Graphics Reference
In-Depth Information
If objects in the environment are far enough away from the CG object, interreflec-
tion between the objects and the CG object can be ignored. And if shadowing
effects are also considered unimportant (which may be the case in the absence
of concentrated light sources) then there is no need for the environment model.
In this case, all the lighting in the environment comes from the HDR radiance
map and the rendering process becomes much simpler. Rendering a CG object
with global illumination using only the radiance map is a straightforward appli-
cation of Monte Carlo path tracing (Chapter 2). Rays reflected off the object are
not directly traced; instead, the radiance carried by a reflected ray comes directly
from the radiance map. Interreflections within the object do, however, involve
secondary path tracing, but only within the object itself.
However, if near-field effects close to the CG object cannot be ignored, they
must be modeled into the scene in which the CG object is rendered. This requires
not only the geometry of the objects, but also their reflectance characteristics.
The geometry could be recovered by using methods IBR (e.g., the architecture
model recovery method of Debevec Taylor, and Malik described in Chapter 5),
but recovering reflectance was a much more difficult problem. Debevec there-
fore made the simplifying assumption that nearby scene objects had Lambertian
surfaces. The method starts by estimating the scene object albedos, then refining
the estimates by rendering the CG object and comparing the result in the com-
posite image. Although this may seem unscientific, the goal of the work was
to create plausible results so this approach is reasonable. In fact, such ad hoc
adjustments to improve visual quality are common in production (and research)
computer graphics—sometimes rendered images lack an aesthetic quality that can
only be added by human intervention. Nevertheless, the problem of recovering re-
flectance characteristics from captured images remained unsolved.
7.2 Environment Maps and IBL
The basic process introduced in Debevec's “Rendering Synthetic Objects into
Real Scenes” paper has been refined for use in more practical situations. In re-
cent years the method of using HDR environment maps as the light source for
rendering computation has become the main method of IBL. Interest in general
environment map research has been part of this trend. Originally, environment
maps were used as a special kind of texture map. However, in modern research
they are being used for lighting in GI computation. This section introduces the use
of environment maps in this more modern context, and explores their relationship
with IBR in general.
