Game Development Reference
models and in spite of algorithmic and software optimisations, the implementation of
tools consumes a lot of machine resources (in terms of time and memory).
These methods are thus very suitable for the calculation of fixed images, but do
not fit at all in the constraints of real time applicable in virtual reality.
Another path of research involves working on the images produced: since the
brain is the ultimate “decision-maker'', let's find out what is it exactly capable of
differentiating. Computer experts and specialists of cognitive sciences then joined their
forces to study the relations between rendering and perception. In other words, if a
rendering method requires very long processing time to microscopically modify the
colour of some pixels whose details will not be perceived by the user, the method needs
to be changed. Thus we shift from the single criterion of realism to a new criterion of
Let's compare it with a traveller who needs to move between two cities some
hundred kilometres from each other. If he uses a tourist plane, an aerial photograph
is certainly very useful for him. But if he uses an automobile, then a standard road
map (with a scale of 1:50,000) is the most suitable for his task. Finally, if he uses an
all-terrain vehicle and takes small track roads, minimising the inclines, a detailed map
(scale 1:5,000) will help him reduce the efforts.
In this example, the aerial photograph is a photo-realistic image, while the maps
are credible images. It is clear that a single best representation does not exist, but some
media are more suitable in certain contexts of task completion.
There are several methods of credible rendering, including some very old meth-
ods. Over the years, a number of artists have invented painting styles different than
realistic painting styles by working on basic motifs (pointillism developed by Seurat),
by ignoring the details of borders, colours or contrasts (impressionism of Monet or
Renoir) or by working on symbols (cubism started by Picasso and Braque, who mainly
ignored the rules of perspective laid by the Renaissance). More recently, comic strip
designers have invented their own styles, often termed as cartoons. All these schools
have influenced computer experts, who derived non-photo-realistic rendering methods
called “expressive rendering'' from these techniques.
15.2 LIGHTING AND SHADING MODELS
15.2.1 Modelling the appearance
For several years, image synthesis has been applied to define reflection models that
simulate the response of any material to a light incidence as exactly as possible. How-
ever, interaction between light and material is a highly complex problem which we
cannot model in its entirety. This interaction can be approached at different scale fac-
tors and with objectives as varied as physical simulation or real-time rendering for
At the microscopic level, this interaction is explained fundamentally by quantum
mechanics (Feynman, 1985). If we go to a higher scale, the mesoscopic scale, the
interaction between light and material can be described entirely by electromagnetism
(Born & Wolf, 1984) and geometric optics.