Depth perception

Seeing with two eyes helps people to judge distances and to see in 3D, but even using one eye, there are many clues (often referred to as visual cues) to give people depth perception. Depth perception using computers is more difficult.

Binocular vision – seeing 3D with two eyes

There are two main binocular cues that help us to judge distance:

• Disparity – each eye see a slightly different image because they are about 6 cm apart (on average). Your brain puts the two images it receives together into a single three-dimensional image. Animals with greater eye separation, such as hammerhead sharks, can have a much greater depth perception (as long as the view from both eyes overlaps the same scene). This can be very useful when trying to catch fast-moving prey.
• Convergence – when looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is. If you hold your finger 20 cm in front of your eyes, your muscles need to work a lot harder than when your finger is 50 cm away.

These binocular cues are most effective for objects up to 6 m away. After this, the amount of eye separation does not give a great enough difference in images to be useful.

3D movies make use of disparity by providing each eye with a different image. However, the brain does not receive any cues from convergence as it normally would. This may cause discomfort for some people.

Monocular cues – 3D information from a single eye

If you close one eye, your vision becomes much less three-dimensional, but there are still many clues that allow you to judge distances. You are still able to pick up a pen, move around without crashing into things and even catch a ball.

Some of these monocular cues are as follows:

• Accommodation – this is the change of focus when you look at a close-up object. The ciliary muscles inside the eye need to work harder to change the shape of the lens inside your eye. The effort required provides the brain with information about distance.
• Sharp focus or blurry – if two objects are at the same distance, they will both appear to be in focus. Objects that are closer or further away will appear blurry.
• Motion parallax – if you move your head, objects that are close to you will appear to move more than those objects that are further away.
• Superposition – objects that appear to move in front of other objects must be closer (a little obvious perhaps, but very useful). You will often see some animals to move their heads from side to side or up and down. This gives important depth information both for motion parallax and for superposition. Try it out!
• Vividness of colours – distant objects often appear less bright and colourful. This is due to the scattering of light as it travels from that distant object. Having more of the atmosphere to travel through means that light will be scattered more, so the colours will not seem as bright.
• Definition and textures – close objects will have a lot of detail and definition apparent. More distant objects will not appear with as much detail. This is very noticeable when looking at a field of grass. Close up, the blades of grass will be noticeable. Further away, the grass is more of a sea of green.
• Relative size – if we already have an idea of the size of two people or objects in a photo, this can give a good clue as to how far apart they are.

Artists use some of these monocular cues to give a perception of distances in a two-dimensional picture.

Creating 3D for movies, robots and security cameras

Computers and robots do not have brains to process these cues from digital images and interpret 3D information. For them, there needs to be an entirely different technology.