9. Constraints: Visual Object Recognition Video Lecture | Artificial Intelligence: A Fundamental Guide - AI & ML

PATRICK WINSTON: You know, it's
unfortunate that politics
has become so serious.
Back when you were little
it was a lot more fun.
You could make fun
of politicians.
Here's a politician some
of you may recognize.
But it's convenient to be able
to vary what this particular
politician looks like.
For example, we can go from
a cookie baker to radical.
[LAUGHTER]
PATRICK WINSTON: We can go
from superwoman to bimbo.
[LAUGHTER]
PATRICK WINSTON: Socialite--
I put socialite into this.
There she is.
Or we can move the slider over
the other way to bag lady.
Alert, asleep, sad, happy.
How does that work?
I don't know.
But I bet by the end
of this hour you'll
know how that works.
And not only that, you'll
understand something about
what it takes to recognize
faces.
It turns out to some theories of
face recognition are based
on the same principles that
this program is based on.
But you can kind of guess
what's happening here.
There are many stored images and
when I move those sliders
it's interpolating
amongst them.
So that's how that works.
But the main subject of
today is this matter
of recognizing objects.
Faces could be the objects,
but they don't have to be.
This could be an object that you
might want to recognize.
And I want to talk to you a
little bit about the history
of this problem and where
it stands today.
It's still not solved.
But it's an interesting exercise
to see how the
attempts at solution
have evolved slowly
over the past 30 years.
So slowly, in fact, that I think
if someone told me how
long it would take to get to
where we are 30 years ago I
think I would have
hung myself.
But things do move slowly.
And it's important to see
how slowly they move.
Because they will continue to
move slowly in the future.
And you have to understand that
that's the way things
work sometimes.
So to start this all off, we
have to go back to the ideas
of the legendary David Marr, who
dropped dead from leukemia
in about 1980.
I say, the gospel according to
Marr, because he was such a
powerful and central figure that
almost anything he said
was believed by a large
collection of devotees.
But Marr articulated a set of
ideas about how computer
vision would work that started
off by suggesting that with
the input from the camera,
you look for edges.
And you find edge fragments.
And normally they wouldn't be
even as well-drawn as I've
done them now.
Or as badly drawn as
I've done them now.
But the first step, then, in
visual recognition would be to
form this edge-based description
of what's out
there in the world.
And Marr called that
the primal sketch.
And from the primal sketch, the
next step was to decorate
the primal sketch with some
vectors, some surface normals,
showing where the faces on
the object were oriented.
He called that the two
and a half D sketch.
Now why is it two
and a half D?
Well, it's sort of 2D in the
sense that it's still
camera-centric in its way of
presenting information.
But at same time, it attempts
to say something about the
three-dimensional arrangement
of the faces.
So the speculation was that you
couldn't get to where you
wanted to go in one step.
So you needed several steps
to get from the image to
something you could recognize.
And the third step was to
convert the two and a half D
sketch into generalized
cylinders.
And the idea is this.
If you have a regular cylinder,
you can think of it
as a circular area moving
along an axis like so.
So that's the description
of a cylinder.
A circular area moving
along an axis.
You can get a different kind
of cylinder if you go along
the same axis but you allow
the size of the circle to
change as you go.
So for example, if you were
to describe a wine bottle.
It would be a function of
distance along the axis that
would shrink the circle
appropriately to match the
dimensions of a wine bottle.
A fine burgundy, I perceive.
In any case, this one once
converted into a generalized
cylinder, when matched against
a library of such
descriptions, results
in recognition.
Great theory, based on the idea
that you start off by
looking at edges and you end
up, in several steps of
transformation, producing
something that you could look
up in a library of
descriptions.
Great idea.
Trouble is, no one could
make it work.
It was too hard to do this.
It was too hard to do that.
And the generalized cylinders
produced, if
any, were too coarse.
You couldn't tell the difference
between a Ford and
a Chevrolet or between a
Volkswagen and a Cadillac.
Because they were
just too coarse.
So although it was a great idea
based on the idea that
you have to do recognition in
several transformations of
representational apparatus,
it just didn't work.
So much later, maybe 15 years
later or so, we get to the
next part of our story.
Which is the alignment theories,
most notably the one
produced by Shimon Ullman,
one of Marr's students.
So the alignment theory of
recognition is based on a very
strange and exotic idea.
It doesn't seem strange and
exotic to mechanical engineers
for a while, because they're
used to mechanical drawings.
But here's the strange
and miraculous idea.
Imagine this object.
You take three pictures of it.
You can reconstruct any
view of that object.
Now, I have to be a little bit
careful about how I say that.
First of all, some of the
vertexes are not visible in
the views that you have.
So, of course, you can't
do anything with those.
So let's say that we have a
transparent object where you
can see all the vertexes.
If you have three pictures of
that, you can reconstruct any
view of that object.
Now I have to be a little
careful about how I say that,
because it's not true.
What's true is, you can produce
any view of that in
orthographic projection.
So if you're close enough to
the object that you get
perspective, it doesn't work.
But for the most part, you can
neglect perspective after you
get about two and a half
times as far away as
the object is big.
And you can presume that
you've got orthographic
projection.
So that's a strange
and exotic idea.
But how can you make
a recognition
theory out of that?
So let me show you.
Well, here's one drawing of the
object, I need two more.
Let's see.
Let's have this one.
And maybe one that's tilted
up a little bit.
It's important that these
pictures not be just rotations
on one axis.
Because they wouldn't form what
you might think of as a
kind of basis set.
So there are three pictures.
We'll call them a, b, and c.
And then we want a
fourth picture.
Which will look like this.
It doesn't have to
be too precise.
And we'll call that
the unknown.
And what we really want to know
is if the unknown is the
same object that these three
pictures were made from.
So let me begin with
an assertion.
I'll need four colors of chalk
to make this assertion.
What I want to do is I want to
pick a particular place on the
object, like this one.
And maybe the same place on
this object over here.
Those are corresponding
places, right?
So I can now write an equation
that the x-coordinate of that
unknown object is equal to, oh,
I don't know, alpha x sub
a plus beta x sub b plus
gamma x sub c plus
some constant, tau.
Well, of course, that's
obviously true.
Because I'm letting you take
those alpha, beta, gamma, and
tau and make them anything
you want.
So although that's conspicuously
obviously true,
it's not interesting.
So let me take another point.
And of course, I can write the
same equation down for this
purple point.
And now that I'm on a roll and
having a great deal of fun
with this, I can
take this point
and make a blue equation.
And you know I'm destined
to do it, so I've
got one more color.
I might as well use it.
Let's just make sure I get
something that works here.
That's this one, that's
this one.
I hope I've got these
correspondences right.
STUDENT: [INAUDIBLE].
PATRICK WINSTON: Have
I got one off?
STUDENT: [INAUDIBLE].
PATRICK WINSTON: Which color?
STUDENT: Blue.
[INAUDIBLE].
PATRICK WINSTON: OK.
So this one goes with this
one, goes with this one.
Is that one wrong?
STUDENTS: Yeah.
PATRICK WINSTON: Oh, oh, oh.
Of course this one, excuse
me, goes down here.
Right?
And then this one
is off as well.
I wouldn't get a very good
recognition scheme if I can't
get those correspondences
right.
Which is one of the
lessons of today.
OK.
Now I've got them right.
And now that equation
is correct.
I think I've got this
one right already.
So now I can just
write that down.
I'm on a roll, I'm just
copying this.
So those are a bunch
of equations.
And now the astonishing part
is that I can choose alpha,
beta, gamma, and tau
to be all the same.
That is, there's one set of
alpha, beta, gamma, and tau
that works for everything,
for all four points.
So you look at that puzzled.
And that's OK to be puzzled.
Because I certainly
haven't proved it.
I'm asserting it.
But right away, there's
something interesting about
this and that is that the
relationship between the
points on the unknown object and
the points in this stored
library of images are
related linearly.
That's true because it's
orthographic projection.
Linearly related.
So I can generate the points in
some fourth object from the
points in three sample objects
with linear operations.
Christopher?
STUDENT: Is that the
x-coordinate of--
PATRICK WINSTON: It's
the x-coordinate.
Christopher asked about
the x-coordinates.
Each of these x-coordinates are
meant to be color coded.
It gets a little complicated
with notation and stuff.
So that's the reason I'm color
coding the coordinates.
So the orange x sub u is the
x-coordinate of that
particular point.
STUDENT: In 3D space?
PATRICK WINSTON: No.
Not in 3D space.
In the image.
STUDENT: So it's a 2D
projection of it?
PATRICK WINSTON: It's a 2D
projection of it, an
orthographic projection.
OK?
So we're looking at drawings.
And those coordinates over there
are the two-dimensional
coordinates in the drawing.
Just as if it were
on your retina.
STUDENT: [INAUDIBLE]
vertexes on the 3D projection
or can curved surfaces also?
PATRICK WINSTON: So he asked
about curved surfaces.
And the answer is that you have
to find corresponding
points on the object.
So if you have a totally curved
surface and you can't
identify any corresponding
points, you lose.
But if you consider our faces,
there are some obvious points,
even though our face are
not by any means
flat like these objects.
We have the tip of our nose
and the center of our
eyeballs and so on.
So if that's true, what does
that mean about recovering
alpha, beta, gamma, and tau?
Can we find them?
[INAUDIBLE], what
do you think?
How do we go about
finding them?
You're nodding your head
in the right direction.
[LAUGHTER]
STUDENT: It's four
equations and--
PATRICK WINSTON: Splendid.
It's four equations
and four unknowns.
Four linear equations
and four unknowns.
So obviously, you can solve for
alpha, beta, gamma, and
tau if you know that these
equations are correct.
So how does that help
us with recognition?
It helps us with recognition
because we can take another
point, let me say this square
point here and this
corresponding square point here
and this corresponding
square point here, and what
can we do with those three
points now?
We've got alpha, beta, gamma,
and tau, so we can predict
where it's going to be
in the fourth image.
So we can predict that that
square point is going to be
right there.
And if it isn't, we're highly
suspicious about whether this
object is the kind of object
we think it is.
So you look at me
in disbelief.
You'd like me to demonstrate
this, I imagine.
STUDENT: Yeah.
PATRICK WINSTON: OK.
Let me see if I can
demonstrate this.
So I'm going to do this in a
slightly simplified version.
I'm only going to allow
rotation around
the vertical axis.
And just so you know I'm not
cheating, there's a little
slider here that rotates
that third object.
Let's see, why are there
just two known
objects and one unknown?
Well that's because I've
restricted the motion to
rotation around the vertical
axis and some translation.
So now that I've spun that
around a little bit, let me
pick some corresponding
points.
Oops.
What's happened?
Wow.
Let me run that by again.
OK.
So there's one point
I've selected
from the model objects.
The corresponding point
over here on the
unknown is right there.
I'm going to be a little off.
But that's OK.
So let me just pick that
one and then that
corresponds to this one.
Krishna, would you like to
specify a point so people know
I'm not cheating.
Pick a point.
Pick a point, Krishna.
STUDENT: Oh, the right?
PATRICK WINSTON: The right?
STUDENT: Yeah.
PATRICK WINSTON: This one?
STUDENT: Yep.
PATRICK WINSTON: Oops.
OK, let's pick it out
on the model first.
Now pick it over here.
Boom.
So all the points are where
they're supposed to be.
Isn't that cool?
Well, let's suppose that the
unknown is something else.
This is a carefully
selected object.
Because the points are all the
correct positions vertically,
but they're not necessarily the
correct positions in the
other two dimensions.
So let me pick this point, and
this point, and this point,
and this point.
And Krishna had me
pick this point.
So let me pick this point.
So if it thinks that the
unknown is one of these
obelisk objects, then we would
expect to see all of the
corresponding points correctly
identified.
But boom.
All the points are off.
So it seems to work in this
particular example.
I find the alpha and beta
using two images.
And I predict the locations
of the other points.
And I determine whether those
positions are correct.
And if they are correct, then I
have a pretty good idea that
I have in fact identified the
object on the right as either
an obelisk or an organ,
depending on which of the
model choices and the unknown
choices I've selected.
So the only thing I have left
to do is to demonstrate that
what I said about
this is true.
So I'm going to actually
demonstrate that what I said
about this is true using the
configuration in this
demonstration.
Because it's much too hard
for me to remember matrix
transformations for generalized
rotation in three
dimensions.
So here's how it's
going to work.
The z-axis is going
up that way.
Or, it's going to be pointing
toward you.
And what I'm going to
do is I'm going to
rotate around this axis.
And what I want to do is I
want to find out how the
x-coordinate in the image
of the points move
as I do that rotation.
So here's the x-axis.
This is the coordinate
that you can see.
Here is the y-axis.
That's in depth, so you can't
tell how far away it is.
And the z-axis--
x, y, z-axis must be pointing
out that way toward you.
So now I'm going to consider
just a single point on the
object and see what
happens to it.
So I'm going to say to myself,
let's put the object in some
kind of standard position.
I don't care what it is.
It can be just random,
just spin it around.
Some position, we'll call that
the standard position, S. And
that means that the x-coordinate
of the standard
position is x sub s.
And the y-coordinate of the
standard position is y sub s.
And now I'm going to rotate
the object three times.
Once to form the a picture, once
to form the b picture,
and once to form
the c picture.
And you can make
those choices.
Those can be anything, right?
So let's say that the a
picture is out here.
So that's the a picture.
The B picture is out here.
And the unknown is
up that way.
And so what I want to know
depends on these vectors.
We'll call that theta sub a,
and this is theta sub b.
And this one is theta sub u.
So I would like to know how
x sub a depends on x
sub s and y sub s.
And I can never remember how
to do that, because I can
never remember the
transformation
equations for rotation.
So I have to figure
it out every time.
And this is no exception.
So what I'm going to say is that
this vector that goes out
to S consists of two pieces.
There's the x part
and the y part.
And I know that I can rotate
this vector by alpha sub a by
rotating this vector and
rotating that vector and
adding up the results.
So if I rotate this vector
by alpha sub a, then the
contribution of that to the
x-coordinate of a is going to
be given by the cosine
of theta sub a
multiplied by x sub s.
So you can just exaggerate that
motion, say, well if I
pitch it up that way then the
projection down on the x-axis
is going to be this length
of the vector times the
cosine of the angle.
Now there's also going to be
a dependence on y sub s.
Let's figure out what
that's going to be.
I've got this vector here.
And I'm going to rotate it
by theta sub a as well.
If I rotate that by theta sub a
and see what the projection
is on the x-axis, that's
going to be given by
the sine of the angle.
But it's going the wrong way, so
I have to subtract it off.
So that's how I don't have to
remember what the signs are on
these equations.
Well, that was good.
And now that I'm off
and running I can
do what I did before.
It makes it easy to
give the lecture.
Because this is going to be x
sub b is equal to x sub s
times the cosine of theta sub
b minus y sub s times the
cosine of theta--
oh, you're letting
me make mistakes.
Shame.
I can generally tell by all
the troubled looks.
But there should be some
shouting as well.
That's the sine and
that's the sine.
And one more time.
x sub u is equal to x sub s
times the cosine of theta sub
u minus y sub s times the
sine of theta sub u.
And I forgot the b up there.
So there are some equations.
And we don't know what
we're doing.
We're just going to stare at
them awhile and see if they
sing us a song.
So let's see if they
sing us a song.
What about x sub
a and x sub b?
These are things that
we see in the image.
These are things that
we can measure.
What about all those cosines
and sines of theta
a's and theta b's.
Well, we have no idea
what they are.
But one thing is clear.
They're true for all of the
points on the object.
Because when we rotate the
object around by angle theta,
we're rotating all of
the points through
the same angle, right?
So with respect to any
particular view of the object--
here we are in the standard
position.
Here we are in position a.
The vectors to all of the
points on the object are
rotated by the same angle when
we go from the standard
position to the a position.
So that means that for all of
the images in this particular
rendering, with a particular
rotation by theta a, theta b,
and theta u, those
are constants.
Now remember this is for
a particular theta a, a
particular theta be, and
a particular theta u.
As long as we're talking about
all of the points for each of
those rotations, those angles
and cosines are going to be
the same for all possible
points on the object.
OK.
So now we go back to our high
school algebra experts and we
say, look at these first two
equations, We've got two
equations and what we can now
construe to be two unknowns.
What are the unknowns
that are left?
We can measure a and b.
Whatever the cosines
are, they're the
same for all the pictures.
So if we treat those as
constants, then we can solve
for x sub s and y sub s.
Right?
We can solve for x sub s and y
sub s in terms of x sub a and
x sub b and a whole bunch
of constants.
But, I don't know, a whole bunch
of constants, let's see.
We can gather up all of those
cosines and ratios of sines
and cosines and all that stuff
and put them all together.
Because they're all constants.
And then we can do this.
We can say x sub
u is equal to--
well, it's going to depend
on x sub a and x sub b.
And by the time we wash or
manipulate or screw around
with all those cosines, we can
say that the multiplier for x
sub a is some constant alpha and
the multiplier for x sub b
is some constant beta.
So that's not a slight
of hand.
That's just linear
manipulation of those equations.
And that's what we wanted to
show, that for orthographic
projection, which this is--
there is no perspective
involved here, we're just taking
the projection along
the x-axis--
we can demonstrate for this
simplified situation that that
equation must hold.
Now I want to give you
a few puzzles.
Because this stuff
is so simple.
Suppose I allow translation
as well as rotation.
What's going to happen?
STUDENT: You just get the tau.
Basically, you get a constant.
PATRICK WINSTON: Yeah, you
add a constant, tau.
But what do we need to do
in order to solve it?
STUDENT: Subtract them
[INAUDIBLE].
You subtract two equations
and then [INAUDIBLE].
PATRICK WINSTON: Let's
see, now we've got
three unknowns, right?
I don't know tau.
I don't know x sub s.
And I don't know y sub s.
So I need another equation.
Where do I get the
other equation.
STUDENT: [INAUDIBLE].
PATRICK WINSTON: From
another picture.
That's why up there I
needed four points.
That covers a situation where
I've got three degrees of
rotation and translation.
Here I got by with just two
pictures in this illustration.
That one involved a tau
translational element, so I
needed three pictures.
And this one's got full
rotation, so I needed four.
So great idea, works fine.
The trouble is it doesn't work
so fine on natural objects.
It works fine on things that are
manufactured because they
all have identical dimensions.
So if I made a million of these
in a factory, I'd have
no trouble recognizing them.
Because all I'd have to do is
take three pictures, record
the coordinates of some of the
points, and I'd be done.
The trouble is the natural
world isn't like this.
And you aren't like
this either.
I don't know, if I'm trying to
recognize faces, it's not that
easy to do all this.
First of all, it's a little
difficult to find the exact
point, the exactly corresponding
points.
I made a mistake in
doing it myself.
And if the computer made a
mistake it would certainly
make an error.
Because it would be using
non-corresponding points to
make the prediction.
So it would be way off.
But this is still in the
tradition of working from
local features in the objects
toward recognition.
So having looked at that theory,
we also find it a
little wanting.
It works great it some
circumstances, doesn't seem to
solve the whole recognition
problem.
Years pass.
Shimon Ullman comes up with
another theory that's not so
much based on edge fragments or
the location of particular
features but rather
on correlation.
Taking a picture of, say,
Krishna's face, taking a
picture of the whole class, and
then using that as a kind
of correlation mask, running it
all over the picture of the
class, seeing where
it maximizes out.
Now that's vague.
I'll explain when I'm talking
about [INAUDIBLE]
correlation in a minute.
But it's basically saying, if
I have a picture of Krishna,
where do I find him?
I'll find him in one place.
But you know what?
Krishna doesn't look
like anybody else.
So I might not find
any other faces.
And if my objective is to find
all the faces, then maybe that
idea won't work either.
Or, to take another example,
here's a dollar bill.
We haven't had raises in quite
well, so this is my last one.
It's got a picture of George
Washington on it.
And I can look all
over the class.
And if I use this is as a face
detector, I'd be sorely
disappointed.
Because I wouldn't
find any faces.
Because thank God, nobody looks
exactly like George
Washington.
So the correlation wouldn't
work very well.
So that idea's a loser.
But wait a minute.
I don't have to look
for the whole face.
I could just look for eyes.
And then I could look for
noses and maybe mouths.
And maybe I could have a library
of 10 different eyes
and 10 different noses and
10 different mouths.
Would that idea work?
Probably not so well.
The trouble with that
one is, I'd find
eyeballs in every doorknob.
There's just not enough stuff
there to give me a reliable
correlation.
So let's make this a little
more concrete by
drawing some pictures.
Halloween is approaching.
So here's a face.
All right?
Here's another face.
So those might be faces in my
pre-recorded library of
pumpkin faces.
Now along comes this face.
What's going to happen?
Well, I don't know.
Let's draw yet another face.
I don't know, that could
be a pretty weird
pumpkin face, I suppose.
But I mean it to be something
that doesn't look very much
like a face.
So if I'm doing a complete
correlation with either of
these faces in my library,
neither one will match this
one very well.
If I'm looking for fine features
like eyes, then I've
got these eyes everywhere.
So it doesn't help very much.
So you can see where
I'm going.
And you can reinvent Ullman's
great idea.
What is it?
We don't look for big features,
like whole faces.
We don't look for
small features,
like individual eyes.
We look for intermediate
features, like two eyes and a
nose, or a mouth and a nose.
So when we do that, then we
can say, now, here are two
eyes and a nose.
Well, that's found
in this one.
And what about the combination
of that nose and that mouth?
Oh, that's over here.
But neither of those features
can be found
in the fourth image.
So that's the Goldilocks
principle.
When you're doing this sort of
thing, you want things that
are not too small
and not too big.
I've got the Rumpelstiltskin
principle up
there, too, by the way.
Because I meant to mention
that Marr was a genius at
naming things.
And even though many of his
theories have faded, he's
still known for these names like
primal sketch and two and
a half D sketch because
he was such an artist
at naming the concepts.
He even got credit for a lot
of stuff that he didn't do.
Not because he was deliberately
trying to get it
inappropriately, but just
because he was so good at
naming stuff.
So we had the Rumpelstiltskin
principle back then.
And now we have the Goldilocks
principle.
Not too big, not too small.
But that leaves us with the
final question, which is, so
if what we want to do is look
for intermediate-size
features, how do we actually
find them in a sea
of faces out there?
See, I might have a library,
I might take 10 of you and
record your eyes.
Take another ten, record
your mouths.
And they may be put together
in a unique way for each of
you out there.
But it's likely that I'll
fin Lana's eyes
somewhere else in a crowd.
And Nicola's mouth somewhere
else in a crowd.
So how do we in fact go
about finding them?
And I mentioned the
term correlation a
couple of times now.
Let me make that concrete.
So let's consider a
one-dimensional face that
looks like this.
Which is a signal.
And I'm going to consider
a one-dimensional image.
And in that one-dimensional
image I've got a
facsimile of the face.
And the question is, what kind
of algorithm could I use to
determine the offset in the
image where the face occurs?
So you can see that one
possibility is you just do an
integral of the signal in the
face and the signal out here
over the extent of the face and
see how it multiplies out.
Or, to make it less lawyerly
and more MITish, let's say
that what we're going to do is
we're going to maximize over
some parameter x the integral
over x of some face, which is
a function of x and the image
g, which is a function of x
minus that offset.
So when the offset, t, is equal
to this offset, then
we're essentially multiplying
the thing by itself and
integrating over the
extent of the face.
And that gives you a very big
number if they're lined up and
a very small number
if they're not.
And it's even true if we
add a whole lot of
noise to the images.
But these are images.
They're not one dimensional.
But that's OK.
It's easy enough to make
a modification here.
We're going to maximize
over translation
parameters x and y.
And these are no longer
functions of just x, they're
also functions of y.
Like so.
So that's basically
how it works.
We won't go into details about
normalization and all that
sort of thing because that's
the stuff of which other
courses remain the custodians.
So would you like to see
a demonstration?
OK.
All right.
So without realizing it, Nicola
and Erica have loaned
us their pictures.
And they are embedded in that
big field of noise.
And it's pretty easy to pick out
Erica and Nicola, right?
Because we are actually pretty
good at picking faces out of
these images.
So let's add some noise.
It's a little harder now.
What I'm going to is I'm going
to run this correlation
program over this whole image
using Nicola's face as a mask
and seeing where the correlation
peaks up, in spite
of all the noise that's
in there.
Boom, there he is.
I don't know, maybe we
can find Erica too.
I forgot where she was.
I can't find her.
There she is.
Unfortunately the parameters
aren't very good here.
Do you see that?
Let me get another
version of this.
I'll just do some real-time
programming.
I've been trying to reset the
parameters so that the images
in the demonstration comes
out clearly up there.
Let's see if this works
a little better.
OK, so let's add some noise.
And let's find Erica.
There she is.
There are some other
things that look a
little bit like Erica.
But nothing looks quite
exactly like Erica.
So let's try Nicola's eyes.
So they stand out pretty
clearly against the
background.
Let's see if we can
find Erica's eyes.
So they stand out pretty
clearly against the
background.
Notice that it also gets
Nicola's eyes.
So two eyes is an
intermediate-size constraint.
It's loose enough that it will
match more than one person.
But it's not so loose
that it's as bad as
looking for one eye.
See, they're all
over the place.
So two eyes and a nose, a mouth
and a nose, that would
be even better as an
intermediate feature.
But it doesn't matter what the
best ones are, because you can
work that out experimentally.
So that's how correlation
works.
And it's just amazing how much
noise you can add and it'll
still pick out the
right stuff.
There's Nicola.
Boom.
Very clear.
Want to add some more noise?
I don't know, I can see it,
but that's because I'm a
pretty good correlator, too.
Boom.
I don't know, let's add
some more noise.
It's just hard to
get rid of it.
It's just amazing how well
it picks it out.
That's good.
That's cool.
Now, but the reason that this
is 30 years and we're still
not done is there are still
some questions.
This is recognizing
stuff straight on.
How is it I can recognize you
in the hall from the side?
Nobody knows.
One possibility is that you have
an ability to make those
transformations.
If so, then that alignment
theory has a role to play.
Another theory is that, well,
after I've seen you once I can
watch you turn your head and
keep recording what you look
like at all possible angles.
That would work.
The trouble is, is there
enough stuff in there?
Maybe.
We don't know.
Now what would it take to
break this mechanism?
Well, I don't know.
Let's just see if we can
break the mechanism.
Let's see if you can recognize
some well-known faces.
Who's that?
Quick.
STUDENT: Obama.
PATRICK WINSTON: Oh,
that's too easy.
We'll see if we can make
some harder ones.
Yeah, that's Obama.
Who's this?
STUDENT: Bush.
PATRICK WINSTON: Oh boy.
You're really good at this.
That's Bush.
How about this guy?
STUDENT: Kerry.
PATRICK WINSTON: OK.
Now I've got it.
Some people are starting
to turn their heads.
And that's not fair.
[LAUGHTER]
PATRICK WINSTON: That's
not fair.
Because you see what's happened
is that if this kind
of pumpkin in theory is correct,
then when you turn
the face upside down you lose
the correlation of those
features that have vertical
components.
So if you have two eyes and a
nose, they won't match two
eyes and a nose when they're
turned upside down.
Well, let's see.
We'll try some more.
Who's that?
STUDENT: Gorbachev.
PATRICK WINSTON: Gorbachev.
Who said that?
Leonid, where are you?
This is Gorbachev, right?
You can recognize him because of
the little birthmark on the
top of his head.
One more.
Who's--
oh, that's easy.
Who is it?
That's Clinton.
How about this one?
Do you see how insulting
it is to be at MIT?
That's me.
[LAUGHTER]
PATRICK WINSTON: And you
didn't even know.
Oh, god.
So this might be evidence for
the correlation theory.
But of course, turning the face
upside down would make it
very difficult to do
alignment, too.
So it would break out alignment
theory, as well.
Let me get that after class,
Was there a mistake, or?
STUDENT: No, no.
I was just curious [INAUDIBLE]
stretching would break the
correlation.
PATRICK WINSTON: If what would
break the structure?
What?
Stretching?
STUDENT: [INAUDIBLE].
PATRICK WINSTON: Elliot asked if
stretching would break the
correlation.
And the answer is, I think,
stretching in the vertical
dimension is worse than
stretching in
the horizontal dimension.
Because you get a certain amount
of stretching in the
horizontal dimension when
you just turn your head.
By the way, since our faces
are basically mounted on a
cylinder, this kind
of transformation
might actually work.
That's a sidebar to the answer
to your question, Elliot.
But now you say, well, OK, so
this is not completely solved.
You can work this out.
But if you really want to work
something out, let me tell you
what the current questions
are in computer vision.
People have worked for an
awful long time on this
recognition stuff and, to my
mind, have neglected the more
serious questions.
It's more serious questions
are, how do you visually
determine what's happening?
If you could write a program
that would reliably determine
when these verbs are happening
in your field of view, I will
sign your Ph.D. thesis
tomorrow.
There are 48 of them there.
And that is today's challenge.
But since we're short on time,
I want to skip over that and
perform an experiment on you.
I want you to tell me
what I'm doing.
STUDENT: [INAUDIBLE].
PATRICK WINSTON: So the best
single-word answer is?
[INAUDIBLE]?
STUDENT: Drinking.
PATRICK WINSTON: OK, this
is not a trick question.
OK, the best single-word
answer.
Christopher, what
do you think?
STUDENT: Toasting.
PATRICK WINSTON: Christopher.
Well, you.
You.
STUDENT: Toasting.
PATRICK WINSTON: What?
Toasting.
OK.
Not a trick question.
What's happening here?
Best single-word answer?
STUDENT: Drinking.
PATRICK WINSTON: Is drinking.
Which pair look more alike?
[LAUGHTER]
PATRICK WINSTON: So that cat is
drinking and nobody has any
trouble recognizing that.
And I believe it's because
you're telling a story.
So our power of storytelling
even reaches down into our
visual apparatus.
So the story here is that some
animal has evidently had an
urge to find something to drink
and water is passing
through that animal's mouth.
That's the drinking story.
So even though they look
enormously different visually,
the stuff at the bottom of our
vision system provides enough
evidence for our story apparatus
so that we can give
the left one and the right one
different labels and recognize
the cat is drinking.
And that's the end
of the story.