Graphics Reference
In-Depth Information
Sphere representing environment
(environment map)
Ray sampling the illumination
from the environment
Rays unable to reach the
environment due to occlusion
Point
p
to be rendered
Figure 7.2
GI computation using an environment map.
7.2.1 IBL Using HDR Environment Maps
An HDR radiance map recovered from Debevec's light probe images provides
the incoming radiance from each direction at the center of the probe. But it could
also be regarded as coming from a distant point on a very large sphere that en-
compasses the whole environment. In this sense, the radiance map is a kind of
HDR environment map. The interpretation is useful, as environment maps are
well established in basic rendering; in fact, environment map lookup is commonly
available in graphics hardware so that it can be employed in GPU shaders.
Figure 7.2 illustrates how an environment map is used in rendering a simple
scene consisting of a sphere sitting on a block. Rendering a point
p
on the surface
of the block including global illumination can be accomplished by Monte Carlo
ray tracing (see Chapter 2). Rays are sampled in directions on the hemisphere of
directions above the surface at
p
. A ray that is unoccluded in the environment
hits the environment sphere, and the radiance carried along the ray thus comes
directly from the environment map. If the ray in a particular direction hits another
object, the GI computation has to be done at the point of intersection with that
object. That computation is done the same way, by sampling over the environ-
ment. The method does not strictly need to use an HDR environment map, but the
dynamic range of illumination in a real environment is usually much larger than
can be stored in an ordinary texture map. Low dynamic range environment maps
have limited use as illumination sources.
