Passive Stereographics

Written by Paul Bourke
January 2000

Introduction

There are many ways of presenting stereographic images, the two dominant computer based methods are active and passive stereo. Both of these methods use eye-wear, "active" refers to glasses with electronic components, "passive" refers to no electronics. This document serves to both present the considerations when planning a passive stereo system and to document some of the components used in passive stereo installations by the author.

Active Stereo Systems

The higher quality active systems are generally frame sequential, that is, the left and right eye images are presented in rapid succession (typically 120Hz) and the LCD shutters on the glasses go alternatively transparent or opaque in perfect synchronisation with the stereo pairs being presented. Whether the glasses are wired or use infared emitters, the end result is that when the left eye image is being presented the left eye shutter of the glasses is transparent and the right eye opaque. The reverse is true when the right eye image is being projected. There are a number of disadvantages of active systems, the main ones are as follows.

  • The cards that support left and right stereo buffers are typically expensive and are currently lagging behind non stereo capable cards in price/performance.

  • The projectors capable of projecting at the high frame rates are limited to CRT, these are bulky and difficult to calibrate. Even among CRT projectors one needs to install fast phosphor tubes (generally green) to support the 120Hz refresh rate. They particularly don't lend themselves to portable systems or to high brightness levels.

  • Active stereo systems in conjunction with scanning CRT projectors have a common problem with ghosting at the bottom of the imaging area. This is essentially unsolvable without considerable expense, the problem is fundamental due to the finite time required for the electron beam to scan from the top to the bottom of the image.

  • In public unattended environments the active glasses are not ideal because they are quite fragile and not particularly cheap. For more than a couple of viewers the cables associated with the cheaper wired glasses become messy.

Passive Stereo Systems

The most common passive stereographic system uses dual projectors with polaroid filters in front of the projectors and matching filters in the glasses. The projectors can driven in many ways, by a single computer with a multiple graphics pipes, by semi independent but synced computers, synced DVD players, slide projectors, etc. This solves the main problems identified above with active systems. If the system is being driven by computer then one can use non stereo capable hardware from which there is a wide choice from the consumer game industry. There is a wider source of suitable projectors, although now one needs two of them. Lastly but critical for public environments, the glasses cost a couple of dollars instead of hundreds.

In what follows the various options for the different components of a passive stereographics system will be discussed. Particular emphasis and recommendations relate to a passive stereo system installed in a visitors center where the aim was to create both high quality content at a reasonable cost.

The images on the right hand column are from a 20 minute passive stereo show, the content was created by staff at the Swinburne Astrophysics and Supercomputing group.

Linear/circular filters

Linear or circular polarisation are the two main alternatives for passive stereo, they have their own relative advantages. The main advantage of circular polarisers is that the audience can tilt/rotate their heads without losing the effect. However, circular polarisers which are made up of a quarter wave plate and a linear polariser have a wavelength dependence (at the 1/4 wave plate). This results in different ghosting for different colours in the content. There seems to be a significant variation is ghosting with circular polarisers from different suppliers. There is a significant variation in the quality of circular filters and glasses from different suppliers, there is also a wide range of pricing.

Note that for circular polarisers, the direction is reversed on reflection. So, left circular light becomes right circular after reflection from the silver screen.

Linear polarisers seem to give a better signal blocking and are much cheaper and easier to acquire. There can be problems mounting the polaroid filters (generally plastic) close to the projector lens if there is significant heat output, the filters can melt. The same applies if the projector pumps hot air out the front. There are solutions where glass plate filters are installed inside the projectors at the factory. These tend to have a significant price penalty, there is no significant quality difference just convenience of not having to externally mount filters. Some internal filter arrangements also have the advantage of keeping the filters clean.

Projectors

There are numerous projector technologies available. CRT projectors are generally bulky, hard to calibrate, have a low light output, and offer no significant benefits for passive stereo. LCD projectors are problematic because their light is already partially polarised. While there are ways to compensate for this it is generally considered too messy and often requires one to manufacture their own glasses since the filters end up at non standard angles (usually +- 45 degrees). DLP projector technology is suitable, they produce unpolarised light, are bright, and can come in small packages.

The projectors chosen for this project were ASK M5, they have a lens separation of just under 7cm although in the final mounting this was set to about 8cm. Another major benefit of these projectors is their relative brightness given their small dimensions, 1100 ANSI lumens, this is important since the polaroid filters attenuate the final light output. Perhaps the only drawback of these projectors is they don't support component video, S-Video was used from the DVD players.

One common design problem when using dual projectors is the infared remote. While one remote will operate both projectors it is important that they don't get out of sync, one on and one off. If the power button is held down until they are both on or off then problems are avoided. Much better would be two wired remotes or serial ports where a computer or simple serial device can control the power and other settings.

In another installation the more recent SIM2-250 projectors were used. These have serial port control over all features and additionally have lens shift capabilities, see later.

Front/Rear projection

While rear projection has advantages there usually isn't the space. Stewart Screens have a very high quality rear projection screen suitable for 3D applications. We chose their front projection 3D silver screen, dimensions 3.33 wide and 4/3 ratio, this was chosen to completely fill one wall of the theatre. In a later installation we used a 4.8m wide screen. A major advantage of rear projection is that one can use fixed lens shift projectors, one mounted on the ceiling and one on the floor.

Lens shift

A major price break point for DLP projectors is whether they have a fixed or adjustable lens shift, the majority have a fixed lens shift designed for mounting on a table (upright) or on the ceiling (upside down). For rear projection fixed lens shift is fine, one projector is on the floor and the other on the ceiling. For front projection there are two main alternatives, the first is to slightly rotate the projectors so their images overlap (one projector mounted above the other), the other is mount the projectors horizontally and clip the top and bottom of the images appropriately. In either case one wants projectors that can be mounted as close to each other as possible, in the first case this reduces the amount of image clipping, in the second case it reduces the amount of keystone distortion. Note that the digital keystone correction provided on most projectors can't be used to correct for toe-in projectors.

Projector housing

Typically a custom frame needs to be designed and built for the two projectors if using front projection. It needs to allow the projectors to be positioned, shifted, and rotated in order to overlap the two images with a minimal keystone error. A consideration here is to avoid the cooling outlet of one projector blowing directly into the inlet of the the other projector. With lens shift projectors it is common to arrange one projector the right way up and the other upside down.

Room

The room details are site dependent, it depends on the available space and the size of the intended audience. The projector placement depends on the throw specification of the projector. An example is given below.

If is my recommendation that the seats be raised towards to back of the room in order to avoid blockage of the screen. Occlusion by people in the foreground greatly diminishes the 3D effect.

Content Creation

The details of content creation are outside the scope of this document. Most of the content created locally is based upon OpenGL, that way it can be used interactively in our active stereo theatre and well as bundled into a non interactive movie for audience based installations. Other rendering and raytracing packages are also capable of creating stereographic content even though not all of them do it well. At the time of writing both Lightwave and SoftImage had serious problems with their builtin stereo support.

DVD playback

One possible media for precomputed (non interactive) content is two DVD disks, one for the left eye and one for the right. The most critical requirement is for the DVD players to be in sync, this is not possible with run of the mill players. We chose the Pioneer DVD 7400 (7300 is the US model), it is overloaded with features but most importantly it can be controlled over a serial interface and it can be started on a contact switch. The first is used to position the players at the desired start frame, the contact start is used to start the two players at the same time. The players will start 3 or 4 fields after this contact, this means the two streams will start either perfectly together or half a frame out. In the testing of the units there was never any noticeable out of sync effect for the duration of the 20 minute show.

All DVD content was created locally, arranged using Final Cut Pro, converted into QuickTime on the Macintosh and burned onto DVD using the Mac G4 and DVD Studio Pro software. The important aspect to the authoring is to ensure the left and right eye tracks match perfectly. The quality from DVDs is surprisingly good if one is in control of the quality settings, our initial concerns with DVD quality arose from the results created by bureau services.

Computer playback

Computer playback software was developed for both interactive and precomputed stereo movies. The key component was a TCPIP socket based communication protocol between the two computers to keep them in sync. Achieving this for interactive OpenGL based applications is straightforward although it adds another consideration to such applications, namely, keeping the internal state of the model consistent between the machines and performing the display refresh at exactly the same time. Playing stereo movies required special hardware (hard disks and raid striping) in order to achieve the target frame rates of between 25 and 30 frames per second.

   











































































































Projector arrangement options for passive stereo

Written by Paul Bourke
April 2001

Stereoscopic projection using passive polaroid glasses generally involves two projectors and the two projected images need to be overlaid precisely. There are a number of ways this can be achieved, the main options will be discussed here along with their relative merits. The figures on the right are a side view of each option showing the projectors (shaded) and screen (red). Except for the first option (manual lens shift) the discussion of the alternatives is driven by the desire to use consumer grade and lower cost projectors.


Lens shift

This is the optimal method. Some projectors, generally the higher end models, allow the image to be shifted vertically (and sometimes horizontally) on the lens. This is usually a manual adjustment, sometimes motorised, and sometimes electronic. Note however that this is not what lower end projectors refer to as electronic shift, lens shift is a mechanical process where the imaging surface is shifted with respect to the lens. The effect is that the image is moved up/down or left/right on the projection surface without any keystone distortion being introduced. The usual approach is to stack the projectors one above the other and use vertical lens shift to overlap the images. Since this feature is usually only found in the better projectors it comes with a price penalty. The better projectors usually have good optics and this combined with the lens shift feature means that pixel perfect image alignment of the stereo pairs is possible, indeed the non-stereo application for this type of lens shift is image alignment for intensity doubling. A few projector manufacturers or OEM partners provide lower end projectors with a fixed lens shift for brightness doubling or stereoscopic projection, these are normally supplied with the two projectors mounted in a cage since the projector separation needs to match the amount the lens shift.


Tilted projectors

This is the "cheap and nasty" approach. The two projectors are normally stacked one above the other, the top projector is rotated slightly downwards and the bottom projector rotated slightly upwards so that the two images overlap. However this introduces a degree of keystone distortion. There are two ways this keystone distortion can be dealth with, the first is to do nothing but to choose very thin projectors and stack them close together and live with the stereo error that is most apparent near the corners. The other approach is the use digital keystone correction and live with the slightly fuzzier image that results. This is normally perfectly adequate for video/dvd based content but less suitable for computer based graphics. It certainly allows one to employ low cost projectors, often the biggest price tag item in a stereoscopic projection environment.


Digital image shift

This approach can be used when crisp (no electronic keystone correction) images are required and one doesn't mind loosing a little vertical resolution. It does however generally require that one is developing the content being projected. Two perfectly parallel mounted projectors stacked vertically will have projected images that are offset by the projector separation distance. One can simply create content that takes this into consideration, this means creating the images offset and with a black fringe on the top or bottom. Most projectors have a vertical shift that could be used to overlap the two images so the content only needs to have the black fringe added, in practice the vertical shift isn't sufficient. Note that this could equally be applied to projectors arranged horizontally but since most projectors have their fan inlet/outlet arranged horizontally this option isn't recommended.


Top/bottom (rear projection)

This approach uses the fact that the vast majority of projectors have a fixed lens shift arranged so the projector can be mounted on the floor or the ceiling, the facility is provided within the projector to flip the image appropriately. If one projector is mounted on the floor and one on the ceiling then the two images can be overlapped without introducing digital keystone correction. The main limitation of this approach is that it is normally (but not always) only suitable for rear projection. This approach is very rarely used because without very long throw projectors the light intensity gradient that exists vertically is in opposite directions and is usually visually disturbing. Long throw projectors and rear projection is normally out of the question for space reasons although it is possible to use mirrors to reduce this.