Elements of visual perception

Elements of visual perception
Dr T M Inbamalar

Structure of the Human Eye
• The eye is nearly spherical in form with an
average diameter of approximately 20 mm.
• It is enclosed by three membranes;
the cornea and sclera outer cover, the choroid,
and the retina.
• The cornea is a tough, transparent tissue that
covers the anterior or front surface of the eye.
The sclera is continuous with the cornea.
• It is the opaque membrane that encloses the
remainder of the eye.

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.
• 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.

Human vision
• When an object is observed, it is first focused through the cornea
and lens onto the retina.
• Retina is a multilayered membrane that contains millions of light-
sensitive cells that detect the image and translate it into a series of
electrical signals.
• These image capturing receptors of the retina are
termed rods and cones
• Cones and Rods are connected with the fibers of the optic nerve
bundle through a series of specialized cells that coordinate the
transmission of the electrical signals to the brain.
• In the brain, the optic nerves from both eyes join at the optic
chiasma where information from their retinas is correlated.
• The visual information is then processed through several steps,
eventually arriving at the visual cortex, which is located on the
lower rear section of each half of the cerebrum.

Human vision
• Our eyes take in light waves for our visual cortex to process into a
perception.
• The light passes through the cornea and is focused onto the retina
with the lens.
• The retina then controls the amount of light that enters the eye.
• Rods and cones are used to process color in the light and dark.
• Rods are responsible for processing images in a dark atmosphere.
• They are more sensitive and account for peripheral vision.
• The cones are prominent in daylight.
• The optic nerve primarily routes information to the cerebral cortex,
where visual perception occurs.

Human vision
• A particularly specialized component of the eye is the fovea
centralis, which is located on the optical axis of the eye in an area
near the center of the retina.
• This area exclusively contains high-density tightly packed cone cells
and is the area of sharpest vision.
• The density level of cone cells decreases outside of the fovea
centralis and the ratio of rod cells to cone cells gradually increases.
• At the periphery of the retina, the total number of both types of
light receptors decreases substantially, causing a dramatic loss of
visual sensitivity at the retinal borders.
• This is offset, however, by the fact that humans constantly scan
objects in their field of view, usually resulting in a perceived image
that is uniformly sharp

Cones and Rods
• In the human eye, there are two distinct visual
systems (based on two types of retinal cells)
• Scotopic vision: not color sensitive, more
sensitive to light and mainly peripheral (rods)
• Photopic vision : color sensitive, less sensitive
to light and mainly central (cones)

Image formation in the Eye
1. Representation of an object
2. Light leaves the object - propagating in all
directions
3. Some of the light leaving the object reaches
the eye
4. Light changes direction when it passes from
the air into the eye
5. Location of Focused Image

Brightness Adaptation and
Discrimination
• The ability of the eye to discriminate between
different brightness levels is an important
consideration in creating and presenting
images.
• The range of light intensity levels to which the
human visual system can adapt is on the
order of 1010 from the scotopic threshold
(minimum low light condition) to the glare
limit (maximum light level condition).

Subjective brightness
• Subjective brightness is brightness as
perceived by the human visual system
• Subjective brightness is a logarithmic function
of the light intensity incident on the eye.
• This characteristic is illustrated in the figure
below, which is a plot of light intensity versus
subjective brightness.

Brightness adaptation
• The long solid curve represents the range of
intensities to which the visual system can adapt.
• In photopic vision alone, the range is about 106.
• The transition from scotopic to photopic vision is
gradual over the approximate range from 0.001
to 0.1 millilambert (-3 to -1 mL in the log scale),
as illustrated by the double branches of the
adaptation curve in this range.

• For a 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 in the
figure.
• 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 to a higher value than Ba.

• The visual system cannot operate over such a
range simultaneously. It accomplishes this large
variation by changes in its overall sensitivity. This
phenomenon is known as brightness adaptation.
• The total range of intensity levels it can
discriminate simultaneously is rather small
compared with the total range.
• For any given set of conditions, the current
sensitivity level of the visual system is called
the brightness adaptation level.

• Two phenomena clearly demonstrate that
perceived brightness is not a simple function
of intensity:
• Mach Bands
• simultaneous contrast

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.

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.

Simultaneous Contrast
• The simultaneous contrast phenomenon: each
small square is actually the same intensity but,
because of the different intensities of the
surrounds, the small squares do not appear
equally bright.
• The hue of a patch is also dependent on the
wavelength composition of surrounding light.
• A white patch on a black background will appear
to be yellowish if the surround is a blue light.

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

Elements of visual perception

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