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include <imhdr.h>
# XT_SAMPLE -- Get sample of pixels.
#
# This routine returns a sample of unmasked pixels from an N-dim image.
# The input is the image pointer, the mask pointer (which may be NULL),
# the array to be filled, the maximum number of sample plixels, and the
# minimum number of lines to sample. The return value is the actual number
# of pixels which will be less than or equal to the specified maximum number.
#
# The intent of this routine is to sample fairly uniformly but efficiently.
# If nlines is zero the total number of pixels, in raster order, is divided
# into uniform steps. But this may end up reading many lines each for a
# few pixels. To be more efficient if nlines is greater than zero then as
# many pixels per line as possible are read to sample at least the requested
# number of lines.
$for (sird)
int procedure xt_sample$t (im, bpm, sample, nsample, nlines)
pointer im #I Image pointer
pointer bpm #I Bad pixel pointer
PIXEL sample[nsample] #I Work array
int nsample #I Maximum number of sample pixels
int nlines #I Minimum number of lines to sample
int nreturn #I Number of pixels returned
long v[IM_MAXDIM], vbuf[IM_MAXDIM]
int i, ip, n, ndim, npix, nc
real p, c, pstep, cstep
pointer buf, bpmbuf
int imgnls()
$if (datatype != s)
int imgnl$t()
$endif
begin
# Determine the number of pixels in the data, the number
# to make up nsample pixels, and the pixel step.
ndim = IM_NDIM(im)
nc = IM_LEN(im,1)
npix = 1
do i = 1, ndim
npix = npix * IM_LEN(im,i)
pstep = real(npix) / min (npix, nsample)
# To insure a minimum number of lines and efficient use of
# pixels in a line, set the column step.
if (nlines == 0)
cstep = pstep
else
cstep = nc / min (min(npix,nsample)/nlines, nc)
# Step through the pixels.
call amovkl (long(1), v, IM_MAXDIM)
nreturn = 0
for (p=(pstep-0.01)/2; p<npix && nreturn<nsample;) {
# Convert pixel number to image vector coordinates.
n = npix; ip = nint(p)
do i = ndim, 1, -1 {
n = n / IM_LEN(im,i)
v[i] = 1 + ip / n
ip = mod (ip, n)
}
# Sample the pixels in the line.
if (nlines == 0)
c = v[1]
else
c = (cstep - 0.01) / 2
if (bpm == NULL) {
v[1] = 1
if (imgnl$t (im, buf, v) == EOF)
break
for (; c<nc && nreturn<nsample; c=c+cstep) {
ip = nint (c)
nreturn = nreturn + 1
sample[nreturn] = Mem$t[buf+ip]
p = p + pstep
}
} else {
v[1] = 1
call amovl (v, vbuf, IM_MAXDIM)
if (imgnls (bpm, bpmbuf, vbuf) == EOF)
break
if (imgnl$t (im, buf, v) == EOF)
break
for (; c<nc && nreturn<nsample; c=c+cstep) {
ip = nint (c)
if (Mems[bpmbuf+ip] == 0) {
nreturn = nreturn + 1
sample[nreturn] = Mem$t[buf+ip]
}
p = p + pstep
}
}
}
return (nreturn)
end
$endfor
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