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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <imhdr.h>
include "iis.h"
# FINDZ -- Estimate the range of greylevels Z1 to Z2 containing a specified
# fraction of the greylevels in the image. The technique is to sample the
# image at some interval, computing the values of the greylevels a fixed
# distance either side of the median. Since it is not necessary to compute
# the full histogram we do not need to know the image zmin, zmax in advance.
# Works for images of any dimensionality, size, or datatype.
procedure findz (im, z1, z2, zfrac, maxcols, nsample_lines)
pointer im
real z1, z2, zfrac
int maxcols, nsample_lines
real rmin, rmax
real frac
int imin, imax, ncols, nlines
int i, n, step, sample_size, imlines
pointer sp, buf
pointer imgl2r()
include "iis.com"
begin
call smark (sp)
call salloc (buf, ncols, TY_REAL)
ncols = IM_LEN(im,1)
nlines = IM_LEN(im,2)
# Try to include a constant number of pixels in the sample
# regardless of the image size. The entire image is used if we
# have a small image, and at least sample_lines lines are read
# if we have a large image.
sample_size = iis_ydim * nsample_lines
imlines = min(nlines, max(nsample_lines, sample_size / ncols))
step = nlines / (imlines + 1)
frac = (1.0 - zfrac) / 2.
imin = frac * (ncols - 1)
imax = (1.0 - frac) * (ncols - 1)
rmin = 0.0
rmax = 0.0
n = 0
do i = 1 + step, nlines, max (1, step) {
call asrtr (Memr[imgl2r (im, i)], Memr[buf], ncols)
rmin = rmin + Memr[buf + imin]
rmax = rmax + Memr[buf + imax]
n = n + 1
}
z1 = rmin / n
z2 = rmax / n
call sfree (sp)
end
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