mation smoothing, edge enhancement.

pnmnlfilt alpha radius [pnmfile]

This program is part of Netpbm.

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 sur-
rounding pixels. Rather than using a square block of surrounding pix-
els (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 effect). 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 determine 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 surrounding hexagon values
is calculated, and the amount of smoothing is made inversely propor-
tional 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 parame-
ter to get the desired effect. This type of filter is best for filter-
ing 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 parame-
ter 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 =

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 smooth-
ing 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 declin-
ing alpha values are more effective than a single pass with a large
alpha value. As usual, there is a tradeoff between filtering effec-
tiveness and loosing detail. Experimentation is encouraged.

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 Jour-
nal, 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,whichistaken-
fromPhilip 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.

The parameters are:
alpha The alpha value (described above), in decimal. May be frac-

radius The radius (described above), in decimal. May be fractional.

pgmenhance, pnmconvol, pnm

Graeme W. Gill

netpbm documentation 24 October 2006 Pnmnlfilt User Manual(0)