Thursday, May 3, 2012

Stereoscopic 3D Basics

Replica of traditional stereo viewer from 3D Concepts
I enjoy taking 3D pictures and watching 3D movies. There have been several posts here which either comment on 3D content or include 3D images. In this post, I would like to explain the basic concepts behind 3D photography and 3D cinematography.

Each of us have two eyes approximately 6 centimeters apart. When both eyes are functioning properly, we see two slightly different scenes in each eye. Our mind combines those scenes and uses the difference between them to determine depth.  

To see how this works, slide back until you are about three feet from your computer screen and hold one finger up about halfway between your face and the screen. Now close one eye and note which words are still visible. Now open your eye, close the other and note which words are visible. At short distances, the difference between the scenes captured by each eye is quite significant.

As our minds process the differences between the two scenes, there are two pieces of information that are used to determine depth.  The first, horizontal parallax, is the difference between the relative positions of items in the two scenes. In our example, we know the finger is closer than the screen because it moves right and left on the screen as we switch from eye to eye.

Note the differences in which people are visible in the backgrounds of the two frames of this stereoscopic 3D image of street performers at Santa Monica pier. The difference in who is hidden is called occlusion revelation. The difference in the relative positions of the tourists is the horizontal parallax.

The second clue to depth is called occlusion revelation. This refers to the small areas of a scene that are occluded or hidden from one eye but revealed to the other. The perception of those small areas and the width of those occluded areas are a major input in our brain’s calculation of depth.

When we view a traditional 2D photography or movie, we can still recognize the depth of the scene through several monoscopic cues. We can make depth judgements based on perspective, the relative size of objects, changes in texture gradients and position relative to the horizon. Occlusion is also the most important monoscopic depth cue because any object that hides must be in front of what is hidden.

3D photography increases the level of realism by adding back in the stereoscopic depth cues. This is done by showing each eye its own unique view of the scene. Stereoscopic 3D requires a way to capture or create the scene with separate right and left eye views and a way to present each of two those views to the appropriate eye. I will explore those methods in future posts.

The stereoscopic 3D image above was captured with this Loreo 3D lens adapter which uses two focusing lenses to project two separate image views onto one camera frame.

What is your favorite 3D movie so far?

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