Bridge geometry - Computational Analysis and Design of Bridge Structures - page 559

reproduce and render such a 3D curve. This subdivision process is the
so-called curve tessellation. As tessellations are for visualization purpose
only, the minimum length of subdivision can be 1 m or a couple of feet.
When tessellating a curve, several different types of points on curve have to
be subdivided. These points, the so-called ensured points include (1) geom-
etry control points where either plane curve or vertical curve changes, (2)
girder or beam section change points, (3) point of interests, (4) support
points, (5) diaphragm points, (6) roadway cross-sectional points, and (7)
control points for superelevation and superwidening. Once the ensured
points are obtained according to geometry theories, points in between any
two consecutive ensured points will be inserted according to a minimum
tessellation segment length.
When the tessellation is for the purpose of plane view, tessellated seg-
ments may contain arcs and straight lines, for arcs can be rendered by gen-
eral computer graphics technologies. When the tessellation is for 3D view,
the tessellated segments can only be straight lines.
When showing roadway surface or deck in 3D, a similar tessellation
process is needed to produce triangle planes in space so that the surface
can be shown as 3D views. In addition to the longitudinal tessellation on
3D roadway centerline, cross sections at each longitudinal tessellation
point will be further evaluated, as discussed in Section 18.3. Each cross-
section curve will be tessellated transversely. The ensured points on trans-
verse curve include (1) geometry control points such as where a parabola
starts or ends; (2) locations of all girder centerlines; and (3) locations
of mainline, road edges, curbs, or medians. Given two transverse seg-
ments on two consecutive cross sections, as shown in Figure 18.12, two
triangle planes can be produced for 3D rendering. Figure 18.12 shows a
roadway in 3D with the wireframe mode so that the tessellated triangles
can be illustrated, whereas Figure 18.14 shows the roadway with the solid
rendering mode.
Figure 18.12 Tessellations of roadway surface or deck.