Exam Results

Mean: 76Median: 79

Exam Results: Grading Policies• Our grading policies as outlined on the website

are as follows:

• Approximately half of the class will get A's & B's; the other half C's & below. – This means if you were above a 79, that is an A or B on

the exam

• Students getting 50% or more of a perfect score on their total score of exams, concept quizzes, and HWs will get at least a C.

Chapter 7: Processing the Image

• The Human Visual System

• Lightness Perception– Brightness– Lightness constancy

• Lateral Inhibition– Edge enhancement– Lightness contrast

The Human Visual System

• The photoreceptors turn light into electrical signals

• The nerve cells in the retina do some low level image processing

• Then transmit the signal to the optic nerve and the brain

Structure of the Retina

← Nerve cells

← Photoreceptors

← Choroid

Light

Brightness vs. Lightness

• In this course, brightness and lightness do not mean the same thing!

• Brightness is the overall light level of a scene, and is related to the intensity of the light source

• It also depends on the degree of dark adaption• The opposite of “bright” is “dim”

Lightness

• Lightness is a property of a material surface and how much light it reflects

• It is independent of illumination and dark adaptation

• The opposite of “light” is “dark”

Lightness Constancy

• Because our perception of light is independent of illumination and dark adaptation, it must depend on the relative lightness of objects compared to their surroundings.

• A white object in the shade may have less light coming from it than a dark object in bright sunlight, but we perceive them as light and dark because of lightness constancy

Lightness Constancy: Comparison• The one below is seen in less illumination

than the one at right • The relative darkness and lightness of

different parts of the image is preserved when there is more illumination

• The lightness of the part of the dog in the white rectangle at left is actually almost the same as the part of the dog in the white rectangle below!

Concept Question: Lightness

• You are sitting outside reading a book and the sun goes behind a cloud. What happens to the brightness and lightness of the white background of the book pages?

A. They both stay the sameB. Brightness stays the same, lightness changesC. Lightness stays the same, brightness changesD. They both change

Lateral Inhibition• How does our visual system

determine relative light intensity?

• Recall that the neural network in the retina is connected laterally.

• This allows the signal at one point on the retina to affect the signal at other points on the retina

Lateral Inhibition

• Increased illumination in one region of the retina diminishes the signal to the brain from a neighboring region

• This makes the total signal sent to the brain relatively insensitive to overall illumination changes, and very sensitive to differences in illumination

Lateral Inhibition• A signal received at A will

send a positive (excitatory) signal to its own neural cell and a negative (inhibitory) signal to B’s neural cell.

• A signal at B will do the reverse.

A B

to the brain

Lateral Inhibition• If the overall light level

increases, the signals from each receptor increase, but so does their ability to inhibit one another.

• The net result is that the change in illumination does not change the total signal to the brain very much.

A B

to the brain

Edge Enhancement• Imagine that only A is

illuminated and there are no lateral neural connections.

• The neural cell at A sends a signal corresponding to the light level, and the neural cell at B sends nothing.

A B

to the brain

Edge Enhancement• Now we add the lateral neural

connections.

• Now the neural cell at B is inhibited by the signal from A, but B does not inhibit A because it is dark.

• This gives a smaller signal at B than if there were no lateral connections, so the difference between A and B is larger.

A B

to the brain

Lateral Inhibition

Each ganglion cell (the top rung of the neural cell system) is connected through other neural cells to a region of photoreceptors that is called its “receptive field”

The ganglion cell receives signals from all of these photoreceptors

Receptive Field: No Light

Photoreceptors:Pink is excitatoryGreen is inhibitory

Intermediate neural cells

Ganglion cell

The ganglion cell produces some background response even when there is no light on its receptive field

Receptive Field: Center Illumination

Photoreceptors:Pink is excitatoryGreen is inhibitory

Intermediate neural cells

Ganglion cell

When the center photoreceptors are illuminated, the ganglion cell response is large

Receptive Field: No Light

Photoreceptors:Pink is excitatoryGreen is inhibitory

Intermediate neural cells

Ganglion cell

When light hits the surround receptors, the response of the ganglion cell is inhibited.

Receptive Field: No Light

Photoreceptors:Pink is excitatoryGreen is inhibitory

Intermediate neural cells

Ganglion cell

If light hits the entire receptive field, the excitation from the center receptors and the inhibition from the surround receptors cancels out, and the ganglion cell response as it does to no light at all

Determining Response

+1

-1

The inner area (excitatory) can be thought of as plus 1The outer area (inhibitory) can be thought of as minus 1

If the illumination is equal on both areas, the effect is +1-1=0

If the center is bright and only half of the outside is bright, the you have +1-1/2=+1/2, so you have a larger response.

Summary of Lateral Inhibition

Concept Questions: Lateral Inhibition•  Shown below are 6 different center-

surround receptive fields in the image on your retina as you look at a dark grey shape on a white background. The receptive fields are labeled A, B, C, D, E, and F. Each receptive field is connected to a different (ganglion) nerve cell.

• Which receptive fields neither enhance nor inhibit their ganglion cell's signal?

a) F, b) B, d) D, e) F, B and D

• Which receptive field inhibits its ganglion cell to send the weakest signal (indicating the darkest region)?

a) A, b) B, c) C, d) D. e) E

• Which receptive field stimulates its ganglion cell to send the strongest signal (indicating the lightest region)?

a) A, b) B, c) C, d) D. e) E

Little light on grey region

Bright lighton white region

The Craik-O’Brien Illusion

Edge Illusions in Art

• The change in reflectivity between the lightest and darkest parts of a painting or photograph is only 10-15 times, while the difference in brightness in a real scene may be 1,000 to 1,000,000 times.

• Artists compensate for this using this same type of edge illusion

From Seurat’s “Bathers at Asnières”

Water gets lighter right before edge

Skin gets darker right before edge

Seurat’s “Poseur en profil”Darker near the edge