Stereoscopic 3D is a huge topic in the film industry right now. All the studios seem to have a stereo film in production whether it be live action or animated, Siggraph had a whole day devoted to it and Dreamworks has said that even during production it only ever views its films in stereo.
So why is it making such a strong comeback after being discarded in the 1980s? One reason is that the technology has improved. The best technology from the 1980s had a film that was printed with the left eye image above the right eye image on a single filmstrip. The film was shown through a projector with an adapter on the lens that projected the images superimposed on one another through orthogonal polarising filters. The audience wore passive linearly polarised glasses where the left lens extinguished the right eye image and vice versa. The audiences complained of headaches and eyestrain that were primarily caused by two flaws in the technology.
Firstly the linear polarization required the viewer to keep their eyes horizontal; if they tilted their head then the image from one eye would leak into the other. Secondly, the projectors were mechanical devices containing cogs and belts, and this inevitably caused some jitter in the movie. Slight variation on a non-stereo film is not a problem, but on a stereo on the jitter confuses the brain and causes headaches.
Moving to digital cameras that have no moving parts has solved the problem of image jitter. To solve the head-tilt problem the technology moved away from linearly polarised filters to circularly polarised filters. The filter can be polarised with either left or right-handedness and the viewers wear low-cost glasses containing analysing filters (a circular polariser mounted in reverse), where one lens extinguishes the right-handed polarised light and the other lens extinguishes the left-handed polarised light. This technique doesn’t exhibit image leakage when the viewer’s head tilts.
All in all the new technology is good and is generally able to convince the brain of the stereoscopic nature of the image.
There is, however, one important cue that the brain uses to determine depth that this system is unable to simulate. When the eyes focus on a nearby object the eye muscles contract. The more they have to contract, the closer the object is. Yet when viewing a stereoscopic image, the eyes are always focused on the projector screen and so the brain gets confused signals as to the depth of an object. Visually an object may appear to be very close to the viewer but the eye muscles are telling the brain that they are still focused on the projector screen in the
It is because of these mixed signals that scenes where objects appear to come out of the scene and close to the viewer are kept to a minimum for fear of causing headaches.
What’s interesting to us in games, is that this polarisation technology is starting to make its way into the living room. Soon there’s going to be an update to Blu-ray standards that will allow it to encode stereoscopic movies – now there are increasing numbers of polarising LCDs that can display them.
The polarising LCDs are normal ones with line sequential micro-polarisers bonded to the display. These micro-polarisers change the polarity of each scan-line so that the even lines are left-circularly polarised and the odd lines are right-circularly polarised.
So to render your game in stereoscopic 3D on one of these displays, you need to render the game from the point of view of each eye and interlace the two images so that the left eye image is on the even lines and the right eye image is on the odd-numbered lines. If your game supports split screen then you’re already doing the hard part and it is a simple extension to render the left eye in the top horizontal split and the right eye in the bottom horizontal split. A small piece of shader code (figure 1) will interlace them together, and six lines of code to support 3D stereoscopic in your game isn’t bad.
That’s not quite the end of the story. The film makers will say that stereo is an art rather than a science, and that they are able to tweak the stereo separation in each scene to ensure the viewer doesn’t get eyestrain. Unfortunately we don’t have that luxury in games, so a lot of experimentation is required to find out what looks good for your particular title.
David Jefferies started in the industry at Psygnosis in Liverpool in 1995, eventually working on Global Domination and WipEout 3. He later moved to Rare where he worked on the Perfect Dark and Donkey Kong franchises. Next came a move down to Brighton to join Black Rock Studio (which was then known as Climax Racing) in 2003. On this generation of consoles he’s been the technical director of MotoGP’06 and MotoGP’07 before starting work on new racer Split/Second.