Lecture 1 Visual Perception
Lecture 1
Visual Perception
Reading
Glassner, Principles of Digital Image Synthesis, sections 1.1-1.6.
Optional
Spencer, Shirley, Zimmerman, and Greenberg. Physically-based glare
effects for digital images. SIGGRAPH 95.
Ferwerda, Pattanik, Shirley, and Greenberg. A model of visual
adaptation for realistic image synthesis. SIGGRAPH 96.
Forming an image
First, we need some sort of sensor to receive and record light.
w
w
w
Is this all we need?
object
Do we get a useful image?
film
Restricting the light
Pinhole Camera
object
barrier
Advantages:
w easy to simulate
w everything is in focus
Disadvantages:
w needs a bright scene (or long exposure)
w everything is in focus
film
Collecting the light
Instead of throwing away all but a single ray, let’s try to collect a bunch
of rays and concentrate them at a single point on the sensor.
To do this, we need to be able to change the path of a light ray.
Fortunately, we have refraction. Light passing from one medium into a
denser one will bend towards the normal of the interface.
air
glass
light ray
air
glass
Stacking prisms
We can use variously shaped prisms to take light rays of
various angles and bend them to pass through a single point.
As we use more and more prisms, the shape approaches a
curve, and we get a lens.
Forming an image with a lens
We can now replace the pinhole barrier with a lens, and we still get an
image.
object
lens
film
Now there is a specific distance at which objects are “in focus”.
By changing the shape of the lens, we change how it bends the light.
Optics
w Focal point - the point where parallel rays converge when passing
through a lens.
w Focal length - the distance from the lens to the focal point.
w Diopter - the reciprocal of the focal length, measured in meters.
• Example: A lens with a “power” of 10D has a focal length of
________.
focal point
focal length
Structure of the eye
w Cornea - a clear coating over the front of the eye:
• Protects eye against physical damage.
• Provides initial focusing (40D).
w Iris - Colored annulus with radial muscles.
w Pupil - The hole whose size is controlled by the iris.
Structure of the eye, cont.
w Crystalline lens - controls the focal distance:
• Power ranges from 10 to 30D in a child.
• Power and range reduces with age.
w Ciliary body - The muscles that compress the sides of the lens,
controlling its power.
Q: As an object moves closer, do the ciliary muscles contract or relax to keep the
object in focus?
Eye geometry
w Emmetropic eye - resting eye has focal point on retina.
w Myopic eye - eye too long (near-sightedness)
w Hyperopic eye - eye too short (far-sightedness)
Retina
w Retina - a layer of photosensitive cells covering 200° on the back
of the eye.
• Cones - responsible for color perception.
• Rods - Limited to intensity (but 10x more sensitive).
w Fovea - Small region (1 or 2°) at the center of the visual axis
containing the highest density of cones (and no rods).
The human retina
nasal
temporal
Photomicrographs at increasing distances from the fovea. The large
cells are cones; the small ones are rods.
Photoreceptive cells
Cone on a stick.
Neuronal connections
Even though the retina is very densely covered with photoreceptors, we have
much more acuity in the fovea than in the periphery.
light
rods
+
cones
+
to brain
+
-
-
+
to brain
In the periphery, the outputs of the photoreceptors are averaged together
before being sent to the brain, decreasing the spatial resolution. As many as
1000 rods may converge to a single neuron.
Demonstrations of visual acuity
With one eye shut, at the right distance, all of these letters should appear
equally legible.
Blind spot demonstration.
Wavelength sensitivity
Electromagnetic radiation comes in all wavelengths, from 10-14 to 108
meters.
The eye is sensitive to EMR with wavelengths from 380 to 780
nanometers (10-9 meters), called “visible light”.
Q: What color is the light at 380nm? at 780nm?
Photopigments
Photopigments are the chemicals in the rods and cones that react to light.
Can respond to a single photon!
w Rods contain rhodopsin, which has peak sensitivity at 500nm.
w Cones come in three varieties: S, M, and L.
Principle of univariance: No information is transmitted describing the
wavelength of the photon. Q: why not?
Transmitting color
Color information is transmitted to the brain in three nerve bundles or
channels:
w Achromatic channel A = M + L
w Red-green chromatic channel R/G = M - L
w Blue-yellow chromatic channel B/Y = S - A
Saturation is perceived as the ratio of chromatic to achromatic
response.
Flicker
The photoreceptive cells provide a time-averaged response:
more photons ➠ more response
Above a critical flicker frequency (CFF), flashes of light will fuse into a single
image.
CFF for humans is about 60 Hz. (For a bee it’s about 300 Hz.)
Q: Do all parts of the visual field have the same CFF?
solid: response at 550nm; gray: response at 659 nm
Adaptation
Adaptive processes can adjust the base activity (“bias”) and scale the
response (“gain”).
Through adaptation, the eye can handle a large range of illumination:
B a c k g ro u n d
M o o n le s s o v e r c a s t n ig h t
M o o n le s s c le a r n ig h t
T w ilig h t
O v e rc a s t d a y
D a y w ith s u n lit c lo u d s
L u m in a n c e (c d /m
0 .0 0 0 0 3
0 .0 3
3
3 0 0
3 0 ,0 0 0
2
)
Luminous efficiency
You can plot the luminous efficiency of:
w Rods (scotopic vision)
w Cones (photopic vision)
as a function of wavelength.
The Purkinje shift refers to the change in peak wavelength perception
between the two types of vision.
Perceptual light intensity
We perceive light intensity as we do sound: on a relative or logarithmic
scale.
Example: The perceived difference between 0.20 and 0.22 is the same
as between 0.80 and ______.
Ideally, to display n+1 equally-spaced intensity levels
I1 I2
In
= =L =
I0 I1
In−1
Noise
Noise can be thought of as randomness added to the signal.
The eye is relatively insensitive to noise.
Mach bands
Mach bands were first dicussed by Ernst Mach, and Austrian physicist.
Appear at C0 or C1 intensity discontinuities.
Also appear when there is a rapid intensity change.
Mach bands, cont.
Possible cause: lateral inhibition of nearby cells.
Q: Why is this summation pattern useful?
Lightness contrast
A related phenomenon is known as:
w lightness contrast
w simultaneous contrast
w color contrast (for colors)
This phenomenon helps us maintain a consistent mental image of the
world, under dramatic changes in illumination.
Summary
Here’s what you should take home from this lecture:
w
w
w
w
w
All the boldfaced terms.
How a camera forms an image.
The basic structures of the eye and how they work.
How light is a form of EMR.
How light intensity is perceived on a logarithmic scale and is a
function of wavelength.
w The eye’s relative sensitivity to intensity discontinuities, but
insensitivity to noise.
w The phenomena of adaptation and lightness contrast.
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