2. Visual Perception
Although the digital image processing field is built on a foundation of
mathematical and probabilistic formulations,
DIP depends human intuition and analysis play a central role in the choice of
one technique versus another, and this choice often is made based on
subjective, visual judgments.
It is important to study human visual perception.
5. Human visual system
Vision is the most advanced human sense. So,
images play the most important role in human
perception.
Human visual perception is very important because
the selection of image processing techniques is
based only on visual judgements.
Our visual system allows us to organize and
understand the many complex elements in our
environment.
The visual system consists of an eye that
transforms light into neural signals, and related
parts of the brain that process the neural signals
and extract necessary information.
6. Elements of visual perception
• The complex physical process of
visualizing something involves the nearly
simultaneous interaction of the eyes and the
brain through a network of neurons,
receptors, and other specialized cells.
7. Structure of the Human Eye
• The eye is nearly spherical in form with
an average diameter of approximately 20 mm.
• The eye, called the optic globe is enclosed
by three membranes namely, the cornea
and sclera outer cover, the choroid, and the
retina.
• The human eye is equipped with a variety
of optical elements including the cornea, iris,
pupil, a variable-focus lens, and the retina.
• Together, these elements work to form images
of the objects in a person's field of view.
9. The Cornea and Sclera outer cover
• The cornea is a tough, transparent strong bulge located
at the front surface of the eye.
• Adult cornea has a radius of approximately 8mm.
• The cornea (convex outer layer )contributes to the image
forming process by refracting light entering the eye.
• The sclera , tough white sheath around the outside of the
eye-ball, is continuous with the cornea.
• It is the opaque membrane that encloses the
remainder of the eye.
• Sclera- White of the Eye
10. Choroid
The choroid layer is located behind the retina directly below the
sclera and absorbs unused radiation.
•This membrane contains a network of blood vessels that serve as
the major source of nutrition to the eye.
•The choroid coat is heavily pigmented and hence helps to reduce
the amount of extraneous light entering the eye and the
backscatter within the optical globe.
At its anterior extreme , the choroid is divided into the ciliary body
and the iris diaphragm.
Ciliary muscle:
The ciliary muscle is a ring shaped muscle attached to the Iris.
It is important because contraction and relaxation of the ciliary
muscle controls the shape of the lens.
11. The choroid and Iris Diaphragm
Iris Diaphragm:
It contracts or expands to control the amount of light that enters
the eye.
The central opening of the iris (the pupil) varies in diameter from
approximately 2 to 8 mm.
The front of the iris contains the visible pigment of the eye,
whereas the back contains a black pigment.
12. Lens
•The lens is made up of concentric layers of fibrous cells
• It is suspended by fibers that attach to the ciliary body.
• It contains 60 to 70%water, about 6%fat, and more protein than
any other tissue in the eye.
Cataracts:
•The lens is colored by a slightly yellow pigmentation that
increases with age.
• In extreme cases, excessive clouding of the lens, caused by the
affliction commonly referred to as cataracts, can lead to poor
color discrimination and loss of clear vision.
•The lens absorbs approximately 8% of the visible light spectrum,
with relatively higher absorption at shorter wavelengths.
•Both infrared and ultraviolet light are absorbed appreciably by
proteins within the lens structure and, in excessive amounts, can
damage the eye.
13. Retina
The retina may be described as the “screen” on which an image
is formed by light that has passed into the eye via the cornea,
aqueous humour, pupil , lens , then the hyaloid canal and finally
the vitreous humour before reaching the retina.
The retina contains photosensitive elements (called rods and
cones) that convert the light they detect into nerve impulses that
are sent onto the brain along the optic nerve.
Optic Nerve:
The optic nerve is the second cranial nerve and is responsible for
vision.
Each nerve contains approx. one million fibres transmitting
information from the rod and cone cells of the retina.
.
14. The Retina
When the eye is properly focused, light from an object outside
the eye is imaged on the retina.
Pattern vision is afforded by the distribution of discrete light
receptors over the surface of the retina.
Light receptors : Rods & Cones
There are two classes of receptors: cones and rods. Rods and
cones operate differently. Rods are more sensitive to light than
cones. They photo receptors are located primarily in the central
portion of the retina, called the fovea, and are highly sensitive
to color.
Visual Axis:
A straight line that passes through both the centre of the pupil
and the Centre of the fovea.
15. photoreceptors
Retina has two kinds of photoreceptors: cones and rods.
Cones
•The cones are highly sensitive to color.
•Their number is 6-7 million and they are mainly located at the
central part of the retina.
•Each cone is connected to one nerve end.
•Cone vision is the photopic or bright light vision.
Rods
• Rods serve to view the general picture of the vision field.
•They are sensitive to low levels of illumination and cannot
discriminate colors. This is the scotopic or dim-light vision.
•Their number is 75 to 150 million and they are distributed over the
retinal surface.
•Several rods are connected to a single nerve end. This fact and
their large spatial distribution explain their low resolution.
16. The larger area of distribution and the fact that several rods are
connected to a single nerve end reduce the amount of detail
discernible by these receptors.
Rods serve to give a general, overall picture of the field of view.
Rods are not involved in color vision and are sensitive to low levels
of illumination.
Humans can resolve fine details with these cones largely because
each one is connected to its own nerve end.
Muscles controlling the eye rotate the eyeball until the image of an
object of interest falls on the fovea.
Cones respond to higher levels of illumination
The number of rods is much larger: Some 75 to 150 million are
distributed over the retinal surface.
Rods and cones
20. Distribution of Rods & Cones
Figure shows the density of rods and cones for a cross section of
the right eye passing through the region of emergence of the
optic nerve from the eye.
21. Types of Vision
Scotopic or dim-light vision
For example, objects that appear brightly colored in daylight
when seen by moonlight appear as colorless forms because only
the rods are stimulated. This phenomenon is known as scotopic
or dim-light vision.
photopic or bright-light vision
Cone vision is called photopic or bright-light vision. Cones
respond to higher levels of illumination; their response is called
photopic vision.
Blind spot / papilla :
The area in which there is no response or presence of light
receptors
The absent of reciprocators is called blind spot.
22. Role of lens in Image formation in the eye
•The major difference between the lens of the eye and an
ordinary optical lens in that the former is flexible.
•The radius of curvature of the anterior surface of the lens >
radius of curvature of its posterior surface.
•The shape of the lens of the eye is controlled by tension in the
fiber of the ciliary body.
To focus on distant objects, the controlling muscles cause
the lens to be relatively flattened and lens will have lowest
refractive power.
•Similarly,these muscles allow the lens to become thicker in
order to focus on objects near the eye and most strongly
refractive.
•The distance between the optical center of the lens and the
retina is called the focal length
•Focal length varies from 17mm to 14mm as the refractive power
of the lens increases from its minimum to its maximum.
25. Image formation in the eye
When the eye focuses on an object farther away than about
3m , the lens exhibits its lowest refractive power.
When the eye focuses on a nearly object. The lens is most
strongly refractive.
The retinal image is reflected primarily in the area of the fovea.
Perception then takes place by the relative excitation of light
receptors, which transform radiant energy into electrical impulses
that are ultimately decoded by the brain.
26. For example, the observer is looking at a tree 15 m high at a
distance of 100 m. If h is the height in mm of that object in the
retinal image, the geometry of Fig.4.2 yields 15/100=h/17 or
h=2.55mm.
The retinal image is reflected primarily in the area of the fovea.
Perception then takes place by the relative excitation of light
receptors, which transform radiant energy into electrical impulses
that are ultimately decoded by the brain.
Calculation of retinal image size
27. The retinal image of height “h” is reflected primarily in the area of
the fovea.
Perception then takes place by the relative excitation of light
receptors, which transform radiant energy into electrical impulses
that are ultimately decoded by the brain.
Object Perception
28.
29.
30. Brightness Adaptation
Human visual system cannot operate over a wide range of light
levels simultaneously.
Rather, it accomplishes this large variation by changes in its
overall sensitivity. This phenomenon is known as brightness
adaptation.
For a given set of conditions the current sensitivity level of the
visual system is called the brightness adaptation level.
Brightness Discrimination
The eye’s ability to discriminate i.e differentiate various intensity
levels since digital Images are displayed as a discrete set of
intensity.
The total range of distinct intensity levels it can discriminate
simultaneously is rather small when compared with the total
adaptation range.
Brightness Adaptation and
Discrimination
31. Brightness Adaptation and
Discrimination
For any given set of conditions, the current
sensitivity level of the visual system is called the
brightness adaptation level, which may
correspond, for example, to brightness Ba .
The short intersecting curve represents the
range of subjective brightness that the eye can
perceive when adapted to this level.
This range is rather restricted, having a level Bb
at and below which all stimuli are perceived as
indistinguishable blacks.
The upper (dashed) portion of the curve is not
actually restricted but, if extended too far, loses
its meaning because much higher intensities
would simply raise the adaptation level higher
than Ba.
32. Weber’s Law
Weber’s law states that the ratio of the
incremental threshold to the background
intensity is a constant.
It predicts a linear relation between
incremental threshold and the background
intensity
Weber Ratio:
Weber ratio = ΔIc /I
– I: the background illumination
– ΔIc : the increment of illumination which is
distinguishable 50% of the time with
background illumination.
– Small Weber ratio indicates good
discrimination
– Larger Weber ratio indicates poor
discrimination
33. Psychovisual effects
The perceived brightness is not a simple function of intensity
•Mach bands- Human Visual system tends to undershoot or
overshoot around the boundaries of regions with different
intensities.
•Mach bands or the Mach effect refers to an optical phenomenon
from edge enhancement due to lateral inhibition of the retina .
This is an inbuilt edge enhancement mechanism of the retina,
where the edges of darker objects next to lighter objects will
appear darker and vice versa, creating a false shadow.
35. Mach Bands
• The Mach bands effect is due to the
spatial high-boost filtering performed by
the human visual system on the
luminance channel of the image
captured by the retina.
• Along the boundary between
adjacent shades of grey in the
Mach bands illusion, lateral inhibition
makes the darker area falsely appear
even darker and the lighter area
falsely appear even lighter.
36. Mach Bands
• In the case of visual information, such
(spatial) differentiation causes gradual changes
in the contrast between an object and its
background to be enhanced, i.e., to become
more visible.
• In human perception, this contrast
enhancement produces what is known as Mach
Bands: between two regions of different intensity a
thin bright band appears at the lighter side and a
thin dark band appears on the darker size.
• These bands are not physically present are
"overshoot" and "undershoot" caused by our
neural circuits in processing a step discontinuity
in illumination.
37.
38.
39. Psychovisual effects
Simultaneous Contrast
Two colors, side by side, interact with one another and
change our perception accordingly. The effect of this interaction
is called simultaneous contrast.
Simultaneous Contrast phenomenon a region is
perceived brightness does not depend simply on its
intensity.
Since we rarely see colors in isolation, simultaneous
contrast affects our sense of the color that we see.
40. Psychovisual effects
For example,
Consider an intense beam of blue light, surrounded by white
light, striking our retinas.
Where the blue light strikes, the blue cones will be stimulated,
overloaded and fatigued. The horizontal cells that link the blue
cones will cause blue cones, outside of but close to the blue
beam, to also become fatigued.
In the surround of the blue beam where the white light falls, the
blue receptors will be fatigued and the white light will appear to
our brain as yellow. (Recall that blue light plus yellow light equals
white light.)
Simultaneous contrast causes the white around the blue to seem
yellow. Similarly, white light around a yellow beam will seem blue
41. Psychovisual effects
Simultaneous contrast affects every pair of adjacent colors.
For our example of the red and blue flowerbeds, the red bed
makes the blue bed seem green because it induces its
complementary color, green, in the blue bed.
The blue bed makes the red bed seem orange because it
induces its complementary color, yellow, in the red bed.
The real colors are not altered; only our perception of them
changes.
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42. Optical illusion
• An optical illusion (also called a visual
illusion is an illusion caused by the
visual system and characterized by a
visual percept that appears to differ from
reality.
• The eye fills in non-existing
information or wrongly perceives
geometrical properties of objects
43. Optical Illusions
Optical Illusions can use color, light and patterns to create
images that can be deceptive or misleading to our brains.
The information gathered by the eye is processed by the brain,
creating a perception that in reality, does not match the true
image.
Perception refers to the interpretation of what we take in through
our eyes. Optical illusions occur because our brain is trying to
interpret what we see and make sense of the world around us.
Optical illusions simply trick our brains into seeing things which
may or may not be real.
44. Optical Illusions
Are the pinwheels moving?
Answer: No, the wheels are
not turning. The Moiré effect
can produce interesting and
beautiful geometric patterns.
