Perspective Projection - Transformations and Projections in Computer Graphics - page 134

speed. In addition, there is no loss of color. The downside is loss of resolution because
each image is displayed on half the rows of the display.
A variation of the line-alternate technique is a lenticular lens. Figure 3.55 shows
theprinciplesofthistechnique. Eachofthetwostereoeyeimagesiscutintonarrow
strips that are then interleaved and viewed through a special lens made of many small
half-circular elements (placed at about 100 elements per inch). Each lens element sends
oneimagestriptothelefteyeandonestriptotherighteye.
The Pulfrich Effect
The Pulfrich effect, described by Carl Pulfrich in 1922, is best explained as an optical
illusion (but then one might argue that any stereoscopic image is an optical illusion).
Imagine an object moving in the plane perpendicular to our line of sight. If we look
at the object with both eyes and dim the light reaching one eye, the object seems to
move out of this plane and to either approach us or recede from us. The simplest way
to observe this effect is to use one sunglass lens, but most pieces of dark glass or plastic,
as well as many optical filters, work fine.
It is easy to demonstrate the Pulfrich effect with a swinging pendulum. When
viewed normally with both eyes, we can verify that the pendulum swings in a plane
back and forth. When a dark lens or filter is placed in front of one eye, the pendulum
suddenly seems to be swinging in an ellipse parallel to the ground. The light has to
be dimmed to one eye only. Dimming the light equally to both eyes results in a dim
pendulum seen swinging in a plane.
Figure 3.54: The Pulfrich Effect.
There are several Web sites with Java applets that illustrate this effect very con-
vincingly through animation. One such site is [Newbold 05], but a Web search for
pulfrich , java ,and animation yields many more.
I have never been able to observe these effects myself, for I have been blind in the left
eye for 16 years as a result of a traumatic (blutigen) injury of the eye suffered when I
was young.
—Carl Pulfrich (1922)