Patent Publication Number: US-11029713-B2

Title: Method and system for expanding the range of working environments in which a 3-D or depth sensor can operate without damaging or degrading the measurement performance of the sensor

Description:
TECHNICAL FIELD 
     At least one embodiment of the present invention generally relates to methods and systems for expanding the range of working environments in which 3-D or depth sensors can operate without damaging or degrading the measurement performance of the sensors and, in particular, to such sensors which have rigid support structures and a plurality of optoelectronic components fixedly supported on the support structures. 
     OVERVIEW 
     Available 3-D or depth sensors fail to meet industrial requirements for performance under typical widely varying industrial conditions of temperature and humidity, as well as performance under hazardous industrial conditions such as are specified for hazardous (classified) environments requiring NEMA Type 7 enclosures.
         1. Ambient temperature changes cause sensor measurements to drift. The magnitude of the thermal drift can be more than 1 mm/1° C. as shown in the graph of  FIG. 5 , which makes measurements too inaccurate for industrial use where temperature conditions can vary.   2. Extremes of ambient temperature and humidity mean that simply cooling a sensor below ambient temperature without careful attention to the atmospheric dew point will lead to surface condensation on the sensor, degrading performance and reliability of the sensor;   3. The requirement for operation under Class 1, Division 1, Groups A-d (NEMA 7, and above) conditions imposes a host of restrictions on the sensor design in terms of the sensor&#39;s operational voltage, air-tightness, etc. that cannot be met with off-the-shelf enclosures. The need for transparent optical windows, careful thermal-stabilization, and mechanical positioning accuracy makes off-the-shelf enclosures inappropriate for industrial use.
 
These constraints have limited the use of active 3-D depth sensors in industrial application environments.
       

     In view of the above, there is a need for a low-cost active 3-D depth sensor working accurately over a wide range of temperature and humidity conditions. Active 3-D depth sensors use light projection methods and optical means to interrogate local surfaces yielding depth information from the environment. Tests show that significant measurement instability results from temperature-induced changes in 3-D sensor optics and 3-D structured light projector(s). This makes sensor measurements inaccurate in an environment that experiences temperature change. 
     There is also a need for an active 3-D depth sensor that is capable of working accurately in a volatile working environment. Volatile working environments impose special restrictions on sensor equipment regarding air-tightness, corrosion resistance, low-voltage and explosion-proof operation. Sensor housings that offer this functionality for an active 3-D sensor, with its specific needs respecting temperature control, unobstructed lines of sight, mechanical positioning accuracy and stability, and optical clarity are not currently known to the art. 
     Very little attention or consideration has been given, to this point, to issues of temperature stabilization and housing design for 3-D or depth sensors. 
     SUMMARY OF EXAMPLE EMBODIMENTS 
     An object of at least one embodiment of the present invention is to provide a method and a low-cost system for expanding the range of working environments in which a 3-D or depth sensor can operate without damaging or degrading the measurement performance of the sensor. The sensor has a rigid support structure and a plurality of optoelectronic components fixedly supported on the support structure. 
     In carrying out the above object and other objects of at least one embodiment of the present invention, the method includes providing an enclosure for the sensor, receiving an electrical signal indicative of a temperature of an interior of the enclosure and determining whether the temperature of the interior is within an operational temperature range of the sensor. The method also includes activating at least one thermoelectric device to either cool or heat the interior of the enclosure based on whether the determined temperature is above or below the range, respectively. 
     The support structure may be thermally conductive. 
     The method may further include transferring heat between the support structure and the at least one thermoelectric device. 
     The optoelectronic components may include a pattern emitter configured to project a known pattern of radiation and at least one detector configured to analyze the radiation pattern as it is cast onto a surface. 
     The enclosure may include an optically transparent window through which the sensor projects and receives visible and near visible radiation. 
     Further in carrying out the above object and other objects of at least one embodiment of the present invention, a system for expanding the range of working environments in which a 3-D or depth sensor can operate without damaging or degrading the measurement performance of the sensor is provided. The sensor has a rigid support structure and a plurality of optoelectronic components fixedly supported on the support structure. The system includes an enclosure for enclosing the support structure and the supported optoelectronic components within an interior of the enclosure and a temperature control circuit including a controller to monitor interior temperature within the enclosure and to regulate temperature within the enclosure to be within an operational temperature range based on the monitored temperature. 
     The support structure may be thermally conductive wherein the control circuit includes at least one thermoelectric device controlled by the controller to transfer heat between the support structure and atmosphere external to the enclosure. 
     The at least one thermoelectric device may comprise at least one Peltier device supported by the enclosure. 
     The control circuit may include a temperature transducer positioned within the interior of the enclosure to monitor the temperature within the enclosure. 
     The temperature transducer may comprise a temperature probe. 
     The control circuit may include at least one heat transfer pipe or bar to transfer heat between the at least one thermoelectric device and the support structure. 
     The control circuit may include a heat dissipation structure connected to each thermoelectric device to dissipate heat from each device to atmosphere external to the enclosure. 
     The control circuit may include at least one element to direct air to flow across the heat dissipation structure. 
     The control circuit may include control logic to determine when the interior temperature is within the operational temperature range. 
     The enclosure may be an airtight, thermally-insulated enclosure. 
     The support structure may comprise a thermally-conductive sensor rail. 
     The enclosure may include an optically transparent window through which the sensor projects and receives visible and near visible radiation. 
     The window may be double-paned. 
     The optoelectronic components may include a pattern emitter configured to project a known pattern of radiation. 
     The pattern emitter may include an IR laser diode source, or other laser source configured to operate at visible or near visible wavelengths. 
     The plurality of optoelectronic components may include a camera configured to view a scene at visible wavelengths. 
     This camera may be configured to reject light originating from the laser source. 
     The plurality of optoelectronic components may include one or more cameras (detectors) configured to view the projected known pattern of radiation at the wavelength emitted by the laser source. 
     At least one embodiment of the present invention stabilizes the temperature sensitive sensor components to reduce measurement instability in 3-D sensors by means of temperature stabilization. 
     Thermally-controlling an active, structured-lighting based 3-D sensor requires solving a number of sub-problems including: detecting sensor temperature, insulating the sensor, controlling the temperature of the sensor internals above or below ambient temperatures, and minimizing and/or eliminating water condensation on sensor components as only provided by the present invention as described below. 
     Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions and claims. Moreover, while specific advantages have been enumerated, various embodiments may include all, some or none of the enumerated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view, partially broken away, of an optical sensor enclosed or housed within an enclosure and mounted on the distal end of a robot arm wherein optically transparent windows of the sensor are shown; 
         FIG. 2  is a block diagram of a 3-D or depth sensor and its enclosure and sensor rail and a temperature control circuit constructed in accordance with at least one embodiment of the present invention and being used in an industrial working environment; 
         FIG. 3  is a front perspective schematic view of an assembly or system including an optical sensor or sensor rail, a hollow protective enclosure having an optical window, and a temperature control circuit constructed in accordance with at least one embodiment of the present invention; 
         FIG. 4 a    is a front schematic view of the camera, emitter and detector of  FIG. 2  mounted on a thermally conductive sensor rail, in which a single detector is configured to view the target object  6 ; 
         FIG. 4 b    is a front schematic view of the camera, emitter and detectors of  FIG. 2  mounted on a thermally conductive sensor rail, in which two detectors are configured to view the target object  6 ; 
         FIG. 5  is a graph of measured target distance (mm) versus temperature (° C.), the graph showing typical distance dependence on temperature; and 
         FIG. 6  is a graph similar to the graph of  FIG. 5 , the graph showing warm-up and extreme temperatures. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     In general, the method and system of at least one embodiment of the present invention provides thermal stabilization of optical components in optical 3D or depth sensors to address the need for sensors that operate in a wide variety of industrial conditions or working environments. The method and system actively control, and thus restrict, thermally-induced changes in the shapes, sizes, and other optical properties of active and passive optical components, such as lasers, lenses, mounts, and optical rails, which play a role in the measurement accuracy of 3D sensors based on time-of-flight or disparity calculations. The following methods, which may be combined or taken separately (as environmental conditions require to achieve 3D sensor thermal stabilization), are provided.
         1. Monitor sensor internal temperature near critical optical components in order to tightly regulate internal sensor temperature.   2. Enclose and insulate the sensor internals to protect them from both humidity and temperature differences with the environment.   3. Use a single detector  34  to view the light projected by the emitter  32  onto the object  6 . Using means known to the art, the light pattern viewed by the detector  34  is compared with a stored representation of the light pattern emitted by the emitter  32  in order to calculate the distance to the object  6  at a plurality of points on the surface of the object  6 .   4. Use two or more detectors  34  to view the light projected by the emitter  32  onto the object  6 . The light patterns viewed by the various detectors  34  are compared with one another, using means known to the art, rather than with a stored representation, in order to calculate the distance to the object  6  at a plurality of points on the surface of the object  6 . The use of multiple detectors  34  in this way eliminates the need to consider the effect of temperature on the pattern projected by the emitter  32 .   5. Regulate sensor temperature by pumping heat into, or out of, the sensor&#39;s internals via an active heat transfer process. As shown in  FIGS. 2-4 , a main optical support rail  20  of a 3-D or depth sensor  10  is connected to Peltier heater/cooler elements  22  via highly conductive heat transfer piping  24 . The Peltier devices  22  actively provide cooling or heating to the sensor rail  20  as directed by the sensor temperature controller. The sensor rail  20  distributes heating or cooling throughout the sensor internals leading to optical temperature stabilization. The main optical support rail  20  is a fixed, rigid internal structure to which internal electro-optical components (i.e. camera  30 , emitter  32  and one or more detectors  34 ) of the sensor  10  are mounted. The purpose of the main optical support rail  20  is to maintain a fixed design geometry between the electro-optical components  30 ,  32  and  34 . The main optical support rail  20  is called the ‘sensor rail’ for short. By making thermal contact with the sensor rail  20 , the geometry of the sensor rail  20  is stabilized and the components ( 30 ,  32  and  34 ) which are mounted upon it are stabilized.
           a. The operation of the sensor  10  is characterized by calibrating it at its stabilized temperature, so excursions from optimum operation due to thermal drift are minimized or eliminated.   
           6. The sensor  10  is capable of operation in hazardous environments by providing an air-tight enclosure  21  with low-voltage operation, insensitivity to corrosive gases, and no external stators or other electro-mechanical components capable of producing ignition sparks. The air-tightness of the enclosure  21  serves both to isolate the sensor internals from ambient temperature excursions, and to protect the sensor  10  from corrosive gases.   7. The dimensions and the material of the air-tight enclosure  21  itself is carefully chosen from thermal modeling calculations to meet the simultaneous requirements of:
           a. Providing an optically-transparent window or windows  9  through which the sensor  10  may project and receive visible and near-visible radiation; and   b. Ensuring that the temperature of the outer surface of the enclosure  21  does not fall below the atmospheric dew point within the environmental, operating range of the sensor  10 .   
               

     One embodiment of a system of the present invention is shown in  FIGS. 1-4 . A thermally isolated box or enclosure  21  is made of 0.5″ polycarbonate plastic with glass wool (not shown) filling the space between the sensor  10  and its optical or sensor rail  20 . One or more optically transparent, double-paned windows  9  made of sapphire or coated high-quality polycarbonate plastic is provided through which visible and near-visible light may be projected and received by the sensor  10 . A pair of Peltier coolers  22  mounted at the outer surface of the box  21  exchange heat from the optical rail  20  with the external atmosphere via copper heat pipes  24  and passive cooling fins  26 . The coolers  22  are controlled by a separate temperature control unit or controller that uses input from a temperature probe  28  to determine if the system should be heated or cooled. The enclosure  21  and all electrical cabling passing through the enclosure  21  are designed to be air-tight. 
     A first issue resolved by at least one embodiment of the present invention is that the sensor  10  needs an optically transparent field of view. However, the window  9  must not compromise the thermal isolation of the sensor  10 . 
     The viewing issue is resolved by using material for the window  9  that is transparent to light in the range 350 to 850 nm to form the double-paned window  9  on one side of the box  21 . This makes the window  9  transparent to both the visible and near-visible light. Making the window  9  double-paned raises the thermal resistivity of the window  9  to the point where the thermal independence of the sensor  10  can be maintained, while minimizing condensation on the outer pane and minimally affecting the optical clarity of the optical path. Materials to use for the window  9  include glass, polycarbonate, quartz, and sapphire (for corrosive atmospheres where transparency of a plastic window may be affected by gases). 
     Another issue is that the channels used to pump heat typically need to have high thermal conductivity as well as allow for the system to be airtight. 
     Heat transfer from sensor internals to energy dissipating units can be done via the heat pipes  21 . Highly conductive materials such as copper or aluminum bar are a preferred material. Copper has a high thermal conductivity of 401 W/(m*K) which allows for very low resistance pumping. The energy transfer pipes, or bars  24 , make direct thermal contact with the internal optical rail  20  (typically made of aluminum), and form an airtight seal with the outside of the enclosure  21 . One or more Peltier coolers  22  make direct thermal contact with the external surface of the heat pipes  24 . The heat pipes  24  are designed in such a way such that the cross sections of the copper bars or pipes  24  matches the cross-section of the Peltier cooler  22  to maximize thermal efficiency. The bars  24  are coated with thermally insulating material in order to maximize energy transfer rates into/out of the sensor  10  as needed. 
     Another issue is that when connecting the heat pipes  24  to the sensor rail  20  there are multiple possible connection sites. The connection site(s) are chosen to maximize the heat pumping efficiency. 
     In a preferred embodiment, the heat pipes  24  are preferably connected to the main sensor rail  20  in two places, directly above the IR sensor  32 , and directly above the RGB camera  30  and IR laser projector  32 . Connecting the heat pipes  24  to the top of the sensor rail  20  maximizes contact area allowing for greater heat flow between the two and less thermal resistance. The pattern emitter  32  and cameras  28  and  30  are the greatest sources of heat generated by the sensor  10 . By connecting the heat transfer pipes  24  adjacent these devices  28 ,  30  and  32  more control of the temperature distribution of the sensor rail  20  is obtained. 
     Another issue is that by having the sensor temperature differ from the ambient temperature one runs the risk of condensation forming on the housing exterior. This has the potential to obstruct the sensor&#39;s view if condensation occurs on the window  9 . Condensation is detrimental to the lifespan of the sensor  10 . Typical adverse conditions are 40° C. and 90% humidity. 
     To prevent condensation, at equilibrium the exterior surface of the enclosure  21  must be at a temperature above the dew point of the ambient environment. This dew point is dependent on both the temperature and humidity of the environment. For example, the dew point is ˜38° C. for an ambient temperature of 40° C. and 90% humidity. The method for achieving an exterior surface temperature above this point is to maximize the casing&#39;s thermal resistance by packing the interior of the sensor enclosure  21  with insulating materials such as glass wool (not shown). Thermal modeling shows that for a 0.5″ polycarbonate box or enclosure  21  and glass wool filler of 0.5″ thickness, or greater, if the interior temperature is kept at 27° C., under the worst conditions (40° C. and 90% humidity) the exterior surface temperature is above the dew point of 38° C. 
     Still another issue is that the heat generated and extracted by the Peltier coolers  22  may need to be dissipated to the surrounding environment. 
     There are three possibilities for doing this: passive external cooling fins, active fan-assisted cooling fins, and externally-controlled air sources to expedite heat dissipation. Depending on requirements, one or more of these following methods might be used.
         1. Passive external cooling fins (i.e.  26  in  FIG. 3 ) are the easiest to implement and do not require external power or controlled air sources. Moreover, the fins  26  require no energy to work and thus are safe for hazardous locations. One drawback is the spatial footprint required for the fins  26 . Further this footprint may need to change based on the environmental operating conditions.   2. Active, fan-assisted cooling provides a small compact form factor capable of dissipating more energy than passive cooling fins of the same size. Thus, active fans provide a greater degree of control over the system energy transfer rates, as well as being able to work in tighter spaces, than passive cooling fins. The drawbacks are that the system has moving components that can break, uses electricity which requires power, and presents electrical sparking challenges that may be difficult to overcome in hazardous locations where explosive compounds may be present at dangerous levels.   3. Externally-controlled air sources provide the benefit of increasing energy transfer rates by blowing air through passive fins or active fans, or across the sensor housing  21  itself. Much greater cooling efficiencies for passive fin or active, fan driven cooling methods are achieved. External air sources can easily be combined with either methods 1 or 2 above, as appropriate.       

     Yet another issue is that in order to keep the optical properties of the sensor  10  fixed one needs to be able to monitor whether the sensor  10  is within its operational temperature range. If the internal temperature is too high, the system needs to be cooled, and if it is too low, it needs to be heated. Knowing whether the sensor  10  is in the operational temperature range is particularly important just after turning the system on. Immediately after startup the internal temperature will most likely be outside this temperature range and will require a warm-up/cool-down period to adjust itself as shown by the graph of  FIG. 6 . 
     This issue is resolved by actively monitor the internal sensor temperature with one or more temperature probes  28 . The digital temperature controller uses data from the temperature probe(s)  28  to command that the Peltier chips or devices  22  add or remove heat from the system, as needed to stabilize the internals within a particular range (e.g. 0.2 degrees Celsius, in a preferred embodiment) of a temperature set point. 
     The sensor electronics are configured so that, if the sensor&#39;s internal temperature is outside the operational temperature range of the sensor  10  (for example, during the initial warm-up or cool-down phase upon initial start-up as shown in  FIG. 6 ) the sensor electronics may provide a READY/NOTREADY signal which may be used to determine when the sensor  10  can be used to measure with its designed accuracy. 
     The temperature control circuit (including temperature probe(s), peltier cooler(s), heat pipe(s), heat dissipation fin(s) and/or fan(s), and control electronics) can be configured to drive the Peltier coolers  22  to both heat and cool the system as needed. The warming-up/cooling-down phase typically lasts around 30-40 minutes and the software or control logic of the temperature controller indicates when it first enters the operational temperature range. 
     The system of at least one embodiment of the present invention includes a 3-D or depth sensor such as 2.5D volumetric or 2-D/3-D hybrid sensor.  FIG. 1  shows a housing or container  21  of the sensor  10  mounted on the distal end of a robot arm  12 .  FIG. 3  shows the sensor  10  positioned near a vehicular body  8  including a hood  6  in an industrial work environment. 
     The sensor technology described herein is sometimes called “3-D” because it measures X, Y and Z coordinates of objects within a scene. This can be misleading terminology. Within a given volume, these sensors only obtain the X, Y and Z coordinates of the surfaces of objects; the sensors are not able to penetrate objects in order to obtain true 3-D cross-sections, such as might be obtained by a CAT scan of the human body. For this reason, the sensors are often referred to as 2½-D sensors which create 2½ dimensional surface maps to distinguish them from true 3-D sensors which create 3-D tomographic representations of not just the surface, but also the interior of an object. 
     In spite of this distinction between 2.5-D and 3-D sensors, people in the vision industry will often speak of 2.5-D sensors as 3-D sensors. The fact that “3-D Vision” sensors create 2.5-D surface maps instead of 3-D tomographs is implicit. 
     Referring again to  FIG. 2 , preferably these sensors each comprise a pattern projector or emitter  32  operating at some wavelength one or more cameras or detectors  34  configured to receive light at wavelength and a visible light, monochromatic or color camera  30  configured to view light at visible wavelengths, but to reject light near wavelength ‘L’. The pattern is projected by the emitter  32  onto the surface of the vehicle and is read by one or more detectors  34  along with the information from the visible light camera  30 . The laser projector  32  operates by means of diffractive optical elements to project several tens of thousands of laser pencils or beams onto a scene to be analyzed. The detector  34  analyzes the scene at wavelength  1 ′ to locate the intersections of the laser pencils with the scene and then uses geometry to calculate the distance to objects in the scene. The visible light camera  30  in a preferred embodiment is used to associate a color or monochrome intensity to each portion of the analyzed image. 
     The pattern emitter  32  may be comprised of an infrared laser diode emitting at 830 nm and a series of diffractive optics elements. These components work together to create a laser “dot” pattern. The laser beam from the laser diode is shaped in order to give it an even circular profile then passed through two diffractive optics elements. The first element creates a dot pattern containing dots, the second element multiplies this dot pattern into a grid. When the infrared pattern is projected on a surface, the infrared light scattered from the surface is viewed by one or more detectors  34  configured to be sensitive in the neighborhood of 830 nm. 
     In addition to the IR sensor  34 , there may be the RGB sensor or camera  30  configured to be sensitive in the visible range, with a visible light, band-pass filter operative to reject light in the neighborhood of 830 nm. During operation, the IR sensor  34  is used to calculate the depth of an object and the RGB sensor  30  is used to sense the object&#39;s color and brightness. This provides the ability to interpret an image in what is traditionally referred to as two and a half dimensions. It is not true 3-D due to the sensor  10  only being able to detect surfaces that are physically visible to it (i.e., it is unable to see through objects or to see surfaces on the far side of an object). 
     Alternatively, the 3-D or depth sensor  10  may comprise light-field, laser scan, time-of-flight or passive binocular sensors, as well as active monocular and active binocular sensors. 
     Preferably, the 3-D or depth sensor  10  of at least one embodiment of the invention measures distance via massively parallel triangulation using a projected pattern (a “multi-point disparity” method). The specific types of active depth sensors which are preferred are called multipoint disparity depth sensors. 
     “Multipoint” refers to the laser projector or emitter  32  which projects thousands of individual beams (aka pencils) onto a scene. Each beam intersects the scene at a point. 
     “Disparity” refers to the method used to calculate the distance from the sensor  10  to objects in the scene. Specifically, “disparity” refers to the way a laser beam&#39;s intersection with a scene shifts when the laser beam projector&#39;s distance from the scene changes. 
     “Depth” refers to the fact that these sensors are able to calculate the X, Y and Z coordinates of the intersection of each laser beam from the laser beam projector  32  with a scene. 
     “Passive Depth Sensors” determine the distance to objects in a scene without affecting the scene in any way; they are pure receivers. 
     “Active Depth Sensors” determine the distance to objects in a scene by projecting energy onto the scene and then analyzing the interactions of the projected energy with the scene. Some active sensors project a structured light pattern onto the scene and analyze how long the light pulses take to return, and so on. Active depth sensors have both emitters and receivers. 
     For clarity, the sensor  10  is preferably based on active monocular, multipoint disparity technology as a “multipoint disparity” sensor herein. This terminology, though serviceable is not standard. A preferred monocular (i.e., a single infrared camera) multipoint disparity sensor is disclosed in U.S. Pat. No. 8,493,496. A binocular multipoint disparity sensor, which uses two infrared cameras to determine depth information from a scene, is also preferred. 
     Multiple volumetric sensors may be placed in key locations around and above the vehicle. Each of these sensors typically captures hundreds of thousands of individual points in space. Each of these points has both a Cartesian position in space and an associated RGB color value. Before measurement, each of these sensors is registered into a common coordinate system. This gives the present system the ability to correlate a location on the image of a sensor with a real world position. When an image is captured from each sensor, the pixel information, along with the depth information, is converted by a computer  12  (i.e.  FIG. 2 ) into a collection of points in space, called a “point cloud”. 
     A point cloud is a collection of data representing a scene as viewed through a “vision” sensor. In three dimensions, each datum in this collection might, for example, consist of the datum&#39;s X, Y and Z coordinates along with the Red, Green and Blue values for the color viewed by the sensor  10  at those coordinates. In this case, each datum in the collection would be described by six numbers. To take another example: in two dimensions, each datum in the collection might consist of the datum&#39;s X and Y coordinates along with the monotone intensity measured by the sensor  10  at those coordinates. In this case, each datum in the collection would be described by three numbers. 
     The computer  12  of  FIG. 2  controls a controller which, in turn, controls a processor, the temperature controller, the camera  30 , the emitter  32  and the detector  34  of the sensor  10 . 
     At least one embodiment of the present invention uses a hybrid 2-D/3-D sensor  10  to measure color, brightness and depth at each of hundreds of thousands of pixels. The collective 3-D “point cloud” data may be presented on a screen  16  of a display  14  (i.e.  FIG. 2 ) as a 3-D graphic. 
     The field of view of the 2-D/3-D sensor  10  can be as wide as several meters across, making it possible for the user to see a hinged part such as a door or the hood  6  relative to the vehicle body  8  in 3-D. The graphic on the screen  16  may look like the 3-D part the user sees in the real world. 
     Embodiments of the invention can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment containing both hardware and software elements. In a preferred embodiment, the invention including control logic is implemented in software, which includes but is not limited to firmware, resident software, microcode, and the like. Furthermore, the invention can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. 
     For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
     A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. Input/output or I/O devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.