MaximumPC 2008 08

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WHITE PAPER

R&D^

EXAMINING TECHNOLOGY AND PUTTING IT TO USE

How 3D game rendering is changing with hardware advancements —ZACK STERN

Raster vs.Rays

WHITE PAPER

Raster vs.Rays

WHITE PAPER

T

he shiny, new hatchback you nudge

in a street race dents slightly on the

driver’s side door. Although you’re

playing a PC game, created with beaucoup

equations, the bend looks almost real. The

3D renderer sculpts all those numbers into

images, with help from the video API (appli-

cation program interface). However, several

completely different rendering techniques

can be the source of those images. Cur-

rently, the hardware and software indus-

tries are debating how to best utilize two

graphics-rendering techniques: ray tracing

and rasterization.

Rasterizing is widely used to render

current 3D games because it strikes a

compromise between real-time processing

demands and pretty pictures. Its regular,

predictable patterns are also suited to

specialized massively parallel processors,

such as GPUs. Essentially, the raster engine

looks at the thousands of 2D triangles that

build a 3D scene and determines which are

visible in the current perspective. With that

information, the engine analyzes the light

sources and other environment details to

light and color pixels onto each triangle.

Ray tracing takes the opposite ap-

proach, borrowing from the way photons

move in the real world. In nature, a light

source creates countless pho-

tons (or rays) that bounce off

objects, take on their color

and properties, and eventu-

ally reach your eye. Ray

tracing reverses the process,

firing its gaze away from the

camera perspective, assessing which objects

are in view. When a ray hits something, the

engine knows to draw a pixel.

THE GRAY AREA

These two techniques further diverge when

adding shadows and other details to a scene.

Rasterized graphics can use a few techniques

to create light and dark, frequently relying

on shadow maps. These guides are created

by rasterizing from the perspective of a light

source, seeing which objects are visible, and

shading the camera perspective based on

this blueprint. A ray tracer calculates shad-

ows just by tracing more beams and seeing

how they bounce. If a beam’s path leads back

to a light source, its pixel is drawn brighter.

If the beam ends without hitting a light, the

engine knows to draw that pixel in shadow.

Ray tracing’s realism—and system

burden—comes from the arbitrary point at

which the engine stops calculating these

bounces. Every time the beam ricochets off

another object, more color, shadow, and

refl ection details can be added back to the

fi rst collision pixel. Fog eff ects can be espe-

cially taxing, requiring the beams to refract

through a mist. The best-looking images can

take billions of rays; that’s just too much

number crunching for today’s CPUs and

GPUs to handle in real time.

And even if those chips could keep up,

other bottlenecks couldn’t keep pace with a

fully ray-traced real-time scene. “It’s just too

hard in terms of memory bandwidth; it’s too

hard in terms of silicon speed,” says David

Kirk, chief scientist at Nvidia. “It’s just too

hard. And I don’t think that’s the goal.”

“Graphics in general is the grand art of

cheating,” Kirk notes, regardless of tech-

nique. “We’re trying to approximate what

nature does—tracing gazillions of photons

around—by doing less work than that,

because even the most sophisticated and

powerful ray tracers don’t trace billions of

rays per second.”

TOOLS FOR THE JOB

“This whole CPU versus GPU distinction is

a little bit artifi cial,” says Bill Mark, senior

researcher at Intel’s Corporate Technology

Group. “Certainly you can build GPUs that

have some CPU-like characteristics. Simi-

HOW IT WORKS

In the real world, light comes from a source and bounces off surfaces. Some of the photons that

reach your eye hit your retina, allowing you to see. Ray tracing works the other way, by casting

rays back from the observer’s viewpoint to determine which pixels it should render.

VIEW POINT

IMAGE

“IF YOU COULD DO ALL RAY

TRACING, WOULD YOU?

I DON’T THINK YOU WOULD.”

In the real world, light comes from a source and bounces off surfaces. Some of the photons that

LIGHT

SOURCE

SCENE

OBJECT

SHADOW

RAY

VIEW

RAY

Rendering Pixels via Ray Tracing