3D TVs, Panasonic VT20, stereoscopyTechnically, 3D TVs work in a similar way to LCD 120 Hz screens that are used with PCs, with just a few small differences. Most TVs currently on the market use active shutter-type glasses that are specific to that TV. Each lens is in fact a giant LCD pixel: a shutter covers first one lens, then the other, 120 times a second. At the same time, the screen, which is synchronised to the glasses, displays 120 frames per second. The glasses/screen pairing therefore shows 60 different frames to each eye.
Unlike 120 Hz LCD screens for PC, which use Dual-DVI connectivity (identical to that used by 30-inch screens), 3D TVs receive their signal via HDMI 1.4a ins. From a physical point of view HDMI 1.4a is similar to HDMI 1.3 with identical clocks and bandwidth. The differences come in terms of the protocol which now allows the transfer of the two images required by stereoscopic 3D (for example, the two images one beside the other or above/below). This protocol can be added to older HDMI peripherals, as with graphics cards that had an HDMI 1.3 connector. The amplifiers and HDMI switches must also be updated to correctly transport the stereo signal.
The VT20 comes with two pairs of active-shutter 3D glasses.
When it comes to 3D Blu-rays, the 1080p24 (1920x1080, 24 Hz per eye) format is used. Two additional formats are however defined by HDMI 1.4a, 720p50 and 720p60 (1280x720, 50 Hz or 60 Hz per eye). Although 24 Hz is fine for the cinema, results at this frequency aren’t very convincing in gaming. With the Playstation 3, Sony has limited 3D support to 720p. We’ve tried both modes across a good few titles and only 720p really seems usable to us. Post-processing effects such as motion blur, which simulates cinematic rendering, are moreover incompatible with stereoscopic technologies. We’ll come back to this.
In our tests we used a Panasonic VT20 3D TV. On paper, plasma technology has a signiicant advantage over LCD screens when it comes to stereoscopic 3D because of the absence of ghosting. The time needed to change the colour of a pixel is not fixed on LCD screens and depends on the colour source and destination. This means that when an image contains strong contrasts (light on a dark background), a halo effect appears around the bright object. In practice, the remains of the image for one eye contaminate the image for the other and vice versa. Very noticeable on the first 120 Hz LCD screens, the effect is now more or less so depending on the model.
As you’ll see further on, we came across a bug in Fallout 3 with one of the solutions on the bit where you choose your character. The character and the menu only appeared in the left eye, with the menu and the circle staying black on the right, as you can see below:
This isn’t right of course but it does allow us to check the ghosting of the TV more easily. We took a photo behind the glasses for each eye. You can see them here:
On the right, we should be seeing a perfectly black screen in the circle and the menu page. However, the character and the menu appear due to ghosting, a phenomenon that you can see when wearing the glasses and shutting first one eye then the other. In practice in games, outside of this bug, the problem is visible mainly on the edges of characters. The effect is relatively noticeable on very dark backgrounds.
On the VT20, although ghosting is imperceptible in most cases, it can be seen in games with a lot of contrast (white or very bright colours on black), with the black areas, where the bright object had been, appearing as dark grey, as you can see on the image above. Much in the same way as a film such as Avatar avoided transitions that were too contrasted, some games will be more convincing than others in stereoscopic 3D (depending on their rendering).
Limitations of stereoscopy
Although considered as the key element in what allows us to see in three dimensions, stereoscopy in fact only simulates one part of what is defined as three dimensional vision. And in fact, to a certain way of thinking, you don’t actually need it: if you close one eye and fix your sight on a scene, simply moving your head allows you to make out the respective positioning of objects in relation to each other because of parallax effects (the objects furthest away move more rapidly than closer objects). There are also various other phenomena which come into play, such as recognition of the size of objects (a building is larger than a screwdriver), visual acuity (simulated/exagerated by depth of field effects in modern games) or accommodation (the eye’s focal length).
The brain uses all these signals and those given by stereoscopy to create 3D vision. This combination of signals means that not everyone perceives stereoscopy in exactly the same way. If, for example, you have one eye that sees significantly better than the other, you’ll have difficulties with stereoscopy. Even though the technology has improved a lot over the last few years (higher resolution, sharpness of screens, higher refresh rates and reduction of left/right eye contamination and so on), stereoscopy is still not a solution that works across the board.