The Basics of Stereo 3D

You'll find here the basics that are necessary to understand all the rest of Convergence3D…

Principles

Shooting stereo is shooting to recreate on screen the feeling of depth the brain gives us in reality, using the small difference of point of view between the right eye and the left eye.
If I put my finger in front of my eyes, then my eyes must tilt in to not see it doubled: this is called convergence: my eyes have to tilt in more to look at an object which is closer, and to tilt in less to look at a far away object. This blends the two images I see (the one of the left eye and the one of the right eye) of that specific object in my eyes. That process is called fusion: I must converge to be able to fuse the two images of an object.
Fusion is an automatic reflex: the brain tries to fuse (by the means of the convergence) every object it sees - and do that many times per second. Depending on the convergence angle your eyes must take to fuse the images of a specific object, the brain infers the distance of that object.
The principle of stereo 3D is to present to the right eye and to the left eye two slightly different images, to force your audience's eyes to take a specific angle of convergence to fuse it - and then, mislead the brain that will deduce the existence of depth.

Stereo 3D Shooting

To do that, we use two synchronized cameras, and we send the image of the left camera to the left eye, and the image of the right camera to the right eye. One can play with the interaxial, which is the distance between the center of the lenses of the two cameras, and with the angulation, which is the cameras' “convergence” (be careful, the camera convergence and the eyes convergence are absolutely independent - in fact, they do not serve the same function).

To hold and align the cameras, we use a 3D-Rig. Ther eis two main types of 3D-rigs:

- the side-by-side rig, where the cameras are, obviously, side by side. This kind of 3D-rig allows large interaxials, but not small interaxials (because of the physical size of the cameras). This is mostly useful for large landscape shots.

- the mirror rig, or beamsplitter rig. One camera film through a semi-transparent mirror, and the other films the reflection in the mirror. These rigs allow small and medium interaxials, useful for most shots, but not very large interaxials (because the equipement would be too large and heavy).

Cameras must be aligned pixel-wise, must have exactly the same brightness and color. They must be perfectly in sync, frame-wise and even scan-wise (an external sync box, plugged in the Genlock inputs of the cameras, is very often used to do that).

Broadcasting

To show the image of the left camera only to the left eye and the image of the right camera only to the right eye, we must use a filtering system. The most common system today are the 3D glasses. There is four main types of glasses

-Anaglyph glasses: the infamous red and cyan glasses. The advantage is that they can work on any surface - any type of screen, and even paper. But they're tiring, do not filter very well, do not reproduce well brightness and colors. They also exists in different combinations of colors, which are more adapted to a specific color range.

-Active glasses: the most common system in European theaters (the XpanD system); also used on personal computers by the Nvidia 3D Vision System. The left image and the right image are projected alternately on the screen, 120 (Nvidia) or 144 (XpanD) times per second. The active glasses (also called “shutter glasses”), made of two LCD filters that can be opaque or transparent, follow in sync - shuttering the eye that shouldn't see the image (left or right) projected on screen while opening the “good” eye.

-Passive polarized glasses: the most common system in the Unites States (the RealD & IMAX 3D systems). The two images are projected almost simultaneously on screen, one on top of the other - but the light of one image is polarized in one direction, and the light of the othe rimage is polarised in the other direction. The polarized filters of the glasses show only the right image to the right eye and the left image to the left eye. This system requires a special silver screen.

-Passive spectral division glasses: this system has similarities with the polarised system, but make use of two diffrent sets of primary colors to sort the two images rather than polarized light. This system (the Dolby 3D system) do not require a special screen.

 
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