three modes.
Alpha trimmed mean filter (0.0 <= alpha <= 0.5)
The value of the center pixel will be replaced by the mean of the 7 hexagon values, but the 7 values are sorted by
size and the top and bottom alpha portion of the 7 are excluded from the mean. This implies that an alpha value of
0.0 gives the same sort of output as a normal convolution (ie. averaging or smoothing filter), where radius will de‐
termine the "strength" of the filter. A good value to start from for subtle filtering is alpha = 0.0, radius = 0.55
For a more blatant effect, try alpha 0.0 and radius 1.0
An alpha value of 0.5 will cause the median value of the 7 hexagons to be used to replace the center pixel value.
This sort of filter is good for eliminating "pop" or single pixel noise from an image without spreading the noise out
or smudging features on the image. Judicious use of the radius parameter will fine tune the filtering. Intermediate
values of alpha give effects somewhere between smoothing and "pop" noise reduction. For subtle filtering try starting
with values of alpha = 0.4, radius = 0.6 For a more blatant effect try alpha = 0.5, radius = 1.0
Optimal estimation smoothing. (1.0 <= alpha <= 2.0)
This type of filter applies a smoothing filter adaptively over the image. For each pixel the variance of the sur‐
rounding hexagon values is calculated, and the amount of smoothing is made inversely proportional to it. The idea is
that if the variance is small then it is due to noise in the image, while if the variance is large, it is because of
"wanted" image features. As usual the radius parameter controls the effective radius, but it probably advisable to
leave the radius between 0.8 and 1.0 for the variance calculation to be meaningful. The alpha parameter sets the
noise threshold, over which less smoothing will be done. This means that small values of alpha will give the most
subtle filtering effect, while large values will tend to smooth all parts of the image. You could start with values
like alpha = 1.2, radius = 1.0 and try increasing or decreasing the alpha parameter to get the desired effect. This
type of filter is best for filtering out dithering noise in both bitmap and color images.
Edge enhancement. (-0.1 >= alpha >= -0.9)
This is the opposite type of filter to the smoothing filter. It enhances edges. The alpha parameter controls the
amount of edge enhancement, from subtle (-0.1) to blatant (-0.9). The radius parameter controls the effective radius
as usual, but useful values are between 0.5 and 0.9. Try starting with values of alpha = 0.3, radius = 0.8
Combination use.
The various modes of pnmnlfilt can be used one after the other to get the desired result. For instance to turn a
monochrome dithered image into a grayscale image you could try one or two passes of the smoothing filter, followed by
a pass of the optimal estimation filter, then some subtle edge enhancement. Note that using edge enhancement is only
likely to be useful after one of the non-linear filters (alpha trimmed mean or optimal estimation filter), as edge
enhancement is the direct opposite of smoothing.
For reducing color quantization noise in images (ie. turning .gif files back into 24 bit files) you could try a pass
of the optimal estimation filter (alpha 1.2, radius 1.0), a pass of the median filter (alpha 0.5, radius 0.55), and
possibly a pass of the edge enhancement filter. Several passes of the optimal estimation filter with declining alpha
values are more effective than a single pass with a large alpha value. As usual, there is a tradeoff between filter‐
ing effectiveness and losing detail. Experimentation is encouraged.
OPTIONS
There are no command line options defined specifically for pnmnlfilt, but it recognizes the options common to all
programs based on libnetpbm (See Common Options .)
REFERENCES
The alpha-trimmed mean filter is based on the description in IEEE CG&A May 1990 Page 23 by Mark E. Lee and Richard A.
Redner, and has been enhanced to allow continuous alpha adjustment.
The optimal estimation filter is taken from an article "Converting Dithered Images Back to Gray Scale" by Allen
Stenger, Dr Dobb's Journal, November 1992, and this article references "Digital Image Enhancement and Noise Filtering
by Use of Local Statistics", Jong-Sen Lee, IEEE Transactions on Pattern Analysis and Machine Intelligence, March
1980.
The edge enhancement details are from pgmenhance(1), which is taken from Philip R. Thompson's "xim" program, which in
turn took it from section 6 of "Digital Halftones by Dot Diffusion", D. E. Knuth, ACM Transaction on Graphics Vol. 6,
No. 4, October 1987, which in turn got it from two 1976 papers by J. F. Jarvis et. al.
PARAMETERS
The parameters are:
alpha The alpha value (described above), in decimal. May be fractional.
radius The radius (described above), in decimal. May be fractional.
SEE ALSO
pgmenhance(1), pnmconvol(1), pnm(1)
AUTHOR
Graeme W. Gill graeme@labtam.oz.au
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at
http://netpbm.sourceforge.net/doc/pnmnlfilt.html
netpbm documentation 24 October 2006 Pnmnlfilt User Manual(1)