|Not a member yet? Register
for full benefits!
Stereoscopic Display Methods
Visual stereoscopy comes in many flavours. Depending on how the video feed
is constructed, and whether or not it was originally designed with stereoscopy
in mind, the technology used to deliver a slightly different positional viewpoint
to the eye, differs greatly.
Stereoscopy at its heart is about delivering in pairs. Most of us possess two
eyes, and for those who do not, work is continually underway to create artificial
replacements. These two eyes sit in general, about two inches apart, measuring
from mid point to mid point. This means that particularly for close objects,
each sees a different angle, and thus a different view. When the two images
are combined we gain a 3D, spatial perspective of the object. If this spatial
location sense is missing, i.e. by delivering the exact same image to both eyes,
then our sense of spatial location ceases to function. Realism and believability
drop through the floor, and interaction becomes a whole lot harder.
Virtual Spatial Display
In VR, from a developer's point of view, the easiest way of producing stereoscopy
is also the most computationally expensive. Normally, each user has a viewpoint
into the virtual environment, a cone of sight that opens up towards the horizon
or back clipping plane, and which contains a rendered view of the world.
To enable stereoscopy, a second view frustrum must be opened up alongside the
first, with a second viewpoint feeding it. Instantly, you are performing twice
the work of rendering the frame, as you must render each frame from scratch
twice. Predictably, this halves the frame rate immediately. Where you might
have been hitting 72 fps before, you are now only getting 36. Older computer
hardware in particular does struggle, as where they may have been seeing a choppy
but still usable 10fps before, now they see only 5.
Because of this, most publicly accessible virtual environments do not utilise
stereoscopy by default. However, many do have the option, or capability to add
it. The frame rate hit can itself be largely negated by doubling the GPU capabilities
on a computer system, and many PCs set up for stereoscopy run two graphics accelerator
cards in parallel.
The problem then, is transferring the two separate viewpoints from the graphic
accelerator cards to the user. This requires specialised hardware, as a standard
monitor display cannot handle more than one viewpoint, at a time, in most situations.
It can handle two viewpoints together in some situations, but again specialised
viewing hardware is necessary in addition, to decode the images produced.
head mounted display is the oldest of the stereoscopic displays. Originally
not even head mounted, but held on a ceiling or floor mounted boom due to
their immense weight, they consist of a unit which is worn on the head. The
originals were like crash helmets with ovens strapped on top. Modern variants
are not much bigger or heavier than a pair of spectacles.
A stereoscopic HMD has a separate monitor display for each eye. Typically
these are LCD or plasma based nowadays, and recreate the world for that eye's
point of view only, blacking out the glare of the image as it passes over
the nose, and blacking out outside light. Sometimes, as with the HMD pictured,
the light suppression system is removable so that the HMD can be used without
restricting access to the outside physical world.
glasses work with shutters that switch on and off many times a second, alternating
between the eyes. This happens too quickly for your brain to process it, but
forces a situation whereby whenever one eye is looking at the display, the
other is looking at a black, closed shutter. The shutter glasses and the display
system ? monitor, holoprojector, retinal laser etc ? work in tandem, either
through wires, or more commonly now, Wi-Fi. The shutter glasses sync their
shuttering speed precisely to the refresh rate of the display system, so that
the display can show images appropriate to the left and right eyes, at appropriate
They are a counter to the problems with doubling the view frustrum. Rather,
the graphics hardware alternates between the two views like toggling a switch.
The result is you still perceive the same fps as a single display, but you
are seeing it through alternate eyes.
Thankfully, this trick works because the brain is intelligent enough to piece
the images together seamlessly.
glasses are basically those 'cheesy' glasses the public generally thinks of
when they picture 3D.Anaglyphs are based on a visual display method that uses
colour differentiation to take a single video stream and split it into two
channels before it reaches the eye. Typically it utilises filter glasses that
mask one eye with a red tint, and the other with a blue or green tint. When
a video source is played that is half green, half red, slightly offset, each
filter allows one half of the image to pass. The two images are the same scene,
slightly offset from one another, which again the brain reassembles into a
single stereoscopic image.
An advantage of anaglyph glasses is that from one displayed image source,
an entire roomful of people with glasses can see the 3D effect.
glasses are not really so different from shutter glasses. The key difference
is that both sides are half-shuttered all the time; they do not flick off
and on. Instead they are polarised such that each half can only accept light
beams whose wavelength is in a precise alignment. Typically, the left eye
looks through vertical polarizing film, right eye looks through horizontal
Two projectors are used with polarised glasses, bringing us back to the two
view frustrums issue and a halving of fps again. One projector projects light
which has a vertical waveform, the other projector, a horizontal waveform.
The two images are out of sync with one another, with one image destined for
the left eye, the other for the right eye. If a user looks at the projected
images without the filter, they see a confusing jumble of two not-quite-aligned
images superimposed and offset, for every item on the display, in the sam
manner as occurs with anaglyphs.
An advantage of polarised glasses is that like with analgyphs, an entire
room full of users can witness the same image, passively in 3D from a single
projector source. Of course, this analogy fails if the view is to be interactive.