Graphics Reference
In-Depth Information
shades, tended to enhance the differences at the edges, emphasizing the faceted
structure.
Given this enhanced sensitivity to edges, it's natural to ask how small an edge
the eye can detect. We can make a drawing of alternating parallel black and white
stripes and move it away from the eye until it appears gray. This turns out to
happen at a distance where two adjacent stripes subtend about 1.6 minutes of arc
(a
minute
is 1
Response
Dark-
adapted
100
Light-
adapted
1
60 of 1
◦
).
The receptors in the eye adapt chemically to the overall brightness of what the
eye is seeing. For many ordinary illumination levels, the eye can detect a ratio of
intensities of about 100:1 within a small area. Figure 5.7 shows that this adaptation
allows us to detect the brightness of arriving light only over a modest range for
each level of adaptation. On the other hand, the eye can adapt very quickly to mod-
est changes in illumination, so you can, for example, quickly search for a pencil
in your dark backpack, even outdoors on a sunny day. Full adaptation to a major
reduction in illumination, however, which involves chemical changes in the recep-
tor cells, requires about a half-hour. After such adaptation, one can detect very low
illumination levels; the ratio of the brightest distinguishable daytime levels to the
dimmest distinguishable nighttime levels is more than 1,000,000:1. Many displays
advertise contrast ratios of 10,000:1; since the eye can only discern ratios of about
100:1, why would such a range be important? Because the adaptation of the eye is
partly
local:
As you stare from your unlit bedroom through a small window to the
sunny outdoors, one part of your eye may be able to distinguish between things
of different brightnesses in the room, while another distinguishes between things
of different brightnesses outdoors. To generate this same percept, a display screen
must be able to present comparable stimuli to the different regions of your eye.
As an example of the extremes of perception, on a clear night you may be able to
see a magnitude-3 star, while also seeing the moon clearly; the stellar magnitude
for the moon is about
Stimulus
/
Figure 5.7: The dark-adapted
eye's response to light “satu-
rates” at a fairly low stimulus
level; the light-adapted eye can-
not detect differences between
various low-light-level stimuli.
12. 5. Since 5 stellar magnitudes represents a factor of 100
in intensity, this represents an intensity range of about 1 million. But if the moon
is reasonably close (in your visual field) to that magnitude-3 star, you'll almost
certainly be unable to see the star.
−
Applications.
The visual system's ability to detect distance to an object
through two different mechanisms—the eye can focus, or the two eyes together
can use parallax, which we'll discuss presently—means that it's possible to have
divergent distance detections when the eyes are fed different data. For instance,
if a user wears a pair of glasses whose lenses are replaced by individual displays,
we can fool the user into seeing “in 3D” by displaying different images on the
two displays, making the user believe that the things seen are at various distances,
creating a “stereo” effect. But to see these two distinct images at all, the user must
focus on the displays, which are just a few inches from the user's eyes (or can
be made to seem more distant with the use of lenses). The two percepts of depth
contradict each other, and this makes many “3D display” experiences unpleasant
for some users.
The adaptation of the eye to surrounding light levels, and the limited dynamic
range within an adapted eye, means that we need not contruct displays with enor-
mous contrast ratios between pixels, although it may be useful to be able to adjust
the
mean
intensity over a large range. On the other hand, it also means that when
we're displaying something very bright, like the sun shining through the leaves
of a tree, we can eliminate most of the detail near the sun, since small variations
in brightness of the leaves will be “masked” by the eye's local adaptation to the
brightness of the sun.
