in this video we're gonna continue our discussion of perception by learning about how we perceive depth that is how we engage in depth perception now to perceive depth we as humans rely on a combination of two different types of cues that tell us about the depth of what we're looking at there are monocular depth cues and binocular depth cues let's break down each of those terms ocular simply means eye mono means one and bi translates to two so monocular depth cues mean depth cues that only require one eye to see binocular depth cues our depth cues that require both eyes I'm going to spend the rest of this video telling you about the various monocular depth cues there are several and both of the binocular depth cues in turn let's start with the monocular depth cues so the first one is relative size all else being equal more distant objects tend to look smaller than objects that are closer to us let's look at this scene of a busy traffic filled highway I get stressed just looking at this I don't know about you to illustrate here so take a look at this Lexus over here and maybe this car right here look at how much of our visual field how much space that these two cars take up these cars are very close to us so relatively their size is much bigger take a look at some of these cars that are further out in the background they take up very little space on our visual field our brain uses this information to say those objects those cars must be further out because they take up less space on our visual fields next we have text your gradient object textures become less apparent farther away close-up object textures are very apparent again let's take a look at a picture to illustrate focus your attention first on some of the flowers and grass that are very close to you some of these over here maybe this area this guy right here look at how detailed everything is how much texture the boundaries between objects are very and well-defined you can tell a lot about the color of the different objects and they don't all blend together but now focus your attention further out on to some of these plants and flowers and grass over here look at how everything just kind of blends together a lot of the textures a lot of the detail is lost again your brain takes into account this information it says okay the further out I'm looking the less textures I'll be able to see and it uses this information to again judge depth next we have interposition and at this point I want to pause to remind you what are we looking at these are all monocular depth cues meaning you can do this with just one eye so if you were to cover up one of your eyes you could still pick up on all of the cues that I'm talking about interposition is the idea that if object a is blocking our view of object B this must mean that object a is closer to us than object B interposition goes by another name which is known by occlusion meaning you know objects can include one another and that tells us about which object is in front or behind of the other object you can look at this very simple scene right here of three different objects we have a blue moon we have a green rectangle and a red star and I can ask the simple question and again you can cover your eye and you'd probably still get this question right hopefully which of these three objects is closest to you well hopefully you said or at least thought the red star right that's in front nothing is blocking it nothing is including it behind that would be what what's behind the red star well probably the green rectangle right in the blue moon in the back because the green rectangle is in front of that blue moon okay that's interposition pretty simple to do next we have the monocular depth cue of linear perspective this one's a little bit different but again you've experienced this parallel lines tend to converge as distance increases now they don't actually converge parallel lines by definition never touch converge means they come together they touch eventually right but the key is that they appear from our perspective like they do converge over great distances eventually meeting at what we call a vanishing point let's take a look at this railroad track here as sort of an illustration because this is a great example first of all railroad tracks are parallel lines right that would be very problematic for the trains on these railroad tracks if they were not parallel lines but notice they converge as you go out into the distance these lines tend to come together notice - there's a vanishing point very far out into the horizon into the distance you kind of it looks like those two lines touch one another and they don't look parallel anymore you can also notice that the distance between the two parallel lines is very far up close but very far out the distance between the two parallel lines is very small very close together so again your brain looks at that it takes into account that information instantaneously to judge depth it knows based on all of these different factors in addition to other ones like texture for example notice the objects textures being very detailed up here in front and very not detailed the further out you go so your brain looks at all of this takes all this into account and says okay all of this must be further out all of this must be closer next we have height in plane this is another simple one that you probably never have thought of before but again you've encountered on a daily basis distant objects tend to appear higher on our visual fields than closer objects let's quickly illustrate with this very nice scene as well so notice this pole right this pole this part of this fence and notice this tree this tree or group of trees or bush or whatever you want to call this this tree is higher on a visual plane than this pole therefore we know that the tree is further away from us and finally our last monocular depth cue light and shadow this is simple the idea that objects cast shadows and those shadows tell us all sorts of things about their three-dimensional shape and from that information given to us by those shadows we can judge depth we can judge all sorts of things we get a lot of information just from that look at this scene here there's just literally light and shadow and almost everything else has been factored out there's no color nothing like that at least not in a useful way and yet we can really the depth of this object alright those are your monocular death cues you can do those with one eye binocular depth cues require two eyes so if you were to cover one of your eyes you literally could not use the information of these different binocular depth cues to judge depth so the first is called binocular disparity because our two eyes are offset they've produced two different images now you don't experience two different images because your brain puts that stuff together into one meaningful sort of you know not disjointed experience you experience the world as if you're filming with a video camera but that's not how it is at all in reality your brain gets two different images we aren't cyclopses right our eyes are offset they aren't on top of one another so here's how your brain uses that information in a really clever way looking at the disparity between those two images can tell you how far away the object is very large disparities between your two eyes between the images your two eyes are producing I should say mean that the object is much closer an object very far out will not produce much of a disparity at all here's an easy way to illustrate this a demonstration I encourage you to do it'll just take a second take your hand and give yourself like a thumbs up first of all okay great confidence booster but also part of the demonstration so give yourself a thumbs up and now extend your arm away from your face as far as you possibly can okay so you're looking right at your thumb as far as you can get it away from your face now sort of alternate closing one eye and then the other so close your left eye and you're looking out your right and then switch right eye is closed now you're looking at your left and switch back and forth really quickly right you'll see that your thumb bounces back and forth a tiny bit now what I want you to do is take your thumb and put it really close to your face as close as you can get it to where you can still focus or fixate on your thumb so where you can still see it without it just being a big blur so look at your thumb now very close to your face and again alternate closing your different eyes you'll see that your thumb jumps around a ton again your brain uses that information it says okay it's jumping around a ton this must be closer to me when it's further away it doesn't jump around at all or very little I should say so it be farther away again you need two eyes in order to do that last but not least we have the binocular depth cue of convergence this one's a little bit different and I think really interesting so you may have noticed when you took your thumb and brought it closer to you if you participated in that demonstration great job if you did if you took your thumb and put it closer to you you probably noticed that you felt a little bit of pressure in your eyes in reality what happened as you brought your thumb very close in front of your face is that your eyes started to converge they in order to fix it on your thumb they have to sort of come together this is what you're seeing on the right hand side of the screen for example if you're focusing on something close to you're close to your face your eyes have to converge if you're looking at something very far away your eyes don't really have to converge now your brain again uses that information it judges the pressure that you're you're feeling physically in your eyes if you fixate on something really close you feel some pressure and your eyes converge and your brain takes into account all of that information to say if I'm converging a lot the object is very close to me if I'm not converging at all the object must be very far away now as a final note there's a lot of different cues hopefully you found these interesting I think they're fascinating that our brains just do this without us asking it's just an automatic thing that we do from a very young age but what I want to mention is that all of these different depth cues both monocular and if you have two eyes the binocular depth cues as well are being used in conjunction at the same time