Real-Time Rendering, Third Edition, 3rd Edition

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Chapter 10

Image-Based Effects

“Landscape painting is really just a box of air with little marks

in it telling you how far back in that air things are.”

—Lennart Anderson

Modeling surfaces with polygons is often the most straightforward way to

approach the problem of portraying objects in a scene. Polygons are good

only up to a point, however. Image-based rendering (IBR) has become a

paradigm of its own. As its name proclaims, images are the primary data

used for this type of rendering. A great advantage of representing an object

with an image is that the rendering cost is proportional to the number of

pixels rendered, and not to, say, the number of vertices in a geometrical

model. So, one use of image-based rendering is as a more efficient way

to render models. However, IBR techniques have a much wider use than

this. Many objects, such as clouds and fur, are difficult to represent with

polygons. Layered semitransparent images can be used to display such

complex surfaces.

In this chapter, image-based rendering is first compared and contrasted

with traditional polygon rendering, and an overview of algorithms pre-

sented. Commonly used techniques such as sprites, billboards, particles,

and impostors are described, along with more experimental methods. Opti-

mizations are described for when the camera and scene are mostly

static.

At the same time, there is more involved in making an image than

simply portraying objects. We may wish to post-process synthesized data

in a wide variety of ways. Image processing techniques are explained for

performing tone mapping, high dynamic range effects, motion blur, depth

of field, and other phenomena. The chapter ends with two related top-

ics, atmospheric effects and using three-dimensional volumetric data for

modeling objects.

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440 10. Image-Based Effects

Figure 10.1. The rendering spectrum. (After Lengyel [763].)

10.1 The Rendering Spectrum

The goal of rendering is to portray an object on the screen; how we at-

tain that goal is our choice. There is no single correct way to render a

scene. Each rendering method is an approximation of reality, at least if

photorealism is the goal.

Polygons have the advantage of representing the object in a reasonable

fashion from any view. As the camera moves, the representation of the

object does not have to change. However, to improve quality, we may wish

to substitute a more highly detailed model as the viewer gets closer to the

object. Conversely, we may wish to use a simplified form of the model

if it is off in the distance. These are called level of detail techniques (see

Section 14.7). Their main purpose is to make the scene display faster.

Other rendering and modeling techniques can come into play as an

object recedes from the viewer. Speed can be gained by using images

instead of polygons to represent the object. It is often less expensive to

represent an object with a single image that can be sent quickly to the

screen. One way to represent the continuum of rendering techniques comes

from Lengyel [763] and is shown in Figure 10.1. We will first work our way

from the left of the spectrum back down to the more familiar territory on

the right.

10.2 Fixed-View Effects

For complex geometry and shading models, it can be expensive to rerender

an entire scene at interactive rates. Various forms of acceleration can be

performed by limiting the viewer’s ability to move. The most restrictive

situation is one where the camera is fixed in position and orientation, i.e.,

does not move at all. Under such circumstances, much rendering can be

done just once.

For example, imagine a pasture with a fence as the static scene, with

a horse moving through it. The pasture and fence are rendered once and