Patent Publication Number: US-9904503-B1

Title: Method and system for application broadcast

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application is a continuation of and claims priority from patent application Ser. No. 14/922,822 titled METHOD AND SYSTEM FOR APPLICATION BROADCAST, filed Oct. 26, 2015, which is a continuation of and claims priority from patent application Ser. No. 14/290,302 titled METHOD AND SYSTEM FOR APPLICATION BROADCAST, filed May 29, 2014, now issued U.S. Pat. No. 9,171,349, which is a continuation of and claims priority from patent application Ser. No. 13/675,138 titled METHOD AND SYSTEM FOR APPLICATION BROADCAST, filed Nov. 13, 2012, now issued U.S. Pat. No. 8,754,898 which is a continuation of and claims priority from patent application Ser. No. 12/562,503 titled METHOD AND SYSTEM FOR APPLICATION BROADCAST, filed Sep. 18, 2009, now issued U.S. Pat. No. 8,310,493, which is a continuation of and claims priority from patent application Ser. No. 10/325,811, titled A METHOD AND SYSTEM FOR APPLICATION BROADCAST, filed Dec. 18, 2002, now issued as U.S. Pat. No. 7,616,208, issued Nov. 10, 2009, each of which is incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to application broadcast and sharing, and more particularly to a method and system for broadcasting an application. 
     BACKGROUND OF THE INVENTION 
     Internet-related networks (e.g., the World Wide Web (Web)) may be used for application broadcast and sharing conferences. An application broadcast and sharing conference may consist, for example, of a conference moderator who may have a particular application (user-oriented, specific-function software) running on a digital processing system (DPS). The conference moderator may wish one or more conference participants, located at remote digital processing systems, to be aware of the moderator&#39;s interaction in regard to the application. Potentially, sharing an application may extend to allowing one or more participants to take control of the application from the moderator. Such application broadcast and sharing, via the Web, has increased dramatically in recent years. 
     Typically, application broadcast and sharing is accomplished by making the pixel data from the moderator&#39;s DPS available on a server DPS which transmits this data through the Internet to remotely locate participant (client) DPSs. The participant DPSs receives the data and causes the corresponding images to be displayed for the participant at the participant&#39;s DPS. Typically, the participant DPS is executing a streaming media playback software, such as Real Player from RealNetworks Inc. of Seattle, Wash. Streaming allows the data to be processed as a steady stream in real time. The presentation from the moderator&#39;s DPS is transmitted to the server on an on-going basis. For example, the system may periodically conduct a screen scrape. A screen scrape may be conducted by performing an exclusive OR (XOR) operation between the pixel data from the current screen and the pixel data from the last screen sent to the server. This XOR operation causes all the pixels that have not changed since the last screen scrape to become blackened, and the rest colored. The server keeps a queue of the screen scrapes, which is a history of what has changed on the moderator&#39;s DPS. To reduce the amount of data that must be stored on the server, the screen scrapes are compressed. For example, a run-length encoding (RLE) compression scheme may be used to compress the data. RLE compression provides a high rate of compression for situations such as this where there are large runs of identical values. The runs are compressed to the value, and a number indicating the length of its run. 
     The participant DPSs download data from the queue as fast as they are able based on the connection and processing capabilities (speeds) of the individual participant DPS. The streaming provides an animation effect, but may allow some participants to fall behind the rest of the conference if their connection and processing speeds are not adequate. Such systems take into account that the Internet is not an ideal medium for real time communication and therefore add buffering which helps to provide a coherent display. This buffering adds a delay of several seconds. Moreover, some current systems queue and create image buffers on a per user basis so that as more participants join a conference, it becomes more difficult for the server to manage the outgoing stream. That is, as the number of participant DPSs is increased, the hardware requirements for a given server DPS may become too great, requiring distribution through streamsplitting. Such systems may split the data stream among several servers to distribute the load to effect scalability. Each streamsplit adds latency to the system reducing the real time affect. 
     Such methods are inadequate for real time conferences in which a telephone system may be employed to describe the application. That is, as the moderator is describing the application over a telephone system, the participants should be viewing those aspects of the application to which the moderator is referring. Delays caused by buffering or scalability factors (e.g., streamsplitting) have a disconcerting effect upon the conference. 
     SUMMARY OF THE INVENTION 
     A method is described for application broadcasting. A display screen of a moderator digital processing system is divided into a plurality of tiles, the display screen containing a plurality of pixels, each tile containing a portion of the plurality of pixels. A display data segment corresponding to each tile is created using a bitmap of the tile stored in memory, each display data segment containing a pixel data of the corresponding tile. The display data segment is stored on a server digital processing system. Each tile is evaluated to determine if the pixel data of the tile has changed. An updated display data segment corresponding to each tile for which the pixel data has changed is created, each updated display data segment containing an updated pixel data of the corresponding tile. The updated display data segment is stored on the server digital processing system such that the corresponding stored display data segment is replaced. 
     Other features and advantages of the present invention will be apparent from the accompanying drawings, and from the detailed description, that follows below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not limitation, by the figures of the accompanying drawings in which like references indicate similar elements and in which: 
         FIG. 1  is a system block diagram of an application broadcast and sharing conferencing system in accordance with one embodiment of the present invention; 
         FIG. 2  is a process flow diagram of a method by which pixel data of a moderator DPS display screen is stored to a server and periodically updated; 
         FIG. 3  illustrates a segmented moderator DPS display screen and the corresponding display data segments stored on the server in accordance with one embodiment of the present invention; 
         FIG. 4  is a process flow diagram of a method by which a server DPS transmits updated moderator DPS display screen data to a participant DPS, in accordance with one embodiment of the present invention; 
         FIG. 5  is a process flow diagram of a method by which a participant DPS receives a current representation of the moderator DPS display screen in accordance with one embodiment of the present invention; and 
         FIG. 6  is a block diagram of a DPS that may be used in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and apparatuses are described for application broadcast and sharing. For one embodiment, pixel data being broadcast via the Internet from a moderator DPS to one or more participant DPSs is segmented into tiles. Display data is generated for each tile. Each display data segment (the display for each tile) contains the pixel data for the tile, which may be compressed, and also contains a tile identifier that may also indicate the tile&#39;s relative display screen position. The display data segments are periodically updated by determining, for each tile, whether the pixel data for that tile has changed. If so, the display data segment for the tile is replaced with the most recent display data segment corresponding to that tile. A time indicator is also included within each display data segment for each tile indicating the time at which the display data segment for that tile was updated (replaced). Only display data segments representing the current version of each tile are available on the server at any given time. A participant DPS requests data from the server and provides the time indicator for the last display data segment it received. The server then transmits the current display data segment for each tile that has been updated subsequently. This allows presentation at the participant&#39;s DPS of the most current version of the moderator&#39;s display screen. 
     In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
       FIG. 1  is a system block diagram of an application broadcast and sharing conferencing system in accordance with one embodiment of the present invention. System  100 , shown in  FIG. 1 , includes a moderator DPS  105  and a server DPS  110 , which are coupled, through Internet  115 , to a plurality of local or remote participant DPSs  120   a ,  120   b , and  120   c . The Internet  115  is a network of networks through which information is exchanged by methods well known to those skilled in the art (e.g., the use of TCP and IP protocols, etc). Typically, the moderator DPS  105  would be presenting an application. The application could be any type of user-oriented, specific-function software, for example, a Windows application available from Microsoft Corporation of Redmond, Wash. 
     A display screen of the moderator DPS  105  is divided into a plurality of tiles. Each tile represents a specific segment of the display screen and corresponds to a specific portion of the display screen pixels. For one embodiment the tiles are configurable, fixed-length, rectangular, pixel blocks (e.g., 128 pixels by 32 pixels). The pixel data for each tile, which may be compressed, is stored on the server DPS  110  as a display data segment. This provides a representation of the moderator DPS  105  display screen data for access by each of the participant DPSs  120   a ,  120   b , and  120   c . An algorithm is employed to quickly discern if any of the pixels within a tile has changed, if so, the new pixel data for that tile is used to create a new display data segment that replaces the display data segment for that tile on the server. Thus, the display screen representation stored on the server DPS  110  is updated. The server DPS  110 , at any given time, is storing only the current version of the pixel data for each tile and therefore is storing only one representation of the moderator DPS  105  display screen. The server DPS  110  is therefore not burdened with storing historical information regarding the display screen, and image buffers are not created for each participant. That is, the amount of memory used on the server DPS  110  is limited to that required to store the current display screen pixel data (i.e. display data segments for each tile) and is independent of the number of participants accessing the data. Thus, greater scalability is possible without introducing additional latency. 
     Included with the pixel data, in the display data segment for each tile, stored on the server DPS  110 , is a time indicator (e.g., timestamp) that indicates the last time the display data segment was updated (i.e., the last time the pixel data for the tile changed). When a participant DPS accesses the data, the server accesses the last time stamp from that participant DPS which is stored on the server. The server DPS  110  then transmits only those display data segments having a later time indicator. However, a tile may be updated several times before the participant DPS has a chance to download the display data segments. In this way the participant DPS receives and processes only that data necessary to display the current version of the moderator DPS display screen. 
     Further details concerning the operation of moderator DPS  105 , server DPS  110 , and the participant DPSs  120   a ,  120   b , and  120   c  are provided below. 
     System  100  also includes a moderator telephone  106  that may be co-located with the moderator DPS  105 , and a plurality of participant telephones  121   a ,  121   b , and  121   c  that may be co-located with the participant DPSs  120   a ,  120   b , and  120   c , respectively. The moderator&#39;s telephone  106  is coupled to each of the participant telephones  121   a ,  121   b , and  121   c  through a telephone network. Telephone network is shown as public switched telephone network  125 , but which may be, in alternative embodiments, any telecommunications network including any analog or digital (e.g., ISDN), wired, or wireless, telecommunications system, or VoIP. The moderator DPS  105  may typically, be controlled by a moderator of a conference. The moderator may also perhaps be talking about the application as it is being presented. Because the participant DPSs,  120   a ,  120   b , and  120   c , only access the most current representation of the display screen, the presentation at the participant DPSs will reflect the moderators comments being transmitted over the telephone network  125  without disconcerting latency. 
       FIG. 2  is a process flow diagram of a method by which pixel data of a moderator DPS display screen is stored to a server and periodically updated. The process  200 , shown in  FIG. 2 , begins at operation  205  in which the display screen of the moderator DPS is divided into segments (tiles). The segmentation of the display screen may be done at the moderator DPS or may, in alternative embodiments, be done at the server DPS. For one embodiment, the moderator DPS display screen is segmented into rectangular tiles with dimensions of 128 pixels by 32 pixels. Typically, after the display screen is so divided, the tile configuration is not changed throughout the conference. Such a configuration results in a fixed coordinate system where each tile may be identified by its coordinates. For example, each tile may be uniquely identified by the x-y coordinate of the top left corner of the tile. 
     At operation  210 , display data segments corresponding to each tile are created and transmitted to the server DPS and stored on the server DPS. Each display data segment contains the pixel data for the corresponding tile. The pixel data from each tile is compressed and stored as a corresponding display data segment together with the tile&#39;s unique identifier. Only one version of the tile is stored on the server at any given time. Each display data segment also includes a time indicator to indicate the last time at which the corresponding tile was changed (updated). 
       FIG. 3  illustrates a segmented moderator DPS display screen and the corresponding display data segments stored on the server in accordance with one embodiment of the present invention. The display screen  300 , shown in  FIG. 3 , has been divided into four tiles  306 - 309 . Each tile has been identified based upon its position on the display screen. For example, tiles  306 - 309  may be identified with the coordinate pairs (0,0), (0,50), (50,0), and (50,50), respectively. The particular identifiers are arbitrary and any system that uniquely identifies each tile may be employed. The pixel data for each tile is encoded (compressed) and transmitted to server  310 . Server  310  contains a display data segment corresponding to each tile  306 - 309 , namely  306   a - 309   a . Each display data segment  306   a - 309   a  contains the encoded pixel data  315  and tile identifier  320  of the corresponding tile. Each display data segment  306   a - 309   a  also contains a time indicator  325  indicating the time at which the display data segment was created. Although display screen  300  is shown divided into four tiles, in practical embodiments, the display screen would be divided into many more, smaller, tiles. In such embodiments, the number of unique colors per tile is less than the number of colors on the screen, allowing for increased compression opportunities as described below. 
     Referring again to  FIG. 2 , at operation  215  each tile is periodically evaluated to determine if the pixel data of the tile has changed. For one embodiment, a processor, having a very fast memory compare routine (e.g., a multimedia extension (MMX) Pentium Processor available from Intel corporation of Santa Clara, Calif.) is used to compare each tile currently displayed on the moderator DPS display screen with the latest version of that tile stored on the server. That is, a bitmap of a tile as it currently appears on the screen, bitmapT.sub.0, is stored in memory. BitmapT.sub.0 is compared with the bitmap of the same tile used to create the corresponding display data segment stored on the server DPS, bitmapT.sub.−1, also stored in memory. Using the memory compare routine it is possible to quickly determine whether the two memory locations are equal or not. For one embodiment, the process of evaluating tiles to determine if a change has occurred is repeated on an on-going basis as often as the constraints of the moderator DPS will allow. Alternatively, the repetition frequency of the tile evaluation process may be based upon the connection and processing speed of the participant DPSs or other system criteria. 
     If bitmapT.sub.0 and bitmapT.sub.−1 are not equal, the tile is designated as a “changed tile” (i.e., the tile has been updated). At operation  220  an updated display data segment is determined for each changed tile. The updated display data segment contains pixel data corresponding to the current bitmap (bitmapT.sub.0) together with an updated time indicator. The tile identifier remains the same throughout the process. The new display data segment is transmitted to the server, and the server replaces the corresponding old display data segment with the new display data segment. The existing display data segment for a tile stored on the server is discarded when the tile is updated. The new display data segment representation of the tile stored on the server and hence the only representation of the tile available to participant DPSs. 
     At operation  225 , if the conference has not ended, the process of evaluating tiles to determine if a change has occurred, determining new display data segments for changed tiles, and replacing old display data segments with the corresponding new display data segments continues. 
     As discussed above in reference to  FIGS. 2 and 3 , the pixel data for each tile is compressed prior to being stored on the server DPS. The pixel data may be compressed through any of several data compression schemes known in the art. The pixel data compression may take place at the moderator DPS or at the server DPS. For one embodiment, the pixel data compression is accomplished at the moderator DPS through a modified RLE compression scheme. Each tile represents a small portion of the display screen. For one embodiment, each tile represents a portion of the display screen that is 128 pixels long by 32 pixels wide. For this relatively small region, the number of colors that are likely to be displayed is much lower than the number of colors that are going to be displayed across the entire screen. For each tile, the number of colors required for the pixel data is between 16 and 64 colors. Therefore, even though the participant DPS display device may provide true color capability (each pixel represented by 24 bits), greater compression may be achieved by representing each pixel within a tile by only 1 byte (8 bits). Thus, for one embodiment of the present invention, a modified RLE encoding may be employed in which the pixels of a tile are encoded using only 1-byte. The 1-byte representation provides 256 colors, which is sufficient for the range of colors within a tile. Moreover, if the color range within a tile is significantly lower, then multiple pixels may be stored per byte. For example, if for a given tile, the color spread is only 16 colors, then only 4 bits are required to encode a pixel and two pixels may be encoded within each byte. This modified RLE encoding scheme is affected by determining the number of colors required to encode a tile and encoding the data as though the display device were set to that color-depth. That is, the color-depth for each tile is dynamically changed as the tile is being encoded. This results in increased compression over typical RLE encoding. 
     For one embodiment, compression may be further increased by doing a second pass compression of the data as it is queued on the network buffer. As described above, a tile that has changed is compressed and transmitted to the server. The system is able to identify and compress the tile faster than the data can be transmitted through the network. 
     Therefore, the updated display data segments are delayed in a network buffer queue on the moderator DPS. While this data is queued, it is possible to conduct a second compression on the outgoing data stream. Also, because tiles may be delayed in the queue, they may get replaced with subsequent updates before being transmitted. 
     The operations described above in reference to  FIG. 2 , and the data compression described above, may be implemented by hardware and/or software contained within the moderator DPS. For example, the moderator DPS may include one or more processors that can execute code or instructions stored within a machine-readable medium that may also be included within the moderator DPS as described in greater detail below in reference to  FIG. 6 . 
       FIG. 4  is a process flow diagram of a method by which a server DPS transmits updated moderator DPS display screen data to a participant DPS, in accordance with one embodiment of the present invention. Process  400 , shown in  FIG. 4 , begins at operation  405  in which the server DPS transmits initial display screen configuration information to the participant DPS. The initial display screen configuration information determines the screen geometry and includes the tile size, and the unique identifier for each tile. The tile identifier may also indicate the tile&#39;s location on the screen and hence provide the screen geometry. At this time, the participant DPS may also receive an initial display data segment corresponding to each tile. 
     At operation  410  the participant DPS contacts the server DPS and requests a data update. Upon receiving the request, the server DPS accesses the last time indicator (e.g., timestamp) from the participant DPS which is stored on the server. The time indicator, indicates the time of the most recent display data segment received by the participant DPS, and, in effect, indicating to the server DPS that its version of the moderator DPS display screen was current as of a given time. 
     At operation  415  the server DPS compares the time indicator, stored for the participant on the server DPS, with the time indicator of each display data segment. To update the participant DPS, the server DPS need only transmit display data segments that have been replaced subsequent to the time indicated by the participant DPS time indicator. 
     If the display data segment for a particular tile has a time indicator that is subsequent to the time indicator stored for the participant DPS, this indicates that the corresponding tile has been changed since the last time the participant DPS accessed the display data segment. If the display data segment has a time indicator subsequent to the time indicator received from the participant DPS, the server DPS transmits that display data segment to the participant DPS at operation  420 . 
     If the display data segment for a particular tile does not have a time indicator that is subsequent to the time indicator stored for the participant DPS, this indicates that the corresponding tile has not been changed since the last time the participant DPS accessed the display data segment. If the tile has not been changed since the last DPS access, the corresponding tile is not transmitted to the participant DPS. 
     At operation  425  the time indicator of another display data segment is compared to the time indicator stored for the participant DPS and the process is continued until all display data segments corresponding to each tile have been compared. 
     The server DPS may be able to determine display data segments having a time indicator subsequent to the time indicator stored for the participant DPS faster than such display data segments can be transmitted to the participant DPS. In such case, the display data segments maybe queued for transmission. The tile corresponding to a queued display data segment may be updated prior to transmission of the display data segment. If so, the server DPS may replace such a display data segment on the queue with the updated version of the display data segment. This ensures that the participant DPS receives the latest version of each tile, even if the tile has changed multiple times since the participant DPS initiated a request for a data update. 
     The operations described above in reference to  FIG. 4  may be implemented by hardware and/or software contained within the server DPS. For example, the server DPS may include one or more processors that can execute code or instructions stored within a machine-readable medium that may also be included within the server DPS as described in greater detail below in reference to  FIG. 6 . 
       FIG. 5  is a process flow diagram of a method by which a participant DPS receives a current representation of the moderator DPS display screen in accordance with one embodiment of the present invention. The process  500 , shown in  FIG. 5 , begins at operation  505  in which the participant DPS contacts the server and requests a data update. 
     At operation  510  the participant DPS receives the display data segments that have been updated subsequent to the most recent prior update. The display data segments include compressed pixel data for corresponding tiles as described above. 
     At operation  515 , the compressed pixel data is decompressed based upon the compression scheme used to compress the data. For example, if the pixel data is compressed using a modified RLE compression scheme as described above, then the participant DPS uses a reverse RLE scheme and takes into account the color depth the data was encoded with. Once the pixel data of a display data segment is decoded the participant DPS has a representation of the changed tile corresponding to that display data segment. 
     At operation  520  the participant DPS uses the tile identifier, contained within the display data segment, to identify the corresponding tile on the participant DPS display screen and then uses the decompressed pixel data to update the participant DPS display screen. For example, for one embodiment the tile identifier is the x-y coordinate of a rectangular tile on the display screen. The participant DPS can thus identify to which tile the updated pixel data applies. 
     The operations described above in reference to  FIG. 5 , may be implemented by hardware and/or software contained within the participant DPS. For example, the participant DPS may include one or more processors that can execute code or instructions stored within a machine-readable medium that may also be included within the participant DPS as described below in greater detail below in reference to  FIG. 6 . 
       FIG. 6  is a block diagram of a DPS that may be used in accordance with one embodiment of the present invention. For example, the DPS  600  shown in  FIG. 6  may be used as a participant DPS, or a Moderator DPS, or a server DPS, etc. Furthermore, the DPS  600  may be used to perform one or more functions of an Internet service provider. The DPS  600  may be interfaced to external systems through a modem or network interface  645 . The modem or network interface may be considered a part of the DPS  600 . The modem or network interface may be an analog modem, an ISDN modem, a cable modem, a token ring interface, a satellite transmission interface, a wireless interface, or other interface(s) for providing a data communication link between two or more DPSs. 
     The DPS  600  includes a processor  605 , which may represent one or more processors and may include one or more conventional types of processors, such as Motorola PowerPC processor (available from Motorola, Inc. of Schaumburg, Ill.), an Intel MMX Pentium processor (available from Intel Corporation of Santa Clara, Calif.), etc. A memory  610  is coupled to the processor  605  by a bus  615 . The memory  610  may be a dynamic random access memory (DRAM) and/or may include static RAM (SRAM). The processor  605  may also be coupled to other types of storage areas/memories (e.g., cache, Flash memory, disk, etc.), which could be considered as part of the memory  610  or separate from the memory  610 . 
     The bus  615  further couples the processor  605  to a display controller  620 , a mass memory  625 , the modem or network interface  645 , and an input/output (I/O) controller  630 . 
     The operations described above in reference to  FIGS. 2, 4, and 5  may be implemented by software stored on mass memory  625  depending on the particular DPS and system configuration. For example, the operations of dividing the moderator DPS display screen into tiles, creating a display data segment corresponding to each tile, determining if the pixel data for a tile has changed, and creating an updated display data segment corresponding to tiles that have changed, may be implemented by software stored upon mass memory  625  of a moderator DPS. The operation of comparing the time indicator of a data request with the time indicator of each display data segment and transmitting only display data segments having a subsequent time indicator may be implemented by software stored upon a mass memory  625  of a server DPS. And the operations of requesting and receiving updated display date segments, decompressing the pixel data of each updated display data segment, and using each tile identifier to apply the updated data to the corresponding display screen tile may be implemented by software stored upon a mass memory  625  of a participant DPS. 
     The mass memory  625  may represent a magnetic, optical, magneto-optical, tape, and/or other type of machine-readable medium/device for storing information. For example, the mass memory  625  may represent a hard disk, a read-only or writeable optical CD, etc. The display controller  620  controls, in a conventional manner, a display  635 , which may represent a cathode ray tube (CRT) display, a liquid crystal display (LCD), a plasma display, or other type of display device. The I/O controller  630  controls I/O device(s)  640 , which may include one or more keyboards, mouse/track ball or other pointing devices, magnetic and/or optical disk drives, printers, scanners, digital cameras, microphones, etc. 
     The DPS  600  represents only one example of a system, which may have many different configurations and architectures and which may be employed with the present invention. For example, Macintosh and Intel systems often have multiple buses, such as a peripheral bus, a dedicated cache bus, etc. On the other hand, a network computer, which may be used as a DPS of the present invention, may not include, for example, a hard disk or other mass storage device, but may receive routines and/or data from a network connection, such as the modem or interface  645 , to be processed by the processor  605 . Similarly, a Web TV system, which is known in the art, may be considered to be a DPS of the present invention, but such a system may not include one or more I/O devices, such as those described above with reference to I/O device  640 . Additionally, a portable communication and data processing system, which may employ a cellular telephone and/or paging capabilities, may be considered a DPS that may be used with the present invention. 
     In the system  600  shown in  FIG. 6 , the mass memory  625  (and/or the memory  610 ) may store media (e.g., applications, video, etc.) that may be processed according to the present invention. Alternatively, media data may be received by the DPS  600 , for example, via the modem or network interface  645 , and stored and/or presented by the display  635  and/or the I/O device(s)  640 . In one embodiment, data may be transmitted across a data communication network, such as a LAN and/or the Internet. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.