10. Image-Based Effects (14/14)

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504 10. Image-Based Effects

Figure 10.43. Fog and water rendered using volume rendering techniques in conjunction

with ﬂuid simulation on the GPU. (Image on left from “Hellgate: London” courtesy of

Flagship Studios, Inc., image on right courtesy of NVIDIA Corporation [205].)

10.16.1 Related Work

A concept related to using layers of textures for volume rendering method

is volumetric textures, which are volume descriptions that are represented

by layers of two-dimensional, semitransparent textures [861]. Like two-

dimensional textures, volumetric textures can be made to ﬂow along the

surface. They are good for complex surfaces, such as landscape details,

organic tissues, and fuzzy or hairy objects.

For example, Lengyel [764, 765] uses a set of eight textures to represent

fur on a surface. Each texture represents a slice through a set of hairs at a

given distance from the surface. The model is rendered eight times, with

a vertex shader program moving each triangle slightly outwards along its

vertex normals each time. In this way, each successive model depicts a dif-

ferent height above the surface. Nested models created this way are called

shells. This rendering technique falls apart along the silhouette edges, as

the hairs break up into dots as the layers spread out. To hide this artifact,

the fur is also represented by a diﬀerent hair texture applied on ﬁns gen-

erated along the silhouette edges. See Figure 10.44. Also see Figure 14.22

on page 682 and Figure 15.1 on page 699.

The idea of silhouette ﬁn extrusion can be used to create visual com-

plexity for other types of models. For example, Kharlamov et al. [648]

use ﬁns and relief mapping to provide simple tree meshes with complex

silhouettes.

The introduction of the geometry shader made it possible to actually

extrude shaded polyline hair for surfaces with fur. This technique was used

in Lost Planet [1025]. A surface is rendered and values are saved at each

pixel: fur color, length, and angle. The geometry shader then processes

10.16. Volume Rendering 505

Figure 10.44. Fur using volumetric texturing. The model is rendered eight times, with

the surface expanding outwards a small amount each pass. On the left is the result

of the eight passes. Note the hair breakup along the silhouettes. In the middle, ﬁn

rendering is shown. On the right is the ﬁnal rendering, using both ﬁns and shells. (Im-

ages from NVIDIA SDK 10 [945] sample “Fur—Shells and Fins” courtesy of NVIDIA

Corporation)

this image, turning each pixel into a semitransparent polyline. By creating

one hair per pixel covered, level of detail is automatically maintained. The

fur is rendered in two passes: Fur pointing down in screen space is rendered

ﬁrst, sorted from the bottom to the top of the screen. In this way, blending

is performed correctly, back to front. In the second pass, the rest of the

fur pointing up is rendered top to bottom, again blending correctly. As the

GPU evolves, new techniques become possible and proﬁtable.

Table of Contents for 10. Image-Based Effects (14/14)

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Table of Contents for
10. Image-Based Effects (14/14)