So you see that shape and shadow are very similar. For the purposes of
CG lighting, we will consider shadow to be cast by geometry in the
scene and shape to be the light that is coming from the light source
before touching any geometry.
Contrast
Contrast refers to the range of difference between the lightest parts of
the image and the darkest parts. An image in which the lightest parts are
much lighter than the darkest parts is considered to have high contrast.
On the other hand, a frame in which the lightest parts and the darkest
parts are similar is considered to have low contrast. For example, on a
bright sunny day, the sunlight shines on a white table. It is nearly blind
-
ing to look at and the shadows below it seem to be pitch black. There is
a great difference between the table and the shadow below. This is high
contrast. If, on the other hand, it is a cloudy day and you look at the
table, you will notice that the white of the table and the light gray of the
concrete below do not seem to be so different. This is low contrast.
The following images demonstrate the difference between low-con
-
trast and high-contrast imagery.
Part I: Lighting Theory ···································
14
Figure 1.9: Even if the wire fence were not visible in the scene, you
would know the shape and proximity of the object casting the
shadow on the child. Shape is a great tool for setting the scene and
giving visual cues to your viewers.
Some of this effect is caused by the dilation of your pupils or the aper-
ture size in your camera. When it is very bright outside, your pupils get
smaller, blocking out much of the light and protecting the retina, and you
must minimize the camera aperture to prevent overexposure. When this
happens, only the brightest light gets through. The concrete under the
table is lit even more than it is on a cloudy, low-contrast day, but your
eye is not registering the light. It has been filtered out by the pupil. This
creates a high-contrast situation. When the sun is obscured by clouds,
there is much less light available. The pupils dilate, allowing more light
in. The concrete beneath the table becomes more easily visible and
more similar in contrast to the table. Essentially what is happening is
that as the pupil or aperture opens and closes, the eye’s or camera’s
light range is changing. It is that light range that determines high or low
contrast.
Why the anatomy lesson? It is important to understand not how the
eye and camera work, but what light is actually there and what situations
create the lighting quality known as contrast so that these can be accu
-
rately recreated (or at least butchered, cheated, and faked) in a CG
environment.
································Chapter 1: Properties of Light
15
Figure 1.10: This image
demonstrates low contrast. All
the illumination and texture
values fall within the middle
range of grays.
Figure 1.11: This image
demonstrates high contrast. All
the illumination and texture
values range from highest (white)
to lowest (black).

Get LightWave v9 Lighting (w/CD) now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.