Depth Perception Cues

Depth perception is concerned with why we see objects in 3D rather than as flat, and how this is accomplished.

This is quite an interesting question if you stop to think about it, because although the eye itself may be a 3D ball the retina is not, it’s flat. So how does a flat surface give us the perception of depth?

The best way to approach this is by using the example of a painting on a flat canvas. Even though just like the retina the canvas is flat, the picture still appears to have depth to it.

This perception of depth is made possible by various cues which provide the onlooker with various types of visual information.

In addition to this, depth perception is also made possible by cues from binocular and monocular vision. So lets look at each of these now.

Binocular Vision

Binocular vision is vision with two eyes, and the main cue for depth perception associated with binocular vision is retinal disparity.

Since the pupils of the eyes are roughly about three inches apart, this means that the right eye gives a slightly different image to that of the left eye.

The disparity (difference) between these two retinal images (retinal disparity) can be used as a cue for depth/distance.

Retinal disparity also provides another example of how the world of sensation differs from that of perception.

For at the level of sensation (seeing through our eyes) we actually sense two images. However at the level of perception (inside our head) we only perceive one image.

This ability of the brain to merge two images together is sometimes called the “zipper effect”, and it is partly as a result of retinal disparity that the images we see appear to be in 3D.

Monocular Vision

The fact that depth perception is possible with only one eye shows that there are other visual cues which allow us to perceive depth, and that this depth is not solely as a result of retinal disparity.

You can test this out for yourself right now by closing one eye and looking around you.

As a result, there are certain monocular cues (described below) which allow you to perceive depth.

Linear Perspective

Linear perspective describes the tendency of parallel lines to appear to converge at the horizon.

This is also known as the Ponzo Illusion, which you can see an example of in the picture below. Notice how the converging lines create depth in the image.

Interposition

Interposition occurs when one object is blocked by another. For example, a card placed in front of another card gives the appearance of the other being behind it.

Shadows

Shadows are differences in the illumination of an image, and help us to see 3D objects by the shadows they cast. If something is 3D it will cast a shadow, if it is 2D it won’t.

Texture Gradient

Texture gradient refers to the level of detail we can see in an image. The closer the image is to us, the more detail we will see.

If it is too close, then that detail will start to become distorted or blurry. Likewise, the further an image is away from us, the less detail we will see it in.

Motion Parallax

Motion parallax describes the tendency when moving forwards rapidly, to perceive differential speeds in objects that are passing by.

A good example of motion parallax occurs when driving. If you see a lamp post in front of you it appears to approach slowly, but just as you are passing it the lamp post seems to flash by quickly in front of you.

If you were to then look back behind you, the lamp post would appear to be slowly moving away from you until eventually it looked stationary.

Size Constancy

Size constancy is the tendency to perceive objects as staying the same size, despite changes in our distance from them.

When that object is near to us, its image on the retina is large. When that image is far away from us, its image on the retina is small. However despite these changes in retinal image size, we perceive the object as the same size.

For example, if you look at the picture below, even though the people in the background are further away they still appear to be of normal size.

To show you just how size constancy works, have a look at the picture below where a cut out of the person has been placed in the foreground.

Even though the cut out picture is exactly the same size as the person in the background, he now appears to be much smaller.

This occurs because when we seen an image our visual system uses various cues from the surrounding environment to make up the image we see and creates a type of visual illusion.

In the example given above, by moving the background figure to the foreground this illusion is broken down, causing us to see the person in an apparently different size.

In the example below, you can see how the brain uses background images to create size constancy.

Again, even though the image always remains the same size, our perception of that image is altered due to the background.

In this example, the background makes the figure appear larger.

Summary

• Depth perception is concerned with why and how we see objects in 3D.

• Binocular vision means seeing with two eyes.

• Retinal disparity refers to the slightly different images we receive in the left and right eye. The difference between these two images is one way in which we perceive depth.

• Linear perspective is the tendency for lines to appear to converge on the horizon.

• Interposition is when one object blocks another behind it.

• Shadows help to give us the perception of depth created by 3D objects.

• We see less detail when objects are further away than when they are close. This is called texture gradient.

• Motion parallax is how objects look to us when we are travelling at speed.

• Size constancy is an illusion created by the brain, which makes things in the distance appear to stay the same relative size.