First announced in the NV35, the UltraShadow technology, upgraded to the second version, appears in the NV40, too. The point of the technology has remained the same: when processing dynamic shadows with the help of the stencil buffer, it’s possible to set border Z values (“depth bounds”) beyond which the shadows from light sources won’t be rendered. Thus, this technology makes possible some computational economy and performance increase in scenes that use real-time shadows rendering.
NVIDIA illustrates UltraShadow II with the following figure, which shows the boundary values (zmin and zmax), beyond which the stencil buffer is not processed:
The opportunity of indicating boundary conditions for rendering shadows coupled with the known capability of the NV40 to speed up when processing the stencil buffer and the Z buffer (that is, to output 32, rather than 16 values per clock cycle) may bring a significant advantage to the NV40 over competitors in games that use dynamic shadows rendering with the help of the stencil buffer.
The advantages of UltraShadow will only be apparent when game developers use this technology explicitly and identify those depth bounds. As for the ability to output many Z values per clock cycle, the NV40 always has this option on, in any game.
Doom 3 is an example of the game that uses dynamic shadows. Moreover, its gameplay is based on shadows. The gaming community has long been anticipating this potential hit, so NVIDIA’s putting its stake on Doom 3 is a good marketing move, at least.