May 2001


Spinning projector creates 3D designs

Mark Fletcher once again delves into the world of the designer's 3D environment and discuses a technology which offers multiple-user 3D design visualisation on your desktop

Give a pixel a Z axis and you get a voxel (volume pixel); put 116 million of them together, project them onto a spinning screen and you can create a clear, 3D, full-parallax image of your computer-based design – floating in space – right on your desktop.

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The technology which has made this possible is the result of an extensive collaboration between 3 US companies. It uses lasers to cast points of light onto a spinning target screen (encased within in a hemispherical dome) the light points then 'add up' to create slices through an image which, when combined, form a 3D image which can viewed at multiple angles by multiple users.

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The volumetric display it offers is targeted at those that need to see their designs in real (rather than pseudo) 3D without the use of glasses and other immersive hardware devices. It offers users a non specialised approach as it accepts image information from many standard sources.

Industries expected to benefit from this approach to visualisation include those involved in life sciences, especially in the design and testing of new pharmaceuticals. The display is expected to allow scientists to intuitively identify new drug targets and design new drugs more rapidly. In the world of engineering, one of the current vogues is rapid prototyping. The display hardware can, in many cases, allow engineers to get as much use as possible out of their visual information before committing resources to physical prototypes. The design could simply be projected using the system and appraised in a way similar to that used at the moment. It is also a lot quicker to create than the deposition process used by many prototyping technologies.

Actuality Systems, one of the collaborating companies, has developed patent pending technologies for 3 primary areas of the technology: the 3D image projector; the high-performance volume graphics rendering computer; and the fast, 3D rasterisation algorithms and interface software.

The company offers us a simplified description of how the device works. Likening it to a flip-book movie is a little unfair but it does offer the best way to describe it. Actuality Systems says that it relies on your persistence of vision to blend a sequence of frames into a life-like result.

The projector displays thousands of images per second which are, in effect, slices through the 3D dataset of the design to be visualised. Several mirrors, all rotating together, project these slices through a precise path onto the screen which is spinning with the mirrors. As the screen rotates and the slices are projected at different screen angles (relative to the viewer) a 3D image is created. This projection system not only allows a pixel to be defined by an X,Y co-ordinate but also, due to the dynamics of the screen and mirrors, it can be given a Z co-ordinate – creating the finished 3D image.

Refreshing at 20Hz, the display contains approximately 116 million addressable voxels. This high resolution display requires a commensurately powerful processor and graphics engine as well as large amounts of memory. This graphics handling capability is handled by the system's electronics which include a raster engine, a graphics memory module and a Texas Instruments projector using DLP Technology in the prototype units.

The raster engine processes the 3D geometry and converts it into a format suitable for the projection systems by using proprietary, high-speed rasterisation algorithms. Actuality Systems quotes an example: "The prototype 3D display will create imagery composed of at least 198 radially-disposed 2D slices. Each slice is a region, 768 pixels across. All of those pixels must be read out in the correct time sequence; then, the pixels become voxels which are addressable regions of an approximately spherical display volume."

Over 6Gb of double-data rate SDRAM is used to store the graphical information in a double buffered architecture, this ensures that the image will not flicker when animated. The system requires a bandwidth as high as 1.8Gb/s for images comprised of 200 slices. The algorithms, created and fine tuned by the company to handle this data rate, are used to handle line drawing, triangle drawing and the removal of hidden surfaces which would clutter the display.

Compatibility problems have been overcome by the unit's ability to be connected to standard windows NT and SGI systems via SCSI connections. It will handle 3D information from many OpenGL-based software applications and the company's application programme interface will allow quick application development for both polygon-based and volumetric data.

The unit offers the user a choice of 8 colours to represent the image which can bee seen through 360 horizontally and 180 vertically. It is only 60cm in diameter and just over 50cm high. Further technical specifications such as brightness, contrast, polygons/sec and unit weight are not available as yet as the unit is still in the stages of final development.

The other two companies in the collaboration are Xilinx, the company which invented the field programmable gate array (FPGA), which supplied its Spartan II FPGA; and Avnet a leading supplier of electronic components, which supplied its total customer solution.

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Volumetric displays

The concept of the volumetric display has been around for a number of years. Many prior designs have utilised the idea of a spinning screen – some projecting light down onto a spinning helix, removing the need for the projector to spin.

Most have been limited in pixel count due to processor power which either demanded that image size had to be small or larger images had to be projected at lower resolutions. Now that processor speed and power is getting up to levels which make this technology more viable, readers can expect a number of new concepts which may offer similar functionality and time savings for different parts of the design process. Another limiting factor has been the availability of high-performance digital projectors which are needed to create the vast amounts of voxels.

Actuality Systems offers a number of considerations that a potential user should take into account when specifying volumetric imaging systems. Resolution and fill ratio is dependent on the number of voxels a system can create. Vector scan-based displays give the lowest percentage fill (typically 1%), while raster scan-based systems such as this one give up to 100% usability of the volume. Viewing angle is another consideration, especially if designs are to be assessed by a number of people. Lenticular screens and concave mirror-based displays tend to offer smaller viewing zones. One advantage of this design is that no matter where you view the image from you will get the impression that you are viewing a volume-filling object from that angle.

Some projection systems will have parallax problems where the image jumps, whereas others may create an 'inside out' 3D quality which indicates the possibility that it is a pseudoscopic image. This phenomenon may make certain systems unsuitable for accurate design and medical appraisal. Finally, by adopting the non-specialist approach, i.e. connectivity to commonly used hardware and software protocols, this concept keeps relative costs lower than a system which may demand a dedicated data supply system.

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