MaximumPC 2004 10

burden on your system’s videocard and

CPU. Let’s take a look at how stencil shad-

ows (sometimes called shadow volumes)

work so you can understand why drawing

them is so hardware intensive.

Check out the screenshot on the pre-

ceding page. The red cube represents

a light source. The area under the cube

should be lit, but there’s a platform

between the light and the floor. You know

the area under the platform will be shad-

owed, but how can you describe this in a

simple computer program? It’s easy. Think

of the light as being the apex of a pyramid

and the four edges of the platform’s top

as lines that intersect the pyramid’s four

walls. The area inside the pyramid will be

illuminated, except for the area beneath

the platform, which will be shadowed.

This is precisely how Doom 3 handles

shadows. In our pyramid example, the

engine actually draws an invisible polygon

where the shadow appears. The engine

draws shadows on any object that enters

the invisible volume polygon. The trick is

that Doom 3 draws these invisible shadow

volumes for every light source and every

object in the game. Even with just two or

three light sources, the number of invisible

polygons quickly gets out of hand—they

may be invisible, but they still consume

precious GPU resources. Considering that

everything from the swinging light to the

muzzle blast of your shotgun is a light

source , that’s a lot to draw.

Do I Look Normal to You?

Dynamic shadows don’t account for all of

Doom 3’s eye candy. Normal maps are also

used to add an extra level of detail to the

character models and lots of the game’s sur-

faces. Here’s how they work.

Bump maps have been used in games

for a long time now. They’re nothing more

than simple grayscale textures that encode

the height of each pixel on the texture. When

applied to an otherwise flat surface, they

convey the illusion of depth. However, bump

maps have a problem. Because they only

modify the height of a flat surface and don’t

actually modify the geometry of the sur-

face, collision and clipping problems occur

when you use bump maps on polygons that

move—like the polys in a human face.

The solution to this problem is the

normal map, which contains not only the

height of each individual pixel on the base

surface, but also encodes the direction

that each pixel is facing. With this informa-

tion, a modern GPU can actually draw the

model with the extra polygons described

by the normal map, and clipping and poly-

gon collision are no longer a problem.

Game designers can create a model with

millions of polygons, then run it through a

filter in their 3D modeling software that will

convert the high-poly model to a low-poly

model and a normal map. In Doom 3, nor-

mal maps and 6,000 poly models are used

to draw monsters that appear to have all the

complexity of the high-res originals!

Oh Shaders, How I Love Thee!

The final stroke in the Carmackian trifecta of

3D engine design are the dozens of little

shader effects strewn liberally throughout

the game. The shockwave effect that occurs

near a rocket or grenade explosion, the

shimmer of heat around steamy pipes, and

the melting-away that claims so many corps-

es are all created with simple pixel and ver-

VIDEOCARD TESTS

GeForce 6800 Ultra (1280x1024 HQ) 81.1 fps

Radeon 9800 XT (1280x1024 HQ) 33.0 fps

GeForce4 Ti 4600 (800x600 MQ) 30.0 fps

GeForce4 MX 460 (640x480 LQ) 13.4 fps

As you can see, there’s a huge perfor-
mance difference between the old, fixed-
function GeForce4 MX card and the state-
of-the-art 6800 Ultra. Not only do you get
a massive performance increase, you also
run at a much higher quality level, with
high resolution textures and much more
nuanced shadows. The Radeon 9800 XT
performed well, but the eight-pipeline card
couldn’t keep up with the 16-pipe 6800.

CPU TESTS

Athlon FX-51 81.0 fps

Athlon XP 3200 64.4 fps

Pentium 4 2.0GHz 33.4 fps

The results of the CPU tests are quite

surprising. The Athlon XP—an underper-

former by today’s standards—performs

admirably. It’s able to keep up with the

CPU-heavy physics and sound engines,

unlike the slow 2GHz Pentium 4 we tested.

SYSTEM TESTS

Athlon FX-51, GeForce 6800 Ultra 81.0 fps

Athlon XP 3200, Radeon 9800 Pro 256MB 48.8 fps

Pentium 4 2GHz, GeForce3 Ti 500 25.1 fps

What’s surprising is that the high-end

system is nearly twice as fast as the mid-

range rig. The lesson is one Maximum PC

has preached for years: New hardware

gives you a huge performance boost,

even compared with a system that’s only

about a year old.

This means that, in order to get the

most Doom 3 performance for your buck,

you should: upgrade your videocard first,

your CPU second, and your memory

third. You should also remember that

pairing an uber-powerful videocard with a

slow-ass CPU will limit your in-game per-

formance, although not as much as we

would have expected.

Ë

OCTOBER 2004 MA XIMUMPC 6

One clever advancement found in Doom 3 is interactive, in-game GUIs that are

similar to Flash animations. When you approach one, your weapon drops and

is replaced by a cursor that lets you punch in codes as if you were standing in

front of the terminal.