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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include "../lib/ids.h"
include <gki.h>
# IDS_CROUND -- coordinate rounding. Since putcell and other similar
# calls are defined to include both the lower-left corner and the upper-right
# corners of the desired rectangle, it is necessary to "round" the
# coordinates so that adjacent rectangles do not have overlapping edges.
# This could have been done by agreeing that the top and right edges of the
# rectangle are not part of it, but this was not done in the GKI definition.
# Hence, here, we adopt the notion that if (for example) the upper y coordinate
# is in the top half of a pixel, that pixel is included and if the lower y
# coordinate is in the bottom half of a pixel, likewise, that pixel is included.
# Otherwise, the pixels are excluded from putcell. The x coordinates are
# treated similarly.
# The code depends on the fact that lower is <= upper, that upper will be
# at most GKI_MAXNDC, and that the device resolution will never be as much
# as (GKI_MAXNDC+1)/2. The last requirement stems from the fact that if
# the resolution were that high, each pixel would be 2 GKI units and
# the "rounding" based on whether or not we are in the upper or lower half
# of a pixel would probably fail due to rounding/truncation errors.
procedure ids_cround(lower, upper, res)
int lower, upper
real res # device resolution
real low, up
real factor
begin
factor = res/(GKI_MAXNDC+1)
low = real(lower) * factor
up = real(upper) * factor
# if boundaries result in same row, return
if ( int(low) == int(up) )
return
# if low is in upper half of device pixel, round up
if ( (low - int(low)) >= 0.5 ) {
low = int(low) + 1
# don't go to or beyond upper bound
if ( low < up ) {
# low already incremented;
# ... 0.2 just for "rounding protection"
lower = (low + 0.2)/factor
# if now reference same cell, return
if ( int(low) == int(up) )
return
}
}
# if "up" in bottom half of pixel, drop down one. Note that
# due to two "==" tests above, upper will not drop below lower.
# 0.2 means drop partway down into pixel below; calling code will
# truncate.
if ( (up - int(up)) < 0.5 )
upper = (real(int(up)) - 0.2)/factor
end
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