Graphics Reference
In-Depth Information
bright
Raw response
There is another set of recently discovered cells in our eyes that respond pri-
marily to light in the blue region of the spectrum; their responses are not carried
by the optic nerve and do not go to the visual cortex. Instead, they are used in
controlling circadian rhythms in mammals.
dim
Inhibition from
all neighbors
The receptors in the eye detect light, provoking a response in the visual sys-
tem; very roughly speaking, each doubling of the arriving light at a receptor gen-
erates the same
increment
of response. If light
B
appears half as bright to you as a
geometrically identical light
A,
then the energy emitted by
B
is about 18% that of
light
A
. A light
C
whose energy is 18% of that from
B
will appear half as bright as
B
, etc. This logarithmic response helps us handle the wide range of illumination
we encounter in everyday life. We discuss the perception of brightness of light
further in Chapter 28. The logarithmic response of the visual system also deter-
mines something about
display
technology: An effective display must be able to
show a wide range of intensities, and this range of intensities should not be divided
into even steps, but rather into even
ratios
of intensity. This notion drives the idea
of gamma correction discussed in Chapter 28.
Brightness
is the name used to
describe the
perception
of light; by contrast, what we've been informally calling
“intensity” of light is more precisely measured in units of radiance, described in
detail in Chapter 26. What we've been saying is that, all other things being equal,
brightness is roughly proportional to the log of radiance.
In general, it's useful to know that the eye
adapts
to its circumstances. When
you're in your bedroom at night, reading, your eyes are adapted to the level of
light in the room, an adaptation that's centered on the intensity
4
of the page you're
looking at; when you turn off the light to go to sleep, everything in the room looks
black, because the page's intensity is now well below the range of intensities to
which your eye has adapted. But a few minutes later you can begin to distinguish
things in your room that are illuminated by just moonlight, as your eye begins to
adapt to the new, lower, light level. If you turn the light on again to resume reading,
the page will initially seem very bright to you, until your eye has readapted.
The receptor cells in the eye do not act entirely independently. When the eye
is generally adapted to ambient illumination, an extra bit of light arriving at one
receptor will not only increase the sensation of brightness there, but also slightly
reduce the sensitivity of the neighboring receptors, an effect known as
lateral
inhibition.
The result of this (see Figure 5.6) is that the edge contrast between
regions of light and dark is enhanced compared to the contrast between the centers
of the regions: The dark side of the edge is perceived as being darker and the
light side as being lighter. This is the origin of the Mach banding discussed in
Section 1.7.
This has an important consequence for computer graphics. In early graphics
systems, polygons were often “flat shaded.” That is, relatively large areas of the
screen were given constant colors. When a shape like a cylinder (approximated by
an extruded polygon) was illuminated by light from one direction, adjacent facets
were assigned differing constant shades depending on how directly they faced the
light source. The eye, instead of blending together the slightly different adjacent
Total response
=
raw
-
inhibition
Figure 5.6: The raw response of
receptors in the bright and dark
regions (in blue, at top), the lat-
eral inhibition amounts (in red,
middle), and their difference—
the actual response—shown in
green at the bottom. Notice the
enhanced contrast at the edge
between light and dark, indicated
by the dotted line.
4. We're using this term informally to describe the amount of light energy leaving the
page and arriving at your eye.
