3D movies

 

    

3D means three-dimensional, i.e. something that has width, height and depth (length). Although our physical environment is three dimensional the movies we watch are only in 2D. The have length and width but not depth..

Humans are able to perceive the spatial relationship between objects just by looking at them because we have a brain that is able to produce depth perception. As each of our two eyes focuses an image onto their retina, two slightly different, two dimensional images of our surroundings are formed. These images are sent to the brain to be processed into a 3D visual experience.

However it's important to note that having vision in both eyes (stereoscopic or binocular vision) is not the only way to see in 3D. People who can only see with one eye (monocular vision) can still perceive the world in 3D, and may even be unaware that they are stereo blind. They are simply missing one of the tools to see in 3D, so they rely on others without thinking about it.

Here are some of the tools humans use for depth perception:

 

  • Stereoscopic vision: Two eyes provide slightly separate images; closer objects appear more separated than distant ones. In order for the brain to work how something is moving in 3D space it processes the images formed in each eye. An object moving closer will appear, on the left eye, to be moving to its right. While the same object will appear, in the right eye, to be moving to the left.

  

 
  • Parallax: As your head moves from side to side, closer objects appear to move more than distant ones.
  

 
  • Accommodation: As you focus on a close or distant object, the lenses in your eyes physically change shape, providing a clue as to how far away the object is. Zonule fibers are specialised nerves located near the lens that pick up the level of tension on the lens by the ciliary muscles and give the brain an idea of how curved the lens has to be in order to focus the object on the fovea.
  
  • Size familiarity: If you know the approximate size of an object, you can tell approximately how far away it is based on how big it looks. Similarly, if you know that two objects are a similar size to each other but one appears larger than the other, you will assume the larger object is closer.
  

 

 
  • Aerial perspective: Because light is scattered randomly by air, distant objects appear to have less contrast than nearby objects. Distant objects also appear less color-saturated and have a slight color tinge similar to the background (usually blue).
  

In order to represent the 3D world on a flat 2D display screen, it's desirable to simulate as many of these perception tools as possible. Although there is currently no way to simulate all of them at the same time, video does use a combination. For example, aerial perspective and size familiarity are automatically captured by the video camera. In CGI scenes, aerial perspective must be added so that distant objects appear less clearly, this is called distance fog, as shown above in aerial perspective.

Of course the addition of stereoscopic images (a separate image for each eye) is a significant improvement—so much so that most people think of stereoscopic films as being 3D, and all others as being 2D.

2D Film & Video

A traditional 2-D video image has width and height but technically it has no depth, i.e. everything in the image is presented at the same distance from the viewer. Still, the viewer does perceive the image as three-dimensional by subconsciously using the techniques listed above—much the same as how stereo-blind people perceive the real world.

3D Film & Video

3D video adds stereoscopic vision, meaning that two separate images are shown simultaneously—one to each eye.

Continue with stereoscopic vision.
Stereoscopic vision is important for the perception of depth. How do people with one eye compensate for this?
For an object to perceived as hurtling towards the head how should the left and right eye image differ over time?
What are zonule fibers and how are they associated with image formation by the brain?
What is parallax and how does the brain use this to represent depth?
Give an example of how the brain uses size familiarity to create depth in a scene?

Click to see a visual representation of the neural pathways originating from the eye and moving through the brain.
a) What is the trochlear nerve responsible for?
b) The oculomotor nerve controls 4 out of 6 of the main muscles of the eye. Why is it important to have a direct connection with nerves originating from the eye?
c) What is the function of the occipital lobe?