Abstract:
Multiple video picture frames are combined into a spatial multiplex video picture frame that may be fully decoded and displayed. The video display of the spatial multiplex video picture frame is a composite combination of all of the video picture frames that have been combined, and may have an appearance such as a mosaic. Multiplexing the video picture frames involves removing picture headers, creating a picture header for the spatial multiplex video picture frame, and altering the headers of individual components of each video picture frame. The new header for the spatial multiplex video picture frame indicates a slice format frame, and headers of the individual components are altered to provide a slice format based picture position for each video picture frame. The headers of the individual components are altered to become slice based, such as in accordance with the ITU-T H.263 video standard, prior to establishing the slice based picture position if the frames are not already of the slice format.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/955,607 filed Sep. 19, 2001 now U.S. Pat. No. 6,956,600. 
    
    
     TECHNICAL FIELD 
     The present invention relates to combining multiple digital video picture frames into a single spatial multiplex video picture frame to produce a single displayed picture that is a composite of several individual pictures. More particularly, the present invention relates to generating the spatial multiplex video picture frame by altering header information of the individual video picture frames being combined. 
     BACKGROUND 
     A motion picture such as broadcast television is made of individual pictures that are rapidly displayed to give the illusion of continuous motion. Each individual picture in the sequence is a picture frame. A digitally encoded picture frame is made of many discrete picture elements, or pixels, that are arranged in a two-dimensional array. Each pixel represents the color (chrominance) and brightness (luminance) at its particular point in the picture. The pixels may be grouped for purposes of subsequent digital processing (such as digital compression). For example, the picture frame may be segmented into a rectangular array of contiguous macroblocks, as defined by the ITU-T H series coding structure. Each macroblock typically represents a 16×16 square of pixels. 
     Macroblocks may in turn be grouped into picture frame components such as slices or groups of blocks, as defined under the ITU-T H.263 video coding structure. Under H.263, a group of blocks is rectangular and always has the horizontal width of the picture, but the number of rows of group of blocks per frame depends on the number of lines in the picture. For example, one row of a group of blocks is used for pictures having 4 to 400 lines, two rows are used for pictures having 404 to 800 lines, and four rows are used for pictures having 804 to 1152 lines. A slice, on the other hand, is a flexible grouping of macroblocks that is not necessarily rectangular. Headers within the encoded video picture bit stream identify and provide important information about the various subcomponents that make up the encoded video picture. The picture frame itself has a header, which contains information about how the picture frame was processed. Each group of blocks or slice within a video picture frame has a header that defines the picture frame component as being a slice or group of blocks as well as providing information regarding the placement of the component within the picture frame. Each header is interpreted by a decoder when decoding the data making up the picture frame in preparation for displaying it. 
     In certain applications, displaying multiple picture frames within a single display is desirable. For example, in videoconferencing situations it is useful for each participant to have a video display showing each of the other participants at remote locations. Visual cues are generally an important part of a discussion among a group of participants, and it is beneficial for each participant&#39;s display to present the visual cues of all participants simultaneously. Any method of simultaneously displaying all the conference participants is called a continuous presence display. This can be accomplished by using multiple decoders and multiple video displays at each site, or by combining the individual video pictures into a single video picture in a mosaic arrangement of the several individual pictures (called a spatial multiplex). 
     Multiplexing picture frames into a single composite picture frame requires some form of processing of each picture frame&#39;s encoded data. Conventionally, a spatial multiplex video picture frame could be created by completely decoding each picture frame to be multiplexed to a baseband level, multiplexing at the baseband level, and then re-encoding for transmission to the various locations for display. However, decoding and re-encoding a complete picture frame is computationally intensive and generally consumes a significant amount of time. 
     The H.263 standard provides a continuous presence multipoint and video multiplex mode that allows up to four individual picture frames to be included in a single bitstream, but each picture frame must be individually decoded by individual decoders or by one very fast decoder. No means of simultaneously displaying the pictures is specified in the standard. Additionally, time-consuming processing must be applied to the picture frames after they have been individually decoded to multiplex them together into a composite image for display. 
     Therefore, there is a need in the art for a method and system that can spatially multiplex multiple picture frames into a single picture frame without requiring each individual picture frame to be fully decoded when being multiplexed and without requiring additional processing after decoding to multiplex the picture frames. 
     SUMMARY 
     The present invention spatially multiplexes several picture frames into a single spatial multiplex video picture frame by manipulating header information for the picture frame components, such as the groups of blocks or slices, containing the picture frame data. A picture header associated with each picture frame is removed and a new picture header is generated that applies to the spatial multiplex video picture frame that is a composite of all of the individual picture frames. The new header provides an indication of a slice format for the spatial multiplex video picture frame. The component headers of each picture frame are altered to set a slice format based picture position for the picture frame within the picture that results from the spatial multiplex video picture frame. The slice format is prevalent within the H.263 standard. Thus, only the component headers need to be decoded and re-encoded to establish the spatial multiplex video picture frame. 
     The spatial multiplex video picture frame results from concatenating the new picture header together with the picture frames having the altered component header information. The spatial multiplex video picture frame may then be decoded as if it were a single picture frame to display the composite of the several individual picture frames. Displaying the spatial multiplex video picture frame allows the individual picture frames to be viewed simultaneously on one display screen. 
     The system that multiplexes the individual picture frames may be a scalable facility such that as the need for picture frame multiplexing increases, the system may be expanded to fill the need. The system includes a plurality of computing devices, such as single board computers, linked to a data packet switch through a serial interface. Each computing device within the system has the ability to combine individual picture frames into a single spatial multiplex video picture frame by altering the headers of the picture frame components to set a slice format based picture position for the picture frames. As the need for additional processing arises, additional computing devices in communication with the data packet switch may be added to provide additional capacity. 
     The present invention may be employed in a networked environment where a processing device, such as a network server, communicates with several client devices, such as videoconferencing devices. The processing device receives the multiple picture frames from various communication channels in the network. For example, the processing device may receive a stream of video picture frames from each participant in a videoconference through the network. The processing device then multiplexes the individual picture frames into a spatial multiplex video picture frame by altering the component header information to produce a slice based picture position for each frame. The spatial multiplex video picture frame is transmitted back through the communication channels of the network where it can be displayed by the display screen of the client devices. 
     The present invention may also be employed in a networked environment where each video site, such as a videoconferencing device, generates video picture frames. The picture frames are transmitted to other video sites in the network, and picture frames produced by other video sites are received. The video site multiplexes the picture frames to produces the multiplexed composite picture frame by altering the component header information to set a slice format based picture position. The video site may then decode the spatial multiplex video picture frame and display it. 
     The various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the drawings and claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a composite picture frame and slice structure, an individual picture frame that may be multiplexed into the composite picture frame, and alternative picture frame structures. 
         FIG. 2  is an exemplary picture layer syntax of a picture frame under the H.263 standard. 
         FIG. 3  is an exemplary group of blocks layer syntax under the H.263 standard. 
         FIG. 4  is an exemplary slice layer syntax under the H.263 standard. 
         FIG. 5  is an operational flow for multiplexing picture frames utilized by one embodiment of the present invention. 
         FIG. 6  is an operational flow of the group of blocks to slice format conversion utilized by the embodiment. 
         FIG. 7 . is a block diagram of an embodiment employing single-point processing in a network environment. 
         FIG. 8  is a block diagram of an embodiment employing on-site processing in a networked environment. 
         FIG. 9  is a block diagram of an embodiment of a scalable multiplexing facility. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a display of a spatial multiplex video picture frame  100  made up of individual picture frames  102 . As shown, the spatial multiplex video picture frame  100  includes sixteen picture frames  102  of individual people participating in a videoconference where the picture frames  102  form a mosaic pattern. Because each participant is always in view, the spatial multiplex video picture frame  100  is referred to as a continuous presence display. As will be discussed below, each individual picture frame  102  of the spatial multiplex video picture frame  100  is initially a normal picture frame  104  that may be displayed in full size on a display screen. The picture frame  104  may be represented as data that is encoded and segmented in various ways. 
     For the example shown, the picture frame  104  may have been transmitted in a quarter-size common image format (QCIF) indicating a pixel resolution of 176×144. In such a case, the spatial multiplex video picture frame  100  is decoded as a 4CIF picture indicating a resolution of 704×576 because it contains sixteen QCIFs where four QCIFs form a CIF size image. It is to be understood that other picture size formats for the individual picture frames  104  and for the spatial multiplex video picture frame  100  are possible as well. For example, the multiplexed image may contain 64 individual QCIF picture frames and therefore have a 16CIF size. 
     The group of blocks format  110  is one alternative for segmenting and encoding the picture frame  104 . The picture frame  104  of the group of blocks format  110  includes one or more rows of picture components known as groups of blocks  124 . In the example, shown, the QCIF frame  104  has three rows of groups of blocks. A picture header  122  is also included. The picture header provides information to a decoder when the picture frame  104  is to be displayed in full size and tells the decoder that the picture frame  104  has a group of blocks format  110 . 
     Each row  124  is made up of an array  112  of macroblocks  128  that define the luminance and chrominance of the picture frame  104 . Each row  124  also includes a header  126  that tells the decoder the position within the picture frame  104  where the row of group of blocks  124  belongs. In the example shown, the group of blocks  124  has two rows of macroblocks  128  because it is intended for the picture frame  104  to be displayed with 404 to 800 total lines. In reality, a group of blocks  124  will have many more macroblocks  128  per row than those shown in  FIG. 1 . 
     As discussed above, the group of blocks format defined by the H.263 standard requires that the row  124  always extends to the full width of the picture. Therefore, a direct remapping of a group of blocks format  110  to a spatial multiplex video picture frame  100  is not possible because the spatial multiplex video picture frame  100  requires individual frames to have a width that may be less than the full width of the picture. In the videoconferencing context, several participants may need to be displayed across the width of the picture as shown in  FIG. 1 , and a group of blocks format  110  does not permit such remapping. 
     An alternative format for segmenting and encoding the picture frame  104  is the slice format  106 , such as defined by the H.263 standard. The slice format  106  is more flexible and does not require each slice to maintain the full width of the picture. The slice format  106  includes one or more picture components known as slices  116  that may or may not extend across the full width of the picture, and a picture header  114  that specifies to the decoder that the picture frame  104  has a slice format. Each slice  116  is made up of a grouping  108  of macroblocks  120 . Each slice  116  also has a slice header  118  that indicates to the decoder the relative position of the slice in the picture  104 . 
     The slice format  106  of the picture frame  104  allows the picture frame  104  to be multiplexed into the composite picture frame  100  with minimal decoding. The spatial multiplex video picture frame  100  may be created in a slice format  130  of many slices  134  corresponding to the slices  116  of the individual picture frames  102  forming the composite. As shown, the slices  134  have a width that is less than the picture width so that multiple slices  134  are provided for each row of slices of the picture. A new picture header  132  is also generated to indicate to the decoder that the picture frame  100  is of the slice format  130  and is of a 4CIF size, 16CIF size, and so on. The header, such as  118 , of each slice  134  is modified to properly position the slice within the spatial multiplex video picture frame  100 . 
       FIG. 2  shows the picture layer syntax  200  that is made up of the picture header included at the beginning of each picture frame as well as the group of block layer or slice layer. The picture layer syntax  200  includes a picture start code (PSC)  202  that signifies the beginning of a new picture frame. A temporal reference (TR)  204  follows in the bitstream and provides a value indicating the timing of display of the picture frame relative to a previous frame and the picture clock frequency. A PTYPE block  206  follows and provides information about the picture such as whether the source format of the picture frame is a quarter-size common image format (QCIF), a CIF format, or other. 
     The picture layer syntax  200  may also include a PLUS HEADER block  208  that contains information about the picture frame, including whether the frame consists of groups of blocks or slices. A PQUANT block  210  provides quantizer information to configure the quantization parameters used by the decoder. An optional continuous presence multipoint (CPM) block  212  signals the use of continuous presence multipoint and video multiplex mode discussed above that permits multiple individual frames to be included in the bitstream. As discussed the CPM mode causes the individual frames to maintain their identities as individual frames and requires that they be individually decoded and then processed to form a single image. A picture sub-bitstream indicator (PSBI)  214  may be included if CPM mode is indicated. CPM mode may be implemented in conjunction with the logical operations of  FIGS. 5 and 6  to provide sub-bitstreams that are themselves multiplexed bitstreams, or CPM may be turned off if only the logical operations of  FIGS. 5 and 6  are desired for providing continuous presence video. 
     A temporal reference for B-picture parts (TRB)  216  may be included if a PB-frame is indicated by the PTYPE block  204  or PLUS HEADER block  208 . A DBQUANT block  218  may also be included if a PB-frame is indicated to indicate the relation of the BQUANT quantization parameter used for B-picture parts in relation to the QUANT quantization parameter used for P-picture parts. A PEI block  220  includes a bit that signals the presence of the supplemental enhancement information (PSUPP) block  222 . PSUPP block  222  defines extended capabilities for picture decoding. The group of blocks (GOB) layer  224  or slice layer  226  then follows in the bitstream. The GOB layer  224  contains each group of block of the picture frame and is discussed in more detail in  FIG. 3 . Slice layer  226  contains each slice of the picture frame and is discussed in more detail in  FIG. 4 . 
     The ESTUF block  228  is included to provide mandatory byte alignment in the bitstream. The end of sequence (EOB) block  234  may be included to signal the end of the sequence of group of blocks or slices. Alternatively, the end of sub-bitstream sequence (EOSBS) block  230  may be included to indicate an end of a sub-bitstream when in CPM mode. An ending sub-bitstream indicator (ESBI) block  232  is included to provide the sub-bitstream number of the last sub-bitstream. The PSTUF block  236  is included to provide byte alignment for the PSC of the next picture frame. 
       FIG. 3  shows the group of blocks layer syntax  300  that is made up of the component header and the macroblocks of the array forming a group of blocks and that would be found in each group of blocks of the group of blocks layer  224  of  FIG. 2 . A GSTUF block  302  is included to provide byte alignment for a group of blocks start code (GBSC)  304 . The GBSC  304  indicates to the decoder the start of a group of blocks. A group number (GN) block  306  indicates the group of block number that defines the position of the group of blocks in the picture frame. A GOB sub-bitstream indicator (GSBI)  308  may be included when in CPM mode to indicate the sub-bitstream number. 
     A GOB frame ID (GFID)  310  is included to indicate the particular frame that the group of blocks corresponds to. GQUANT block  312  provides quantizer information to control the quantization parameters of the decoder. A temporal reference indicator (TRI) block  314  is included to indicate the presence of a temporal reference when operating in a reference picture mode. A temporal reference (TR) block  316  is included to provide a value indicating the timing of display of the group of blocks relative to a previous group of blocks and the picture clock frequency. A temporal reference for prediction indication (TRPI) block  318  is included to indicate the presence of a temporal reference for prediction field (TRP)  320 . The TRP field  320  indicates the temporal reference to be used for prediction of the encoding. 
     A back channel message indication (BCI) field  322  is included to indicate whether a message is to be delivered from the decoder back to the encoder regarding conditions of the received coded stream. A back channel message (BCM) layer  324  contains a message that is returned from a decoder to an encoder in order to tell whether forward-channel data was correctly decoded or not. A macroblock (MB) layer  326  contains a macroblock header and the macroblock data for the group of blocks. 
       FIG. 4  shows the slice layer syntax  400  that is made up of the component header and the macroblocks of the array forming a slice and that would be found in each slice of the slice layer  226  of  FIG. 2 . An SSTUF block  402  is included to provide byte alignment for a slice start code (SSC) block  404  indicating the beginning of a slice. A first slice emulation prevention bit (SEPB 1 )  406  is included to prevent start code emulation after the SSC block  404 . A slice sub-bitstream indicator (SSBI) block  408  is included when in CPM mode to indicate the sub-bitstream number of the slice. A macroblock address (MBA) field  410  is included to indicate the first macroblock of the slice as counted from the beginning of the picture in scanning order to set the position of each slice in the picture frame. 
     A second slice emulation prevention bit (SEPB 2 ) block  412  is also included to prevent start code emulation after the MBA field  410 . An SQUANT block  414  is included to provide quantizer information that controls the quantization parameters of the decoder. A slice width indication (SWI) block  416  is provided to indicate the width of the current rectangular slice whose first macroblock is specified by the MBA field  410 . A third slice emulation prevention bit (SEPB 3 )  418  is included to prevent start code emulation after the SWI block  416 . A slice frame ID (GFID)  420  is included to indicate the particular picture frame that the slice corresponds to. The TRI field  422 , TR field  424 , TRPI field  426 , TRP field  428 , BCI field  430 , BCM layer  432 , and MB layer  434  are identical to the fields of  FIG. 3  that go by the same name. 
     The operational flow of the process  500  for multiplexing individual picture frames containing the GOB syntax  300  or the slice syntax  400  into a single picture frame is shown in  FIG. 5 . In this embodiment of the operational flow, it is assumed that the single picture frames are originating from encoder devices and are being processed by one or more decoder devices after transfer, such as through a network medium as shown in the systems of  FIGS. 7 and 8 . The process  500  begins at call operation  502  where the two devices passing the picture data establish a common mode of operation suitable for generating continuous presence video. The common mode of operation includes a consistent usage of header information so that, for example, back channel messaging is employed between the encoder and decoder or other enhanced capabilities are realized. After communication is established, start operation  504  causes one device of the connection to broadcast a start indicator that allows synchronization of transmission of the individual picture frames from the various sources, such as the remote locations of the video conference. 
     Once the picture frames to be included in the multiplexed frame have been received, header operation  506  reads the picture layer header, such as shown in  FIG. 2 , for each individual picture frame and discards them. This requires that only the picture header be decoded. A single new picture layer header that applies to the spatial multiplex video picture frame is created and encoded at header operation  506 . The single new picture layer header provides in the PTYPE field  206  an indication that the spatial multiplex video picture frame is of a size capable of including the number of individual frames being multiplexed. The PLUS HEADER field  208  of the new picture header is configured to indicate a rectangular slice format. 
     After substituting the new picture header, the component header of one of the individual frames is interpreted at read operation  508  in preparation for subsequent processing discussed below including conversion to a slice format and repositioning within the multiplexed image. Query operation  510  detects whether the picture header read in header operation  506  for the current picture frame indicates a group of blocks format. If a group of blocks format is detected, then conversion operation  512  converts the group of blocks headers into slice headers. Conversion operation  512  is discussed in greater detail below with reference to  FIG. 6 . If a group of blocks format is not detected, then the conversion operation  512  is skipped since a slice format is already present in the picture frame. 
     After finding or converting to a slice format, macroblock operation  514  alters the MBA  410  within each slice of each picture frame to position the slice within a particular region of the spatial multiplex video picture frame. For example, one individual picture frame must go in the top left-hand corner of the multiplexed picture so the top-leftmost slice of that picture frame is given an MBA  410  corresponding to the top left-hand corner position. The component header is also re-encoded at this operation after the MBA  410  has been altered. The slice is then inserted into the proper location in the continuous presence picture stream by concatenating the bits of the slice with the bits already present in the picture stream including the new picture header at stream operation  516 . The picture stream may be delivered as it is being generated at transmit operation  518  wherein the current slice is written to an output buffer and then transmitted to a network interface. 
     After writing the slice to the output buffer, query operation  520  detects whether the last slice was the end of the continuous presence or spatial multiplex video picture frame. If it was not the last slice of the multiplexed frame, then flow returns to read operation  508  where the header of the next group of blocks or slice to be included in the spatial multiplex video picture frame is read. If query operation  520  determines that the last slice was the end of the spatial multiplex video picture frame, then flow returns to header operation  506  wherein the picture headers for the next set of individual picture frames are read and discarded. 
       FIG. 6  shows the operational flow of the conversion operation  512 . Conversion operation  512  begins at alignment operation  602  where the GSTUF field of the GOB syntax  300  is converted to an SSTUF field of the slice syntax  400  by adjusting the length of the stuff code to provide byte alignment of the next code element. At start code operation  604 , the GBSC  304  is maintained because it is already identical to the SSC  404  needed in the slice syntax  400 . At prevention operation  606 , the SEPB 1   406  is inserted into the bitstream to later prevent start code emulation when being decoded. 
     Translation operation  608  converts the GSBI  308  to the SSBI  408 . During this operation, GSBI ‘00’ becomes SSBI ‘1001’, GSBI ‘01’ becomes SSBI ‘1010’, GSBI ‘10’ becomes SSBI ‘1011’, and GSBI ‘11’ becomes SSBI ‘1101’. At MBA operation  610 , the GN  306  is replaced by an MBA  410  chosen to place the slice in its designated location within the composite picture frame resulting from multiplexing the individual picture frame bitstreams. Prevention operation  612  then places a SEPB 2  into the bitstream to prevent start code emulation. At quantizer operation  614 , GQUANT is maintained in the bitstream after SEPB 2  because GQUANT is already identical to SQUANT  414 . 
     Slice operation  616  then sets the width of the slice, or SWI  416 , to the width of the GOB in terms of the number of macroblocks. This is possible because the slice structure selection (SSS) field (not shown) of the PLUS HEADER field  208  of the picture syntax  200  of  FIG. 2  has been set to the rectangular slice mode in header operation  506  of  FIG. 5 . Prevention operation  618  then inserts a SEPB 3  into the bitstream to prevent start code emulation when the slice is decoded. At GFID operation  620 , the GFID  310  is maintained in the bitstream after SEPB 3  because it is already identical to GFID  420 . In substitute operation  622 , all remaining portions of the GOB syntax  300  are maintained in the bitstream because they are also identical to the remaining portions of the slice syntax  400 . 
       FIG. 7 . shows one network environment for hosting a continuous presence videoconference. A server  702  communicates through bi-directional communication channels  716  with client devices  704 ,  706 ,  708 , and  710 . Each client device, such as a personal computer or special-purpose videoconferencing module, is linked to a camera  712  or other video source and a video display  714 . The client devices transmit sequences of encoded picture frames produced by the camera  712  or other video source to the server  702  though the communication channels  716 . The server  702  then employs the processes of  FIGS. 5 and 6  to combine all of the encoded picture frames into an encoded spatial multiplex video picture frame. The server  702  then transmits the spatial multiplex video picture frame back through the communications channels  716  to the client devices where it is decoded and displayed on each display screen  714 . Thus, the client devices may include encoder and decoder processing but do not need to include the multiplexing processing discussed above. 
     Four client devices are shown only for exemplary purposes, and it is to be understood that any number of client devices may be used subject to the limitation on the total number of individual frames to be included on the display  714 . It is also to be understood that each individual frame to be included in the multiplexed frame through the processes of  FIGS. 5 and 6  does not have to be of the same size, such that one frame may occupy more screen area than others. For example, the frame showing the person currently speaking in a videoconference may be enlarged relative to frames showing other participants. One skilled in the art will recognize that negotiation between participating devices can be established such that mode switching can occur to permit one or more participants to provide one image size (e.g., QCIF) while other participants provide a different image size (e.g., CIF), subject to the ability to combine the image sizes into a composite that will fit on the intended display. Furthermore, it is to be understood that the server  702  may customize each videostream being returned to each client device  704 ,  706 ,  708 , and  710 , such as by removing the frame provided by the recipient client device from the spatial multiplex being returned or creating the spatial multiplex from some other subset. 
     The communication channel between the client devices  704 ,  706 ,  708 , and  710  and the server  702  can be of various forms known in the art such as conventional dial-up connections, asymmetric digital subscriber lines (ADSL), cable modem lines, Ethernet, and/or any combination. An Internet Service Provider (ISP) (not shown) may be provided between the server  702  and each client device, or the server  702  may itself act as an ISP. The transmissions through a given channel  716  are asymmetric due to one picture frame being transmitted to the server  702  from each client device while the server  702  transmits a concatenation of picture frames forming the multiplexed bitstream back to each client device. Therefore, ADSL is well suited to picture frame transfer in this network configuration since ADSL typically provides a much greater bandwidth from the network to the client device. 
       FIG. 8  shows an alternative network configuration where each client device  802 ,  804 ,  806 , and  808  has its own processing device performing the operations of  FIGS. 5 and 6 . Each client device is linked to a camera  810  or other video source and a display  812 . A bi-directional communication path  814  interconnects each client device to the others. The bi-directional communication paths  814  can also be of various forms known in the art such as conventional dial-up connections, asymmetric digital subscriber lines (ADSL), cable modem lines, Ethernet, and/or any combination. One or more ISPs (not shown) may facilitate transfer between a pair of client devices. 
     Each client device generates an encoded picture frame sequence that is transmitted to the other client devices. Thus, each client device receives an encoded picture frame from the other client devices. The client device may then perform the multiplexing operations discussed above to create the spatial multiplex video picture frame that is displayed. 
     Multiplexing the individual picture frames together at each client device where the spatial multiplex video picture frame will be displayed allows each client device to have control over the spatial multiplex video picture frame it will display. For example, the client device can choose to exclude certain picture frames or alter the displayed size of particular picture frames. In a videoconference, the client device may choose to eliminate the picture frame that it generates and sends to others from the spatial multiplex video picture frame that it generates and displays. Because each client device performs the multiplexing operations, the communication paths  814  carry only the individual picture frame sequences generated by each sending client device rather than spatial multiplex video picture frame sequences. 
       FIG. 9  shows an example of a scalable multi-point conferencing facility  900 . The facility includes a packet switch  902 , such as a multi-gigabit Ethernet switch, linked to several processing modules, such as single board computers (SBCs)  904 ,  906 , and  908 . An SBC generally refers to a computer having a single circuit board including memory, magnetic storage, and a processor for executing a logical process such as those of  FIGS. 5 and 6 . The processing modules may include general-purpose programmable processors or dedicated logic circuits depending upon the performance necessary. Because the operations of  FIGS. 5 and 6  to be performed by the processing modules requires only decoding of header information, programmable processors are adequate for continuous presence processing in real time for most implementations. 
     The processing modules are linked to the packet switch  902  through high-speed serial interfaces  910 , such as Fast/Gigabit Ethernet. The packet switch  902  receives encoded picture frame sequences from client devices, such as discussed with reference to  FIG. 7 , but possibly from several videoconferencing sessions. The packet switch  902  may then send all picture frame sequences corresponding to a particular videoconference to one of the processing modules  904 ,  906 , or  908 . The processing module multiplexes the picture frames to generate a spatial multiplex video picture frame and sends the spatial multiplex video picture frame sequence back to the packet switch  902 . The packet switch  902  then delivers the spatial multiplex video picture frame sequence back to client devices of the particular videoconference. 
     Thus, the scalable multi-point conferencing facility  900  can provide multiplexing services for multiple videoconference groups simultaneously. As the number of videoconference groups at any given time increases or decreases, the processing modules employed by the packet switch  902  can be added or removed from active service and made available for other duties when not needed by packet switch  902 . 
     Although the present invention has been described in connection with various exemplary embodiments, those of ordinary skill in the art will understand that many modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.