% @(#)createfilt.hlq	17.1.1.1 (ESO-IPG) 01/25/02 17:43:27 
%++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
%.COPYRIGHT  (c)  1990 European Southern Observatory
%.IDENT      createfilt.hlq
%.AUTHOR     KB, IPG/ESO
%.KEYWORDS   MIDAS, help files, CREATE/FILTER
%.PURPOSE    On-line help file for the command: CREATE/FILTER
%.VERSION    1.0  09-DEC-1985 : Creation, KB
%----------------------------------------------------------------
\se
SECTION./FILT
\es\co
CREATE/FILTER					09-DEC-1985   KB
\oc\su
CREATE/FILTER frame [dim_specs] [frame_specs] [filt_type] [coefs]
	create filter frame
\us\pu
Purpose: 
          Create a filter image (e.g. to be used in the Fourier domain).
\up\sy
Syntax:
          CREATE/FILTER frame [dim_specs] [frame_specs] [filt_type] [coefs]
\ys\pa
          frame = name of filter frame
\ap\pa
          dim_specs = specification of dimensions of filter frame
\\
              (a) NAXIS, NPIX(1),...,NPIX(NAXIS)
\\
              (b) "=" (equal sign) to indicate that filter frame shall inherit
                  dimensions from an existing frame;
\\
               defaulted to 1,512 (i.e. to option (a))
\ap\pa
          frame_specs = more specifications for the filter frame
\\
               for case (a) this par. has the form:
\\
               starts,steps = START(1),...,START(NAXIS),STEP(1),...,STEP(NAXIS);
\\
               for case (b) this par. is:
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               ref_frame = reference frame, take START, STEP and all other
                    standard descriptors from that frame;
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               defaulted to 0.0 for START(i) and 1.0 for STEP(i)
\ap\pa
          filt_type = type of filter used:
\\
               BLPF for Butterworth lowpass filter,
\\
               ELPF for exponential lowpass filter,
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               BHPF for Butterworth highpass filter,
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               EHPF for exponential highpass filter;
\\
               filt_type is defaulted to BLPF
\ap\pa
          coefs = coefficients for the filter,
\\
               number and meaning depends on the filt_type
\ap\no
Note: 
          For BLPF, the coefficients represent Do, n, factor; as used in the
          formula:
\\
          f(x,y) = 1. / (1. + factor*(D/Do)**2n)
\\
          where  D := sqrt( (x-xc)**2 + (y-yc)**2 ) the Euclidean distance
          from the center coordinates (xc,yc), default for
\\
          Do = (xstep+ystep)*(xdim+ydim)/16., n = 1 and factor = 1.
\\
          For ELPF the coefficients represent Do, n, factor; as used in the
          formula:
\\
          f(x,y) = exp( -factor*(D/Do)**n); defaults as above.
\\
          For BHPF, the coefficients represent Do, n, factor; as used in the
          formula:
\\
          f(x,y) = 1. / (1. + factor*(Do/D)**2n) if:  (x,y) .NE. (xc,yc)
\\
          f(x,y) = 0.                            if:  (x,y) .EQ. (xc,yc)
\\
          defaults as above.
\\
          For EHPF the coefficients represent Do, n, factor; as used in the
          formula:
\\
          f(x,y) = exp( -factor*(Do/D)**n) if:  (x,y) .NE. (xc,yc)
\\
          f(x,y) = 0.                      if:  (x,y) .EQ. (xc,yc)
\\
          defaults as above
\\
          Use the MIDAS procedure fftfilt.prg in APP_PROC to easily apply these
          filters to FFT's of images.  For more info use HELP/APPL fftfilt.
\on\sa
See also:
          CREATE/IMAGE, CREATE/RANDOM
\as\exs
Examples:
\ex
          CREATE/FILTER newfilt 2,128,128
          Create Butterworth lowpass filter newfilt.bdf with 128*128 pixels,
          start values 0.,0. and stepsize 1.,1. (the default values).
\\
          Note that NPIX(1) and NPIX(2) are powers of 2, so that the
          filter may be easily applied to Fourier transforms of images.
\xe \sxe