Dynamic Tessellation - Practical Rendering and Computation with Direct3D 11 - page 447

Figure 9.35. Example of silhouette enhancement.
9.2.1 Curved Point Normal Triangles
Chapter 4 introduced the history of tessellation in modern computer graphics. In particular,
it mentioned the TruForm feature in ATI Technology's Radeon 8500 GPU and the fact that
this was an early attempt at consumer hardware tessellation. Despite its various merits, the
technology never attained critical mass at the time, due in part to fierce competition, as well
as to software advances.
The obvious benefit of this algorithm both, originally and now, is the improved visual
aesthetics. However, it isn't just the continuity and smoothness of the surface itself that is
significant. It is also worth noting the silhouette. Take Figure 9.35 as a demonstration of the
silhouette enhancements; the key is that the right side does not present an obviously artifi-
cial outline comprised of triangle edges. Advances in recent years with per-pixel lighting
and bump mapping can greatly improve the perceived quality of a model's interior, but
they still leave a reminder that we are merely seeing a handful of triangles, something the
human brain is prone to pick up on and smash the illusion.
Although the Direct3D 8 API didn't explicitly expose details about the algorithm
it used, it did resolve down to what was discussed in the "Curved PN Triangles" paper
(Vlachos, Peters, Boyd, & Mitchell, 2001) which ATI Technologies released with its own
hardware implementation. Direct3D 11 removes the main barriers to the acceptance of
Vlachos et. al's Curved PN Triangles algorithm from first time around, and the remainder
of this chapter will discuss an implementation of the original approach, along with several
potential improvements.
Algorithm Overview
A key advantage of the algorithm is that it requires no additional data to be provided by
the application. This was crucial for commercial success and ease of use when applied to
consumer hardware of the era (even though that success didn't happen then!). The inten-
tion was that developers could flip a switch in the form of an API render state, and the
hardware would transparently apply the algorithm, producing smooth surfaces without any