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Pnmnlfilt User Manual(0)			      Pnmnlfilt	User Manual(0)

NAME
       pnmnlfilt  - non-linear filters:	smooth,	alpha trim mean, optimal esti-
       mation 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 op-
       erating 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  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  ef-
       fect, 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 im-
       age.  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 ef-
       fective	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 en-
       hances edges. The alpha parameter controls the amount of	edge  enhance-
       ment,  from  subtle (-0.1) to blatant (-0.9). The radius	parameter con-
       trols 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.  Sev-
       eral passes of the optimal estimation filter with declining alpha  val-
       ues are more effective than a single pass with a	large alpha value.  As
       usual, there is a tradeoff between filtering effectiveness  and	losing
       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 en-
       hanced 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 Enhance-
       ment 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.

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

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

SEE ALSO
       pgmenhance(1), pnmconvol(1), pnm(5)

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(0)

NAME | SYNOPSIS | DESCRIPTION | References: | SEE ALSO | AUTHOR | DOCUMENT SOURCE

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