If swept_object is a polyline and there is no closure tag,
then
sweep connects n+1 successive copies of the
polyline (including the original) with four-sided polygons, each pair
of copies giving rise to a “polygon strip.” If there are m
points in the original polyline, then (m-1)n polygons are
formed by the sweep. We call these body polygons.
In this manner, sweep forms a
two-dimensional surface from from a one-dimensional polyline.
The order of vertices
produced by sweep is important. If a
polygon's vertices do not appear in counter-clockwise order in the
final image, the polygon will be culled
(unless cull=false is
set). If the points in the k'th copy of the polyline are
P_1, P_2, ..., P_m, and the points in the
next copy, the (k+1)st, are P_1', P_2', ...,
P_m', then the vertex order of the generated polygons is
Body polygon 1: P_2 P_1 P_1' P_2'
Body polygon 2: P_3 P_2 P_2' P_3'
...
Body polygon m-1: P_m P_m-1 P_m-1' P_m'
Options of unclosed line sweeps
are copied to each output polygon.
Options of the swept line are ignored.
When there is a closure tag,
then sweep connects n
successive copies of the polyline (including the original) with
four-sided body polygons just as the case with no closure tag. It then
connects the last copy back to the original to form a ribbon-shaped
surface that closes on itself with two holes remaining.
Finally, the sweep adds two more polygons to seal the holes and form a
closed surface that, depending on the sweep transforms, may
represent the boundary of a solid. In this manner, sweep forms
the boundary of a three-dimensional object from a one-dimensional
polyline. We call these hole-filling polygons ends.
The order of vertices of end polygons
is important for correct culling
as described above. If P_1^1, P_1^2, ...,
P_1^n are the n copies of the first polyline point and
P_m^1, P_m^2, ... ,P_m^n are the n
copies of the last polyline point, then the end polygon vertex order
is
End polygon 1: P_1^n, P_1^n-1, ... ,P_1^1
End polygon 2: P_m^1, P_m^2, ... ,P_m^n
If there are no options on the swept line,
then the sweep
options
are copied to each output polygon. If the swept line does
have options, these are copied to corresponding body polygons; the
sweep options are copied to the end polygons. In this manner, body
and ends may be drawn with different characteristics such as
fillcolor.