PHYSICS AND THE VISUAL ARTS
Notes on LESSON 5
Things to remember from the lecture, demonstrations, and lab exercise.
Brightness is the overall illumination. It can be measured with a
light meter, but to you eye it is subjective depending on your state of
adaptation. Lightness is the appearance of individual surfaces. It
does not depend on brightness or adaptation. Lightness constancy
is that behavior of our visual system that recognizes the lightness
values independently of the actual lighting or lighting changes. Remember
Land's "Mondrian" that you saw and the gray squares that looked different even though they actually reflected
the same amount of light.
Lateral inhibition is the behavior of the photoreceptors to inhibit the
sensitivity of neighboring receptors due to the light striking the first
ones. This behavior contributes to several visual illusions, to edge
enhancement, and it contributes to the effect of simultaneous lightness contrast.
When the lightness changes quickly with position, lateral inhibition acts to improve
the appearance of the edge between the lighter and darker regions. This behavior is
known as edge enhancement. The corrollary is that the eye is relatively
insensitive to gradual changes in lightness.
The lightness of an area is affected by its surroundings. For example, a gray
square surrounded by a darker region looks lighter than the same square when
surrounded by a lighter region. This behavior is known as simultaneous lightness
contrast.
You saw some illusions that are associated with lateral inhibition. A particularly strong illusion is the
Hermann grid in which gray dots appear at all of the intersections of the white lines except the one you are
focusing on.
Illusions are images that deceive our senses. Physical illusions
such as mirages or mirror effects are readily explained by optics.
Visual (or optical) illusions
occur when our perceptions don't agree with reality. Illusions are often
employed in art.
Illusions include ambiguous or multistable figures, figure-ground reversal, and
distortions. There are also impossible figures that are locally correct or
reasonable, but globally they are inconsistent and thus impossible. See for example
the impossible triangle on p. 317 of your text. Or recall the slide of the
impossible box that you saw in class.
Negative after images result from adaptation of the stimulated
region of the retina. Remember our two level dynamic model for
adaptation.
The eyes are in constant motion. If an image is fixed on the eye (the eye
is held motionless or the image moves with the eye) it fades away.
A brief flash of light is seen as delayed (latency) and last longer
(persistence) than the actual flash itself. That it, a positive after
image may remain. It is this
persistence of vision allows us to see movies and TV as a smooth
continuous motion rather than a succession of still frames.
Remember the colorful "magic wand" that flashed. When the wand was swung rapidly in a circle you could see the
individual colors in space due to the persistence of you vision. What colors did you see?
If you can, look again at the demonstration of the wand on the dvd. This time pause the image and then step
through frame by frame. You will see that in each frame the camera only sees one color at any time, but when
the movie is playing normally, you will see all three colors at once. This is the effect of persistence.
The critical flicker fusion frequency is about 50 Hz. A light blinking at a faster rate is seen as continuous.
A light blinking more slowly than the cff is seen as blinking.
Why don't movies shown at the standard rate of 24 frames per second have
noticeable flicker? What about television with its frame rate of 30 per
second? What causes the effect in movies in which the wheels of a car or
wagon seem to be turning backward?
Depth Perception.
One of the grandest illusions in all of art is the representation of depth. In the
hand of some artist, the sensation of solidity and/or depth can be amazingly
realistic. How can they do that?
The depth that we see is due to many different cues. However the dominant cue is stereopsis,
the fusing of two disparate images in our brains. Because the eyes are separated by
approximately 65 mm horizontally,
the retinal image formed in one eye is slightly different than that in the other eye.
However, the brain combines the two images into one so that you perceive only a
single three-dimensional image, the cyclopean image.
Nevertheless, we are used to perceiving depth when looking at flat, two-dimensional
pictures and photographs. How can that be? We interpret depth from the cues that
are given in the image. We call these cue psychological cues. They are:
Psychological Depth Cues
a. Linear perspective
b. Aerial perspective
c. Overlap
d. Shade and shadow
e. Texture gradient
f. Retinal image size
We discussed each of these cues in the lecture. Be sure that you know what each of them is.
We also demonstrated three-dimensional images using anaglyphs in which the two channels are printed on top of
each other in constasting colors (red/blue or red/green). For example, the right-eye image is printed in red
and the left-eye image is printed in blue. If the left eye looks through a red filter is sees only the image
that was printed in blue. Similarly, the right eye looking through a blue filter sees only the image that was
printed in red. That way, each eye sees only the image intended for it and the brain fuses the two images into
one image with depth.
You also saw examples of random-dot stereograms. These random-dot patterns were invented by Bela Julesz in the
late 1960s to prove that stereopsis was a sufficient cue to depth to create a three-dimensional image in the
absence of all other cues. You also had a chance to reverse the red/blue glasses so that the left eye saw the
right-eye image and vice versa. In that case the depth was inverted giving what is called a pseudoscopic
image.
We described three-dimensional imaging methods in several ways.
Binocular systems (two channels, one for each eye)
- 1. Stereoscopes
- a. Viewmaster
- b. Stereo slide viewers
- c. Stereo viewers for parallel prints (a, b, c are all the same thing)
- d. Red/green, red/blue viewers (anaglyphs)
- 2. Movies
- a. Separation by color (anaglyph)
- b. Separation with polarization
- 3. Television
- a. Analyglyph
- b. Shuttered glasses (not discussed)
- c. Passive glasses and shuttered screen (to be discussed in Lesson 7)
A question you should be able to answer. Why is horizontal parallax preferred to vertical parallax
in binocular systems?
Links to other sites that have anaglyph or other 3-D information.
Good anaglyph photos.
A NASA site that should link to 3-D images from
space. Check other NASA links as well.
Another link to 3-D images.
Two links for illusions: link 1 and
link 2.
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Last Modified: 09/12/04
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