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.TH FILT 1 "Ultrasonics Laboratory"
.SH NAME
filt - Filter Projection Data
.SH SYNOPSIS
filt [options...]
.SH DESCRIPTION
This program filters projection data 
that so the data can be back projected.
The filter functions for first,
second, third and fifth generation
scanners can be used.
The program allows all the parameters of
the scanner to be set from the command line.
In addition the type of filtering
can be specified.
.PP
The command line options control the program.
The parameters can be set
by listing the variable name,
an equals sign and then
the new value.
Depending on the
variable,
this value should be either 
a number or a file name.
The parameters and their default values
are:
.TP 13
n=100
Number of samples per projection.
.TP
k=1 
Number of projections.
.TP
gamma=22.5 
Half the fan width for third and
fifth generation scanners (in degrees).
.TP
m=8 
Log base 2 of the size of each projection.
For proper results this
number should be greater than two times
the base two logarithm of
.B n.
.TP
t=1
Half the 
length of each projection.
The sampling interval for parallel and
equi-spatial scanners is given by
2t/(n-1).
.TP
r=1 
The radius of the scan circle for third
and fifth generation scanners.
.TP
nfl=100
The number of flashes for fifth
generation scanners.
This option also invokes the 
.B -5
option.
.TP
if=for003.dat
is the projection input data.
The data should be single precision floating point
numbers.
.TP
of=for002.dat
is the filtered projection data.
.PP
There are also many flags that can be set from
the command line.  
Several control the type of filtering to use.
If none of these flags are used then
the parallel algorithm is used to
filter first generation data.
.TP 13
-x
use Lax's radial algorithm to filter
third generation data.
.TP
-5
filter data from a fifth
generation scanner.
.PP
There are several other flags
that control other features of the
program.
They are listed below with their default
values.
.TP 13
-e(on)
The data is from a scanner with
evenly spaced detectors.
This only applies to second,
third and fifth generation data.
.TP
-a(off)
The data is from a scanner with
detectors that subtend equal angles.
This only applies to second,
third and fifth generation data.
.TP
-h(off)
Apply a Hamming window to the projection data.
.TP
-p(off)
Do not print the program parameters.
.TP
-t(on)
Generate the filter function in the time domain
when calculating the filter.
.TP
-f(off)
Use a frequency domain representation
of the filter.
See C.R. Crawford, and A.C. Kak, "Aliasing Artifacts in CT Images,"
TR-EE 79-25, July 1979.
.TP
-s(off)
save the resulting frequency domain
filter in the file 
.B filter.
.SH FILES
.TP 13
for003.dat
Default input file for the projection data.
.TP
for002.dat
Default output file for data that is
ready to be backprojected.
.TP
filter
File where the resulting filter is stored.
See the 
.B -s 
flag.
.SH SEE ALSO
.B back(1)
and
.B scan(1).
.SH DIAGNOSTICS
All file open errors and inconsistent
parameters are flagged.
.SH BUGS
Second generation data is not handled correctly.
The cos(delta*gamma) term is not included.