Hello everybody as you know that this course
Photogeology in Terrain Evaluation is taken
over by professor Javed N Malik from Department
of Earth Sciences, IIT Kanpur this is Absal
Khan I am your TA teaching assistant in this
course and I am going to help you ah in conducting
labs and your related practicals ok . So,
our today's topic is stereoscopic parallax
.
So, why we get parallax in our stereo pairs;
in aerial photographs ok . So, have you ever
wondered about this phenomena that uh animal
can see the depth or not ok the depth perception
is an important phenomena which the human
have this ability because they are having
their both eyes in the front of their skull
ok , but in case of animals they are having
their eyes; most of the animals they are having
their eyes one eye on the right side of the
skull and left eye on the left side of the
skull .
So, that is why they do not have this ability
of depth perception . We when we see some
area or some object, we can define in our
own terms in our brain that the ah what is
the height of that object or a building ok
and what is the relief over an area ok . So,
this ability is we get on to see on the stereo
pair of these aerial photographs .
So, when we see the left photograph with our
left eye and right photograph with our right
eye ok. So the observation of the same area
or the same point from different point of
observations . So, here suppose this is a
common point on these both photographs point
A is common here also and here on the both
photographs . So, but we are looking over
point a from the 2 different point of observations
. So, that is why we are able to see stereoscopic
parallax over this stereo pair .
Because there will be relative changes in
the positions of the objects .
So, how stereo is done is created as you know
that for creating a stereo vision we must
have a stereo pair means the photograph the
aerial photograph taken through some airborne
sensor or a camera ok of the same area, but
some overlapping part ok; this you know well
.
So, this overlapping part gives us a stereo
vision of an area . So, now, what we can do
here? In a general case I can tell you suppose
uh this is a pen in my hand ok . So, if I
see background beyond this pen ok from one
I will close my left eye; then I close my
right eye .
So, what will be the changes in the background
you can see; there will be a shift in position
of the objects ok which are placing away from
this pen ok beyond behind this pen . So, this
is called the effect of stereoscopic parallax
and this parallax of course, this is important
for creating a stereo vision without the presence
of this effect ; you cannot see 3-D images
of the 2 overlapping photographs or a stereo
pair . So, what is required to make a stereo
is impossible?
As you know that the most important thing
is our eyes then we can have some aid like
the stereoscope or some other technical equipment
ok to see these stereo pair from with this
instrument or equipment ok .
So to see the stereo vision of a stereo pair
we also need a pair of images ok . So, to
see the stereo vision we need a pair of images
that will be united to form a 3-D image an
image is required for the left eye and another
image will be seen by the right eye . So,
then there is possibility to make a stereo
vision .
So, these 2 images are called a stereo pair
you know them . So, they must have to be taken
by a camera onboard up ah an airplane or a
sensor onboard ah satellite and to include
an overlap to make stereo vision possible
.
So, here you have an example in which the
left eye is seeing looking over this area
and the right eye also this common object
ok . So, but we have 2 different photographs
number 1, number 2 or you can say left or
right ok, but we have this common object or
building anything on these 2 photographs .
So, when we see from the left eye and the
right eye on these 2 separate photographs
, we can an image is is created in the in
our brain actually . So, that image is called
the 3-D vision or this is that will be of
course, an imaginary form of this object which
you can perceive in 3-D into your brain here
is other example .
Suppose this is a terrain ok and undulating
terrain of course, so, but how you can see
the relief of this terrain ? When you see
the 2 different photographs of the same terrain
taken by a camera onboard an aircraft . So,
here these 2 photographs are the stereo pair
; so, in this way you will have depth perception
what is the meaning of depth perception ?
On the single photograph you will also you
will be able to see the undulations, but you
will not have ability of depth perception
until you have a 3-D image of that area .
So, these 2 photographs will give you 3-D
image of this area and this part will show
like here in this photograph this is a ground
ground level or you can also say this is a
datum plane in case of the ah photographic
geometry and these are presenting the highest
elevations in this area ok; you can say these
are the peaks of hills ok . So, peaks and
grounds you are able to see the depth in this
area ok so, where; which point is higher,
and which point is lower.
And this you will learn when you will do your
practicals in which you will learn how to
orient the photograph ok . Because if you
will orient correctly then only you will be
able to see the correct depth perception;
otherwise if you if suppose if you have oriented
the photograph in the opposite manner so,
then only you will see a depth perception
which will be the negative of the real depth
real ground variations ok .
Like in in that case the valleys will be expressed
in the form of hills and the hills will be
expressed in the form of valleys . So, you
can say the a negative depth perception 
ok negative depth perception .
So, here a example of a stereo pair from Tongerio,
Belgium ok . So, in this photograph you can
see this much of area covered in the red colour
rectangle is the overlap which you can see
in 3-D and that is why these 2 photographs
are called the stereograph .
So, here is ah live practical in front of
you; how parallax effect works . You you can
see when we see this bottle putting behind
this spoon and uh stretch our arm and having
this spoon in our hand ok. From left eye we
will see that the bottle is on the right side
and from right eye we will see bottle on the
left side ok.
So, this is uh the effect of parallax only
and this you can see in your daily life everywhere,
anywhere ok . So, this is a very common effect,
but most of us do not know ok
. So, the left image is seen by the left eye
and the right image is seen by the right eye
. And when you ultimately close an eye, it
looks as if the spoon is jumping from one
location to the other ; this is because of
the parallax phenomena . So, the parallax
it is the apparent shift in the position of
an object due to a shift in the position of
the observer because in this case ; we are
the observer we are looking over this spoon
; so, we are the observer .
So, because there is a shift in the position
of the observer . So, in our case we are the
observer, but in actual our eye is the observer
. Suppose this is the left eye and this is
the right eye . So, in this case of parallax
phenomena these eyes are the observer . So,
when we see on the single eye in that case
this the left eye will be observer .
And we close our left eye ; so, in that case
our right eye will be observer . So because
there is an apparent shift in the position
of the observer; so, that is why it resulted
into an apparent shift in the position of
the objects which is putting behind that and
why this is so? That I will tell you when
ah conduct the practicals in which I will
explain what is the eye base and what is the
air base ok .
Suppose over an area a common area the first
photograph is this and second photograph taken
by the same camera is this; photograph 1,
photograph 2 , but at point when the camera
was taking photograph on the lens the position
of the lens were here , but at point at the
time of taking photographs number 2; the position
of the lens was here .
So, this is the result of this phenomena is
resulted into the parallax because in this
case this is the distance between the 2 lenses
that if the same lens , but of course, from
a different from 2 different positions .
So, in this case this will be your airbase
ok and in this case when we are looking with
our eyes left hand right eye, but from they
are ah positioned at different ah places . So,
that is why this is our eye base the distance
between these 2 eyes is called the eye base
ok . So, let us move ahead.
Here an example of a high building from Cairo,
Egypt . So, this building; the same building
was taken by Ikonos satellite from 2 different
positions of its sensors ok. So, the same
phenomena appears when an airplane or a satellite
is flying over an area and taking photographs
from certain features or buildings with an
interval of a few seconds ok does from a different
location .
So, there was an interval of only few seconds,
but that gap has resulted in to difference
in the position of the observer .
So, have a look over these 2 images taken
by the Ikonos satellite sensor having a gap
of few seconds , but from the 2 different
positions . So, these 2 images; images number
1 and number 2 you can see both images are
seen differently ok .
The shape of this building and the shape of
this building is totally different . So, that
is the this is because of the parallax these
are the 2 images of the same building here
this high rise building from Cairo, Egypt
ok . So, the apparent displacement of the
building caused by the change in the point
of observation is the parallax .
So, let us understand parallax in terms of
geometry suppose this is a terrain and we
are observing any object that is named as
a here ok . So, we are observing is this this
object these are our lenses position of the
lenses lens 1, lens 2 . So, one thing you
remember ah there are 2 types of photography
in some kind of photograph is we have ah 2
cameras mounted at different angle ok .
So, they are taking the photograph of the
same area, but from different positions ok
because they are mounted at at a distance
over an aircraft . So, in that case we set
2 different photographs forward and afterward
ok. So, and in other kind of photography there
may be a single camera which is taking photograph
of ah a same plane ok through flying over
here , but with a single camera and it is
, but it is having some overlap that is set
with the camera aperture timing and the speed
of the aircraft ok .
So, coming back to over this geometry suppose
there is a point A object A which we have
to observe over this terrain . So, these are
taken by the 2 lenses lens 1 and lens 2 here
. So, each 2 vertical aerial photographs are
taken consecutively along a flight line with
some overlap of coverage between the 2 . So,
then height can be calculated .
So, here in this case point A is imaged on
to the 2 frame photographs at a l and a r
al means this is for left this is for right
al and ar. So, this is a r and this point
is a l and these are having the X coordinate
origins at the principal points or which is
called the perspective center or the sensors
O 1 and O 2 O 1 and O 2; this you may call
your principal point also because this is
the perspective center representing it ok
.
In very simple terms we can ah calculate our
parallax by the difference of these 2 points
like the distance where this object A is imaged
over an photograph here and here. So, in this
photograph it is imaged over here and on this
photograph it is imaged over here .
So, this is the distance from the your perspective
center or the principal point . So, this your
X l and this your X r . So, simply by taking
the difference of these 2 lens you can get
parallax p is equal to X l minus X r .
Another very in simple form if you want to
have mathematical formula to compute heights
or whatever is given to you like eye base
or airways or you need to calculate the parallax
then you can calculate by these 2 ah formulas.
Here you can see a very simple geometry ah
which has created 2 similar triangles like
L 1, L 2 and A and another triangle is your
L 1, O 1 and a l . So, by these 2 similar
triangles you can have this formula ok which
is representing that the parallax at point
A can be shown graphically by transferring
the right image point to the left image as
shown in the figure .
And we can use the parallax to obtain information
about the height of point A; observing the
similarity between triangles we may infer
that B your as I explained earlier your B
here is what? Airbase because this is the
distance between the 2 lenses L 1 and L 2
this is your air base and this is the this
H; capital H, you know well that it represents
the altitude of the flight from the datum
surface .
So, this is your datum surface here ; so,
this is H and this is the height of the object
ok this h is small h . So, how we can get
this length? By subtracting this H from altitude
capital H minus h which is given here ok . So,
by taking the similarity in the geometry of
these 2 triangles , we can state that air
base B by H minus h is equal to B is the parallax
divided by focal length where is the focal
length?
As you know that from the photographic negative
to the distance up to the lens is known as
your focal length small f . So, B by H minus
h is equal to p by f . So, from here you can
derive this formal also ; suppose you have
to calculate the height of the object in indirect
case you can take it here and you can calculate
accordingly .
So, concluding this topic what we can do is
like what we can conclude is this confirms
that the parallax and proximity are inversely
related ok . You know well proximity means
the closeness of the object . So, these are
inversely related the closer the less ok .
And the large parallax implies close proximity
meaning objects height is great ok and by
the small parallax implies a distant object.
So, having an understanding of this topic
we will move ahead to over practicals ok.
Thanks .
