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Lesson#7
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HUMAN INPUT-OUTPUT CHANNELS-1
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As the aim of this lecture is to introduce you the study of
Human Computer
Interaction, so that after studying this you will be able to:
. Understand role of
input-output channels
. Describe human eye
physiology and
. Discuss the visual
perception
7.1 Input Output channels
A person’s interaction with the outside world occurs through
information being
received and sent: input and output. In an interaction with a
computer the user
receives information that is output by the computer, and
responds by providing input
to the computer – the user’s output become the computer’s input
and vice versa.
Consequently the use of the terms input and output may lead to
confusion so we shall
blur the distinction somewhat and concentrate on the channels
involved. This blurring
is appropriate since, although a particular channel may have a
primary role as input or
output in the interaction, it is more than likely that it is
also used in the other role. For
example, sight may be used primarily in receiving information
from the computer, but
it can also be used to provide information to the computer, for
example by fixating on
a particular screen point when using an eye gaze system.
Input in human is mainly though the senses and out put through
the motor control of
the effectors. There are five major senses:
. Sight
. Hearing
. Touch
. Taste
. Smell
Of these first three are the most important to HCI. Taste and
smell do not currently
play a significant role in HCI, and it is not clear whether they
could be exploited at all
in general computer systems, although they could have a role to
play in more
specialized systems or in augmented reality systems. However,
vision hearing and
touch are central.
Similarly there are a number of effectors:
. Limbs
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. Fingers
. Eyes
. Head
. Vocal system.
In the interaction with computer, the fingers play the primary
role, through typing or
mouse control, with some use of voice, and eye, head and body
position.
Imagine using a personal computer with a mouse and a keyboard.
The application you
are using has a graphical interface, with menus, icons and
windows. In your
interaction with this system you receive information primarily
by sight, from what
appears on the screen. However, you may also receive information
by ear: for
example, the computer may ‘beep’ at you if you make a mistake or
to draw attention
to something, or there may be a voice commentary in a multimedia
presentation.
Touch plays a part too in that you will feel the keys moving
(also hearing the ‘click’)
or the orientation of the mouse, which provides vital feedback
about what you have
done. You yourself send information to the computer using your
hands either by
hitting keys or moving the mouse. Sight and hearing do not play
a direct role in
sending information in this example, although they may be used
to receive
information from a third source (e.g., a book or the words of
another person) which is
then transmitted to the computer.
7.2 Vision
Human vision is a highly complex activity with range of physical
and perceptual
limitations, yet it is the primary source of information for the
average person. We can
roughly divide visual perception into two stages:
. the physical
reception of the stimulus from outside world, and
. the processing and
interpretation of that stimulus.
On the one hand the physical properties of the eye and the
visual system mean that
there are certain things that cannot be seen by the human; on
the other interpretative
capabilities of visual processing allow images to be constructed
from incomplete
information. We need to understand both stages as both influence
what can and can
not be perceived visually by a human being, which is turn
directly affect the way that
we design computer system. We will begin by looking at the eye
as a physical
receptor, and then go onto consider the processing involved in
basic vision.
The human eye
Vision begins with light. The eye is a mechanism for receiving
light and transforming
it into electrical energy. Light is reflected from objects in
the world and their image is
focused upside down on the back of the eye. The receptors in the
eye transform it into
electrical signals, which are passed to brain.
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The eye has a number of important components as you can see in
the figure. Let us
take a deeper look. The cornea and lens at the front of eye
focus the light into a sharp
image on the back of the eye, the retina. The retina is light
sensitive and contains two
types of photoreceptor: rods and cones.
Rods
Rods are highly sensitive to light and therefore allow us to see
under a low level of
illumination. However, they are unable to resolve fine detail
and are subject to light
saturation. This is the reason for the temporary blindness we
get when moving from a
darkened room into sunlight: the rods have been active and are
saturated by the
sudden light. The cones do not operate either as they are
suppressed by the rods. We
are therefore temporarily unable to see at all. There are
approximately 120 million
rods per eye, which are mainly situated towards the edges of the
retina. Rods therefore
dominate peripheral vision.
Cones
Cones are the second type of receptor in the eye. They are less
sensitive to light than
the rods and can therefore tolerate more light. There are three
types of cone, each
sensitive to a different wavelength of light. This allows color
vision. The eye has
approximately 6 million cones, mainly concentrated on the fovea.
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Fovea
Fovea is a small area of the retina on which images are fixated.
Blind spot
Blind spot is also situated at retina. Although the retina is
mainly covered with
photoreceptors there is one blind spot where the optic nerve
enter the eye. The blind
spot has no rods or cones, yet our visual system compensates for
this so that in normal
circumstances we are unaware of it.
Nerve cells
The retina also has specialized nerve cells called ganglion
cells. There are two types:
X-cells
These are concentrated in the fovea and are responsible for the
early detection of
pattern.
Y-cells
These are more widely distributed in the retina and are
responsible for the early
detection of movement. The distribution of these cells means
that, while we may not
be able to detect changes in pattern in peripheral vision, we
can perceive movement.
7.3 Visual perception
Understanding the basic construction of the eye goes some way to
explaining the
physical mechanism of vision but visual perception is more than
this. The information
received by the visual apparatus must be filtered and passed to
processing elements
which allow us to recognize coherent scenes, disambiguate
relative distances and
differentiate color. Let us see how we perceive size and depth,
brightness and color,
each of which is crucial to the design of effective visual
interfaces.
Perceiving size and depth
Imagine you are standing on a hilltop. Beside you on the summit
you can see rocks,
sheep and a small tree. On the hillside is a farmhouse with
outbuilding and farm
vehicles. Someone is on the track, walking toward the summit.
Below in the valley is
a small market town.
Even in describing such a scene the notions of size and distance
predominate. Our
visual system is easily able to interpret the images, which it
receives to take account
of these things. We can identify similar objects regardless of
the fact that they appear
to us to be vastly different sizes. In fact, we can use this
information to judge distance.
So how does the eye perceive size, depth and relative distances?
To understand this
we must consider how the image appears on the retina. As we
mentioned, reflected
light from the object forms an upside-down image on the retina.
The size of that
image is specified as visual angle. Figure illustrates how the
visual angle is calculated.
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If were to draw a line from the top of the object to a central
point on the front of the
eye and a second line from the bottom of the object to the same
point, the visual angle
of the object is the angle between these two lines. Visual angle
is affected by both the
size of the object and its distance from the eye. Therefore if
two objects are at the
same distance, the larger one will have the larger visual angle.
Similarly, if two
objects of the same size are placed at different distances from
the eye, the furthest one
will have the smaller visual angle, as shown in figure.
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Visual angle indicates how much of the field of view is taken by
the object. The
visual angle measurement is given in either degrees or minutes
of arc, where 1 degree
is equivalent to 60 minutes of arc, and 1 minute of arc to 60
seconds of arc.
Visual acuity
So how does an object’s visual angle affect our perception of
its size? First, if the
visual angle of an object is too small we will be unable to
perceive it at all. Visual
acuity is the ability of a person to perceive fine detail. A
number of measurements
have been established to test visual acuity, most of which are
included in standard eye
tests. For example, a person with normal vision can detect a
single line if it has a
visual angle of 0.5 seconds of arc. Spaces between lines can be
detected at 30 seconds
to 1 minute of visual arc. These represent the limits of human
visual perception.
Law of size constancy
Assuming that we can perceive the object, does its visual angle
affect our perception
of its size? Given that the visual angle of an object is
reduced, as it gets further away,
we might expect that we would perceive the object as smaller. In
fact, our perception
of an object’s size remains constant even if its visual angel
changes. So a person’s
height I perceived as constant even if they move further from
you. This is the law of
size constancy, and it indicated that our perception of size
relies on factors other than
the visual angle.
One of these factors is our perception of depth. If we return to
the hilltop scene there
are a number of cues, which can use to determine the relative
positions and distances
of the objects, which we see. If objects overlap, the object
that is partially covered is
perceived to be in the background, and therefore further away.
Similarly, the size and
height of the object in our field of view provides a cue to its
distance. A third cue is
familiarity: if we expect an object to be of a certain size then
we can judge its distance
accordingly.
Perceiving brightness
A second step of visual perception is the perception of
brightness. Brightness is in fact
a subjective reaction to level of light. It is affected by
luminance, which is the amount
of light emitted by an object. The luminance of an object is
dependent on the amount
of light falling on the object’s surface and its reflective
prosperities. Contrast is
related to luminance: it is a function of the luminance of an
object and the luminance
of its background.
Although brightness is a subjective response, it can be
described in terms of the
amount of luminance that gives a just noticeable difference in
brightness. However,
the visual system itself also compensates for changes in
brightness. In dim lighting,
the rods predominate vision. Since there are fewer rods on the
fovea, object in low
lighting can be seen easily when fixated upon, and are more
visible in peripheral
vision. In normal lighting, the cones take over.
Visual acuity increases with increased luminance. This may be an
argument for using
high display luminance. However, as luminance increases, flicker
also increases. The
eye will perceive a light switched on and off rapidly as
constantly on. But if the speed
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of switching is less than 50 Hz then the light is perceived to
flicker. In high luminance
flicker can be perceived at over 50 Hz. Flicker is also more
noticeable in peripheral
vision. This means that the larger the display, the more it will
appear to flicker.
Perceiving color
A third factor that we need to consider is perception of color.
Color is usually
regarded as being made up of three components:
. hue
. intensity
. saturation
Hue
Hue is determined by the spectral wavelength of the light. Blues
have short
wavelength, greens medium and reds long. Approximately 150
different hues can be
discriminated by the average person.
Intensity
Intensity is the brightness of the color.
Saturation
Saturation is the amount of whiteness in the colors.
By varying these two, we can perceive in the region of 7 million
different colors.
However, the number of colors that can be identified by an
individual without training
is far fewer.
The eye perceives color because the cones are sensitive to light
of different
wavelengths. There are three different types of cone, each
sensitive to a different
color (blue, green and red). Color vision is best in the fovea,
and worst at the
periphery where rods predominate. It should also be noted that
only 3-4 % of the
fovea is occupied by cones which are sensitive to blue light,
making blue acuity
lower.
Finally, we should remember that around 8% of males and 1% of
females suffer from
color blindness, most commonly being unable to discriminate
between red and green.
The capabilities and limitations of visual processing
In considering the way in which we perceive images we have
already encountered
some of the capabilities and limitations of the human visual
processing system.
However, we have concentrated largely on low-level perception.
Visual processing
involves the transformation and interpretation of a complete
image, from the light that
is thrown onto the retina. As we have already noted, our
expectations affect the way
an image is perceived. For example, if we know that an object is
a particular size, we
will perceive it as that size no matter how far it is from us.
Visual processing compensates for the movement of the image on
the retina which
occurs as we around and as the object which we see moves.
Although the retinal
image is moving, the image that we perceive is stable.
Similarly, color and brightness
of objects are perceived as constant, in spite of changes in
luminance.
This ability to interpret and exploit our expectations can be
used to resolve ambiguity.
For example consider the image shown in figure ‘a’. What do you
perceive? Now
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consider figure ‘b’ and ‘c’. the context in which the object
appears allow our
expectations to clearly disambiguate the interpretation of the
object, as either a B or
13.
However, it can also create optical illusions. Consider figure
‘d’. Which line is
longer?
A similar illusion is the Ponzo illusion as shown in figure
Figure c
Figure a
the Muller Lyer illusion
concave
convex
Figure d
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Another illusion created by our expectations compensating an
image is the
proofreading illusion. Example is shown below
The way that objects are composed together will affect the way
we perceive them, and
we do not perceive geometric shapes exactly as they are drawn.
For example, we tend
to magnify horizontal lines and reduce vertical. So a square
needs to be slightly
increased in height to appear square and line will appear
thicker if horizontal rather
than vertical.
Optical illusions also affect page symmetry. We tend to see the
center of a page as
being a little above the actual center – so if a page is
arranged symmetrically around
the actual center, we will see it as too low down. In graphic
design this is known as
the optical center.
These are just a few examples of how the visual system
compensates, and sometime
overcompensates, to allow us to perceive the world around us. |
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