between natural viewing and s-3D viewing are discussed, as well as the potential consequences of these differences.
4.1 Principles of stereoscopic-3D imaging
By essence, the illusion of depth perception in s-3D cinema is created by presenting a different image to each eye. Each image is physically located on the screen and all the viewers in the room look at the same image pair. In this section, we discuss how these two images can be captured, stored, transmitted digitally, and reproduced to the spectators. The reader uninterested with technical aspects can safely skip directly to the section discussing s-3D perception (Section4.1.4).
4.1.1 Stereoscopic-3D capture
The digital capture of an s-3D image requires the use of two synchronized 2D image sensors. Ideally, the optical systems will be placed side by side, with the lenses separated by a distance similar to that between the eyes of an average person (Figure 4.1(a)). In a review by Dodgson [2004], this interocular distance has a mean value of 63 mm. The distance between the two camera lenses is called the interaxial distance. For example, in the Panasonic AG-3DA1 integrated s-3D camera, the two lenses are separated by approximately 60 mm.
However, it is not always possible to integrate the equivalent of two cameras into one. This might be because the optical systems are too large to approach the mean human interocular distance, or because one wants to control the interaxial distance for artistic reasons. The interaxial distance controls the amount of dis-parity of each depth plane (see Section 6.3 for the mathematical development).
The greater the interaxial distance, the greater the disparity at a given depth. The solution to fine-tune the interaxial distance is to mount two regular 2D cameras on a rig. Usually, one camera in the pair is oriented directly towards the point of interest in the scene, and the other is oriented perpendicular to that direction. A beam-splitter then transmits half of the incident light to one image sensor, and reflects the other half to the second sensor (Figure 4.1(b)).
It is also possible to generate an s-3D image pair from one view only, through a process called 2D-to-3D conversion. However, the discussion of this process is outside the scope of this thesis.
(a) Side-by-side camera. (b) Beam-splitter rig camera.
Figure 4.1: Dual camera stereoscopy: two different s-3D cameras obtained from two regular 2D cameras. After [Lipton,1982].
4.1.2 Stereoscopic-3D transmission
The movie file nowadays is transmitted digitally. In cinemas, the digital equiva-lent of the optical film is the Digital Cinema Package (DCP∗).
For use with a PC, a stereoscopic video file can be created in a large number of ways. The simplest way, which can be achieved using regular 2D video encoding, is to encode each eye into a separate video file, and possibly combine them in a container like matroska† or MPEG’s MP4 (MPEG-4 part 14). However, this method is highly inefficient in terms of storage, because a lot of information in one eye can be found in the other. Therefore, an encoding that specifically supports s-3D (or more generally multiview coding) reduces the file size.
The codec H.264 (MPEG-4 part 10) supports multiview video coding, with its Annex H. A pair of corresponding 2D videos can be encoded in a single s-3D stream with double resolution. Half of the pixels in one frame of this resulting stream represents the left view and the other half represents the right view. One can visualize this by playing the s-3D stream with a regular 2D video player.
The pixels from the left and right views are ordered according to a given rule.
Common rules include side-by-side, top-bottom, and row-interleaved. These rules are illustrated in Figure 4.2. The s-3D stream might be reduced to the same resolution as the original 2D videos, such as 1280 × 720‡ or 1920 × 1080§. This results in a loss of half the resolution in each eye. At the same time, the s-3D video can be processed by certain hardware as if it were a regular 2D video.
∗http://dcimovies.com/specification/index.html, last accessed 30/09/2013
†matroska.org, last accessed 30/09/2013
‡Also known as 720p.
§Also known as 1080p.
4.1. PRINCIPLES OF STEREOSCOPIC-3D IMAGING
(a) Side-by-side. (b) Top-bottom.
(c) Row-interleaved.
Figure 4.2: Common s-3D frame-compatible formats where a white circle represents the sample from one view and a black cir-cle represents the sample from the other view. After [Vetro et al., 2011].
4.1.3 Stereoscopic-3D reproduction to the spectator
In the movie theater, the projection system must “bring” the two different images to the correct eye of the viewer. Three different multiplexing techniques dominate the cinema market for s-3D reproduction, namely polarization multiplexing, time multiplexing, and wavelength multiplexing.
Polarization multiplexing Inexpensive polarization filters can effectively sep-arate two light beams with different polarizations. However, polarization technology requires a special type of screen (silver screen or aluminized screen) which preserve the polarization of the incident light. Linear
po-larization is used in IMAX 3D theaters and circular popo-larization is used in theaters equipped with RealD 3D or MasterImage systems. Circular polar-ization has the advantage over linear polarpolar-ization that the user can rotate his head around its front/back axis without loosing any light intensity. It is interesting to note that s-3D in movie theaters can be obtained with a single projector, and the polarization of the images is time-sequential, thanks to a special apparatus (a modulator called the ZScreen for RealD 3D, and a polarization wheel for MasterImage).
Time multiplexing Time multiplexing is achieved through high frame rate pro-jectors and active liquid crystal shutters in the glasses. The projector alter-nates between left views and right views faster than the frame rate of the movie itself, which is usually 24 frames per second or, very recently, 48. In a technique called triple flash, each view is projected three times, alternating the left and right view, in the lapse of one movie frame. Accordingly, the glasses obstruct the light in one eye and let it pass in the other. Synchro-nization with the projector is achieved using an infrared or a radio signal.
This solution is commercialized for theaters by the company XpanD.
Wavelength multiplexing The idea behind wavelength multiplexing is to use different set of filters for the left image and the right image, at the projector, so that each of the red, green and blue components has a different wave-lengths corresponding to each eye. The glasses are the complement of the filters at the projectors. This system is passive and has the advantage over polarized light that it does not require a polarizing screen. Dolby has com-mercialized this patented technology, developed by Infitec, under the brand Dolby 3D.
The probability to encounter one technology or another depends on the loca-tion of a given s-3D theater. According to data from Jones [2009], the northern American market is dominated by RealD polarized systems (more than 75% of the equipped rooms) while the European market is split between the different technologies: around 50% of theaters are equipped with RealD polarized systems, and the other 50% are almost equally equipped with either XpanD or Dolby 3D systems.
The reproduction equipment is responsible for two important image artifacts, namely crosstalk and flicker. These two image artifacts are discussed in Ap-pendix B.