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Program: MFILTER

Purpose: Apply a median (or average) filter to a (sub)set

Category: CALCULATION, TRANSFER

File: mfilter.c

Author: M.G.R. Vogelaar

Keywords:

INSET= Give set, subsets:
Maximum number of subsets is 2048.

BOX= Give box in ..... [entire subset]

OUTSET= Give output set (, subsets):
Output set and subset(s) for the result. The number of
output subsets is the same as the number of input sub-
sets.

MEDBOX= Give sizes of median box: [3,3]
Sizes must be odd. A size equal to 1 means no
median filter in that direction.

** AVERAGE= Use average instead of median Y/[N]
By repeatedly applying this boxcar average one can
emulate cubic spline smoothing (use it 4 times).
More applications rapidly emulates a 2d gaussian with
standard deviation = N * MEDBOX / 12.

** INCLUDE= Include central pixel in calculation? [Y]/N

** ABSOLUTE= Cutoff as absolute value? Y/[N]
If ABSOLUTE=N (the default), the cutoff will be
interpreted as a relative value.

CUT= Cutoff for replacing the centr.pix. by the med.val.[0.0]
There are two modes for the cutoff. The modes are
specified in ABSOLUTE=
See description.

** REPLACE= Replace blanks by local median? [Y]/N
Default, a blank will be replaced by the median of
the 'median' box. If REPLACE=N, a blank is NOT
replaced.


Description: An input set given with INSET= is filtered with a
median filter and put in OUTSET=. For each pixel in
BOX= the median is calculated in a l x m neighborhood
where l and m are specified in MEDBOX=.
There are some restrictions for the sizes in MEDBOX=
1) the sizes must be odd.
2) The size in Y direction must be smaller than the
number of lines in some buffer (calculated by the
program).
3) The size in X-direction cannot exceed the width of
the subset.
4) The total number of pixels in the 'median' box
cannot exceed the internal buffer size (calculated
by the program).

The median is calculated by sorting the data in the
'median' box. This can take a while for big boxes.
An alternative (without sorting) is in development.
You can exclude the value of the central pixel
with INCLUDE=NO (default, this pixel will be included).
At the edges of the frame, not all neighbor pixels can
contribute to the median.

If the value of a pixel differs more than CUT= from
the median, its value will be replaced by the median,
otherwise, it will not be changed. The cutoff can be
given as an relative or absolute value:

1) ABSOLUTE=Y, the cutoff is absolute, i.e. if the
difference between an image value and the local
median is greater than CUT= , replace the image
value by the median, else keep the image value.
Expressed in statements:

diff = |image value - local median|
if diff > cutoff
return median
else
return image value

2) ABSOLUTE=N (the default), the cutoff is a relative
value and is in the range 0..1. The statements are:

diff = | (image value - local median) / median |
if diff > cutoff
return median
else
return image value


Blank pixels are replaced by the local median if
REPLACE=Y (default).
If REPLACE=N, blank pixels will not be replaced.


Notes:

Example: mfilter
MFILTER Version 1.0 (May 13 1993)
MFILTER INSET=HVC1
Set HVC1 has 2 axes
RA from -207 to 207
DEC from -236 to 236
MFILTER BOX=
BOX range for set HVC1 :
RA from -207 to 207
DEC from -236 to 236
MFILTER OUTSET=hvcmed
Set hvcmed has 2 axes
RA from -207 to 207
DEC from -236 to 236
MFILTER MEDBOX=5 3
MFILTER CUT=0.1

==================== MFILTER ====================
Median filter in 5 x 3 neighborhood
using cutoff value: 0.100000 in relative mode
Name of input set: HVC1
Name of output set: hvcmed
MFILTER processed data in 14.57 sec (7.64 cpu sec)
MFILTER +++ FINISHED +++



Updates: May 6, 1993: VOG, Document created.
May 27, 1998: DoKester, averaging added
Feb 1, 2000: JPT, Increased number of subsets.