Graphics Reference
In-Depth Information
Figure 9.2.
Diffuse reflection geometry.
Figure 9.3.
Specular reflection
geometry.
The ratio A
1
/A
2
specifies the part of the light that will diffuse equally in all directions,
but there is one other assumption. Any light coming from
behind
the surface will be
assumed not to contribute any diffuse light. To put it another way, any surface that
faces away from the light, that is, where
N
•
L
< 0, contributes nothing. It follows that
r
d
= max (
N
•
L
,0).
Putting this all together gives us
()
=
() ()
+
() ()
Bouknight's reflectance model
:
I
l
I
l
k
l
I
l
kr
l
(9.1)
a
a
p
d
d
Next, we want to include a specular component into our intensity function. See
Figure 9.3. Given a light ray
L
and the normal
N
to a plane, the formula for the “mirror
direction”
R
of a light ray hitting the plane is clearly
(
(
)
)
RL L NLN
=-
2
- ∑
.
(9.2)
One assumes that the specular component has the form
()
I
kr
,
p
s s
where r
s
is the specular reflectance factor and the
specular reflection coefficient
k
s
is
really a function of the
angle of incidence
q but is usually set to a constant between 0
and 1. In the case of a perfect mirror,
