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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+# CRT_TWEAK_NDC -- Alter the input ndc endpoints so that the ratio
+# of device_pixels to image_pixels is an integer.  This procedure insures
+# an integer replication (or decimation) factor.  For replication, 
+# ndevice_pixels_spanned / nimage_values_output = an_integer.  For
+# decimation, ndevice_pixels_spanned / nimage_values_output = 1 / an_integer.
+# The NDC coordinates returned also represent integer device pixels.
+
+procedure crt_tweak_ndc (nvalues, ndc_start, ndc_end, device_res)
+
+int	nvalues			# Number of image values to output
+real	ndc_start, ndc_end	# NDC endpoints - changed on output
+int	device_res		# Full device resolution
+
+int	ndevice_elements, first_device_element, last_device_element, int_extra
+int	dev_pixel_1, dev_pixel_2, desired_dev_elements, desired_inverse
+real	ndc_extra, extra, real_extra
+double	gki_tweak
+
+begin
+	gki_tweak = double (32767) / double (32768)
+	if (int (ndc_end) == 1)
+	    ndc_end = gki_tweak
+	first_device_element = ndc_start * device_res 
+	last_device_element  = ndc_end * device_res
+	ndevice_elements = (last_device_element - first_device_element) + 1
+
+	if (mod (ndevice_elements, nvalues) != 0) {
+	    # Calculate amount to be altered
+	    real_extra = real (ndevice_elements) / real (nvalues)
+	    if (real_extra > 1.0) {
+	        # Tweak to get an integer replication factor
+	        int_extra  = ndevice_elements / nvalues
+	        extra = real ((real_extra - int_extra) * nvalues)
+	        ndc_extra = extra / device_res
+	        ndc_start = ndc_start + (ndc_extra / 2.0)
+	        ndc_end   = ndc_end   - (ndc_extra / 2.0)
+	    } else {
+		# Tweak to get an integer decimation factor
+		real_extra = real (nvalues) / real (ndevice_elements)
+		desired_inverse = int (real_extra) + 1
+		desired_dev_elements = nvalues / desired_inverse
+		extra = desired_dev_elements - ndevice_elements
+		ndc_extra = real (extra) / real (device_res)
+		ndc_start = ndc_start - (ndc_extra / 2.0)
+		ndc_end   = ndc_end   + (ndc_extra / 2.0)
+	    }
+	}
+
+	# Now have ndc coordinates of starting and ending pixel such
+	# that the replication or decimation factor is
+	# an integer.  Now insure that the ndc coordinates refer to 
+	# integer device pixels  so that truncation later in the
+	# processing doesn't alter this replication factor.  In what
+	# follows, note that dev_pixel_1 and dev_pixel_2
+	# are 0-based; dev_pixel_1 is the first pixel to be filled, and 
+	# dev_pixel_2 is the first pixel NOT to be filled, in accordance
+	# with Richard's notes.
+
+	dev_pixel_1 = ndc_start * device_res
+	dev_pixel_2 = (ndc_end  * device_res) + 1
+
+	ndc_start = real (dev_pixel_1) / real (device_res) / gki_tweak
+	ndc_end = real (dev_pixel_2) / real (device_res) / gki_tweak
+end