483when they have lived for a user-defi ned length of time, or as decided by
some other user-specifi ed criteria.
Particle eff ects could be rendered using regular triangle mesh geometry
with appropriate shaders. However, because of the unique characteristics
listed above, a specialized particle eff ect animation and rendering system is
always used to implement them in a real production game engine. A few ex-
ample particle eff ects are shown in Figure 10.58.
Particle system design and implementation is a rich topic that could oc-
cupy many chapters all on its own. For more information on particle systems,
see [1] Section 10.7, [14] Section 20.5, [9] Section 13.7 and [10] Section 4.1.2.
10.4.2. Decals
A decal is a relatively small piece of geometry that is overlaid on top of the reg-
ular geometry in the scene, allowing the visual appearance of the surface to be
modifi ed dynamically. Examples include bullet holes, foot prints, scratches,
cracks, etc.
The approach most oft en used by modern engines is to model a decal as
a rectangular area that is to be projected along a ray into the scene. This gives
rise to a rectangular prism in 3D space. Whatever surface the prism intersects
fi rst becomes the surface of the decal. The triangles of the intersected geom-
etry are extracted and clipped against the four bounding planes of the decal’s
projected prism. The resulting triangles are texture-mapped with a desired
decal texture by generating appropriate texture coordinates for each vertex.
These texture-mapped triangles are then rendered over the top of the regular
scene, oft en using parallax mapping to give them the illusion of depth and
with a slight z-bias (usually implemented by shift ing the near plane slightly)
so they don’t experience z-fi ghting with the geometry on which they are over-
Figure 10.59. Parallax-mapped decals from Uncharted: Drake’s Fortune.
10.4. Visual Effects and Overlays