Game Development Reference

In-Depth Information

Figure 9.1.
A blast mark decal applied to a curved surface. (
Image from the C4 Engine,

courtesy of Terathon Software LLC.
)

9.2.1 Decal Mesh Construction

We begin with a point
P
that lies on an existing surface and a unit normal direc-

tion
N
that is perpendicular to the surface at that point. The point
P
represents the

center of the decal and may be the point at which a projectile has hit the surface

or the point where a character's foot has stepped upon the ground. A unit tangent

direction
T
must also be chosen to determine the orientation of the decal. This

configuration is illustrated in Figure 9.2.

Given the point
P
and the directions
N
and
T
, we have an oriented plane that

is tangent to the surface geometry at
P
. We can carve a rectangle out of this plane

that represents the area of our decal by constructing four boundary planes that are

parallel to the normal direction
N
. Let
w
and
h
be the width and height of the de-

cal. Then the 4D vectors corresponding to the four border planes are given by

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