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diff --git a/sys/imio/imioff.x b/sys/imio/imioff.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<syserr.h>
+include	<config.h>
+include	<imhdr.h>
+include	<imio.h>
+include	<mach.h>
+
+# IMIOFF -- Initialize the physical dimensions of a new image.  Compute and set
+# the absolute file offsets of the major components of the pixel storage file.
+
+procedure imioff (im, pixoff, compress, devblksz)
+
+pointer	im			# image descriptor
+long	pixoff			# file offset of first pixel
+int	compress		# if set, do not align image lines
+int	devblksz		# FIO device block size
+
+real	impkden, envgetr()
+long	offset, temp1, temp2, imnote()
+int	ndim, dim, sz_pixel, lblksize, pblksize
+errchk	imerr
+
+include	<szpixtype.inc>
+
+begin
+	sz_pixel = pix_size[IM_PIXTYPE(im)]
+	pblksize = max (devblksz, SZ_VMPAGE)
+
+	if (compress == YES)
+	    lblksize = 1
+	else
+	    lblksize = devblksz
+
+	# Set the offset of the pixel storage area.  Compute the physical
+	# dimensions of the axes of the image.  If image compression is
+	# selected, the logical and physical lengths of the axes will be
+	# the same.  Otherwise, the physical length of each line of the
+	# image will be increased to fill an integral number of device blocks.
+
+	IM_PIXOFF(im) = pixoff
+	call amovl (IM_LEN(im,1), IM_PHYSLEN(im,1), IM_MAXDIM)
+	call amovl (IM_LEN(im,1), IM_SVLEN(im,1), IM_MAXDIM)
+
+	ndim = IM_NDIM(im)
+
+	# If ndim was not explicitly set, compute it by counting the number
+	# of nonzero dimensions.
+
+	if (ndim == 0) {
+	    for (ndim=1;  IM_LEN(im,ndim) > 0 && ndim <= IM_MAXDIM;
+	    ndim=ndim+1)
+		;
+	    ndim = ndim - 1
+	    IM_NDIM(im) = ndim
+	}
+	IM_NPHYSDIM(im) = ndim
+
+	# Make sure dimension stuff makes sense.
+	if (ndim < 0 || ndim > IM_MAXDIM)
+	    call imerr (IM_NAME(im), SYS_IMNDIM)
+
+	do dim = 1, ndim
+	    if (IM_LEN(im,dim) <= 0)
+		call imerr (IM_NAME(im), SYS_IMDIMLEN)
+
+	# Set the unused higher dimensions to 1.  This makes is possible to
+	# access the image as if it were higher dimensional, and in a way it
+	# truely is.
+
+	do dim = ndim + 1, IM_MAXDIM
+	    IM_LEN(im,dim) = 1
+
+	if (lblksize > 1) {
+	    temp1 = pixoff + IM_LEN(im,1) * sz_pixel
+	    temp2 = temp1
+	    call imalign (temp2, lblksize)
+
+	    # Only block lines if the packing density is above a certain
+	    # threshold.  Alignment is disabled if compress=YES since lblksize
+	    # will have been set to 1.
+
+	    iferr (impkden = envgetr ("impkden"))
+		impkden = IM_PACKDENSITY
+
+	    if (real(temp1-pixoff) / real(temp2-pixoff) >= impkden)
+		IM_PHYSLEN(im,1) = (temp2 - pixoff) / sz_pixel
+	}
+
+	# Set the offsets of the histogram pixels and the bad pixel list.
+	# The HGMOFF offset marks the end of the pixel segment.
+
+	offset = imnote (im, IM_LEN(im,1))
+	call imalign (offset, pblksize)
+	IM_HGMOFF(im) = offset
+
+	offset = offset + (MAX_HGMLEN * SZ_REAL)
+	call imalign (offset, lblksize)
+	IM_BLIST(im) = offset
+end
+
+
+# IMALIGN -- Advance "offset" to the next block boundary.
+
+procedure imalign (offset, blksize)
+
+long	offset
+int	blksize, diff
+
+begin
+	diff = mod (offset-1, max (1, blksize))
+	if (diff != 0)
+	    offset = offset + (blksize - diff)
+end