Alright, so our visual system has figured out what things in the world seem to
represent objects and perhaps it's even been able to recognize what those given
objects are. The next problem it faces or I should say
a problem that its simultaneously solving, is where those objects are.
And there's some really cool things about why we have two eyes and what two eyes do
for us in that regard that I really want to make the subject of this lecture.
Alright let's do it. So Week 3: Lecture 5, Perceiving Where.
I, I start off here with just this example to try to get you to realize how
impressive again, we are in some regards without maybe always appreciating it.
If we, you know, think of somebody playing baseball, and hundreds of feet
away, somebody hits a ball. The outfielders almost immediately seem
to have a sense of where that ball will end up.
By watching a little bit of its trajectory, they start to adjust and run
back or run forward and ultimately, they're able to zero right in on where
the ball is going to land. And they're able to, you know, perform
fantastic catches like this little guy right here.
and so the question really is, how do we do this?
It, it's like we're able to calculate trajectory physics somehow without even
knowing what the formulas are. Well, let me kind of go a little slow
and, and begin by saying we can do a lot with one eye.
That's why I call this For You Pirates. in fact, when I talk about this issue in
class, one of the things that I do, and that I suggest you maybe try, is I will
throw something back and forth with another student in front of the class.
And we will do that for a while with two eyes, you know, both eyes open.
And we'll ask everybody to watch how relatively graceful we are, how adept we
are at catching these things quite easily.
And then we'll close one eye. And what you see is that when you close
one eye, you can still do it. You can still figure out where an object
is. That is, you can still perceive depth,
because that's what we're really talking about here.
Where is that, in the world? How deep?
How far away from me is it? so you can still do that with one eye,
but you cannot do it as well. Okay?
So you tend to see the gracefulness disappear, and suddenly you know, people
are making jerky movements to try to catch the ball and they're missing a lot
more often. So, you know, give that a try.
and then ask yourself, okay, so why this, why two eyes?
What, what does that extra eye do for me? Okay, well, here's the things we're using
with one eye. We're using things like perspective.
So, we have some sense of how big people are relative to other things in the
scene. This woman behind here is actually, if
you cut her out and copied her and pasted her right beside the guy, this is what
you would get. and so, you know, really, the image that
she's casting is very small relative to the image that he's casting on our
retinas. But our, our brain knows what size humans
should be. So, how it interprets this is that you
know, unless we stick her right beside him like this where, where it's awful
hard to perceive her far away. If we can provide some kind of
environmental support that she might be further back then that's how the brain
perceives it. So in the normal situation we see her as
you know, maybe a little shorter than him.
But probably not much shorter, just very far away relative to us.
So we use things like perspective, which incorporates knowledge and memory.
and that helps us know how far away it is.
So we can imagine if were throwing a ball back and forth.
The, the image that, that ball casts on our retina is actually getting bigger and
bigger and bigger and bigger and bigger as it comes towards our eye.
And we can detect that and we translate that, not that the ball is getting larger
but rather that it's coming closer. So we use things like perspective all the
time. There's other things that change as well,
with depth. For example, if you look at these rocks,
you'll see the kind of, the size of them. This is the perspective thing again.
You know, these rocks look really big. The ones over here look much smaller.
But again, we don't really think they're much smaller, we think they're just
further away. But we also have a lot more detail.
If you look at the close ones you see a lot more detail on closer objects.
A mot, lot less detail on further objects.
And as you go further and further back, you know, the mountains almost smooth out
quite a bit. So we, we use that how, how granular how
coarse or, or smooth something is that give us an idea of depth.
Of course we use things like interposition, so we know that this rock
is in front of this rock because its literally occluding this rock.
We can see more of this rock, then we can now say this one or this one.
and that tells us that this one is in front.
So we have all of these things that work just as well with one eye as they do with
two eyes. We have one more that I want to show you
because I think it's really cool. and that's shading, and it, and it makes
a really important point. So, look at first of all this figure on
the left. And what you see is really these balls
are positioned in the exact same position relative to the grid underneath it.
and if you're not convinced of that, check, check out this top one and notice
that there's about two and a bit squares. There's about two and a bit squares here.
Okay, they're really the same. All we've changed in these figures is
where the shadow lies. Does it lie right underneath the ball?
Or does it progressively become further underneath the ball.
So that's what's changing across these images.
But what we perceive as changing is where these balls are.
Suddenly, you know, these ones seem to be going up, but staying the same distance
from us. So, this one isn't any further from us
than this one. It's just higher in the air.
Whereas in A, we perceive these as gradually getting further and further
from us receding in the distance. So, let's think about this [INAUDIBLE]
shadow. Where, where, what's that come from?
Well, it comes from the sun, the world we live in.
What we're used to seeing. so the critical point being here, we have
learned certain regularities of the world that tip us off to where things are, and
we've kind of imcorporated those now into our perceptual system so that we can kind
of reverse engineer things. And by looking at things like shadow, we
can figure out where things are. Lemme show you a very dramatic example of
this. so look at this first, and actually I'm
going to leave PowerPoint, which is never as easy as I want from here.
So, I'm going to go really fast into the future, huh.
Seems to be the only way I can do this. Okay.
So here's the slide that I was just showing you.
I'm going to make this a little bigger. Now, when you look at this slide what do
you see. You should see 1, 2, 3, 4, 5, 6, 6 of
these should feel convex, that is they curve out towards you.
Whereas the rest should seem concave, they, they curve away from you.
They go into the distance. But look what happens, so now this is not
magic. Okay, take a good look at what you see.
Six popping out the rest blending in. I am now just going to take this object
and I'm going to spin it around, til I put it upside down.
And now watch what happens. Boom, did you feel that.
These six that were popping out are now concave.
The rest feel convex. They feel like they're coming out towards
us. Let me put it back, and there you go.
There's the reverse. When we put it back, we now see that six
of them are coming out towards us and the rest are going away from us.
So, this is one of these illusions of depth.
and well, it's not really an illusion, it, it's really just how our system is
working. Why is it working this way?
Well, we live in a world where the sun is above us.
And so think of a cave. If a sun is above us and shining into the
mouth of the cave, what do you see? Will you see shadow across the top?
And then brightness across the bottom, where the sun is able to hit and
penetrate. But if you have something convex, the
reverse is true. You see light, you know, think of a bowl.
You see light hitting the top, where it's bowing out, and then the shadow
underneath, because the bowl, bowl comes out and casts a shadow underneath it.
So this is just the way concave and convex things work in the world.
But now when we see something that's two dimensional, but shaded in just the right
way, we see it as concave or convex. And we can you know, completely flip that
perception literally by flipping the figure.
And everything changes. you might ask, well what if we just did
this then? What if we just move 90 degrees?