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pnmnlfilt(1) AMIGA (5 February 1993) pnmnlfilt(1)
NAME
pnmnlfilt - non-linear filters: smooth, alpha trim mean,
optimal estimation smoothing, edge enhancement.
SYNOPSIS
pnmnlfilt alpha radius [pnmfile]
DESCRIPTION
This is something of a swiss army knife filter. It has 3
distinct operating modes. In all of the modes each pixel in
the image is examined and processed according to it and its
surrounding pixels values. Rather than using the 9 pixels in
a 3x3 block, 7 hexagonal area samples are taken, the size of
the hexagons being controlled by the radius parameter. A
radius value of 0.3333 means that the 7 hexagons exactly fit
into the center pixel (ie. there will be no filtering
effect). A radius value of 1.0 means that the 7 hexagons
exactly fit a 3x3 pixel array.
Alpha trimmed mean filter. (0.0 <= alpha
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
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 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
Page 1 (printed 3/1/94)
pnmnlfilt(1) AMIGA (5 February 1993) pnmnlfilt(1)
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 >=
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
filtering effectiveness and loosing detail. Experimentation
is encouraged.
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.
Page 2 (printed 3/1/94)
pnmnlfilt(1) AMIGA (5 February 1993) pnmnlfilt(1)
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.
SEE ALSO
pgmenhance(1), pnmconvol(1), pnm(5)
BUGS
Integers and tables may overflow if PPM_MAXMAXVAL is greater
than 255.
AUTHOR
Graeme W. Gill graeme@labtam.oz.au
Page 3 (printed 3/1/94)