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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
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