Patent Publication Number: US-7215345-B1

Title: Method and apparatus for clipping video information before scaling

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of digital video, and, more specifically, to digital video applications in a network environment. 
   Sun, Sun Microsystems, the Sun logo, Java and all Java-based trademarks and logos are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and other countries. All SPARC trademarks are used under license and are trademarks of SPARC International, Inc. in the United States and other countries. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems, Inc. 
   2. Background Art 
   Computers and computer networks are used to exchange information in many fields such as media, commerce, and telecommunications, for example. One form of information that is commonly exchanged is video data (or image data), i.e., data representing a digitized image or sequence of images. A video conferencing feed is an example of telecommunication information which includes video data. Other examples of video data include video streams or files associated with scanned images, digitized television performances, and animation sequences, or portions thereof, as well as other forms of visual information that are displayed on a display device. It is also possible to synthesize video information by artificially rendering video data from two or three-dimensional computer models. 
   The exchange of information between computers on a network occurs between a “transmitter” and a “receiver.” In video applications, the information contains video data, and the services provided by the transmitter are associated with the processing and transmission of the video data. An issue in all network applications is utilization of network bandwidth. For video applications, bandwidth is an even greater concern due to the large amounts of data involved in transmitting one or more frames of video data. For example, consider a raw workstation video signal of twenty-four bit RGB data, sent in frames of 1280×1024 pixels at sixty frames per second. The raw workstation video represents approximately 240 MBps (megabytes per second) of continuous data. Even for smaller frame sizes, video data can represent a significant load on a network, resulting in poor video performance if the required bandwidth cannot be provided. Further, other applications on the network may suffer as bandwidth allocations are reduced to support the video transmission. To provide a better understanding of video and its limitations, a general description of computer graphics and video technology is given below. 
   General Video Technology 
   In digital video technology, a display is comprised of a two dimensional array of picture elements, or “pixels,” which form a viewing plane. Each pixel has associated visual characteristics that determine how a pixel appears to a viewer. These visual characteristics may be limited to the perceived brightness, or “luminance,” for monochrome displays, or the visual characteristics may include color, or “chrominance,” information. Video data is commonly provided as a set of data values mapped to an array of pixels. The set of data values specify the visual characteristics for those pixels that result in the display of a desired image. A variety of color models exist for representing the visual characteristics of a pixel as one or more data values. 
   RGB color is a commonly used color model for display systems. RGB color is based on a “color model” system. A color model allows convenient specification of colors within a color range, such as the RGB (red, green, blue) primary colors. A color model is a specification of a three-dimensional color coordinate system and a three-dimensional subspace or “color space” in the coordinate system within which each displayable color is represented by a point in space. Typically, computer and graphic display systems are three-phosphor systems with a red, green and blue phosphor at each pixel location. The intensities of the red, green and blue phosphors are varied so that the combination of the three primary colors results in a desired output color. 
   An example of a system for displaying RGB color is illustrated in  FIG. 1 . A frame buffer  140 , also known as a video RAM, or VRAM, is used to store color information for each pixel on a video display, such as CRT display  160 . DRAM can also be used as buffer  140 . VRAM  140  maps one memory location for each pixel location on the display  160 . For example, pixel  190  at screen location X 0 Y 0  corresponds to memory location  150  in VRAM  140 . The number of bits stored at each memory location for each display pixel varies depending on the amount of color resolution required. For example, for word processing applications or display of text, two intensity values are acceptable so that only a single bit need be stored at each memory location (since the screen pixel is either “on” or “off”). For color images, however, a plurality of intensities must be definable. For certain high end color graphics applications, it has been found that twenty-four bits per pixel produces acceptable images. 
   Consider, for example, that in the system of  FIG. 1 , twenty-four bits are stored for each display pixel. At memory location  150 , there are then eight bits each for the red, green and blue components of the display pixel. The eight most significant bits of the VRAM memory location could be used to represent the red value, the next eight bits represent the green value and the eight least significant bits represent the blue value. Thus, 256 shades each of red, green and blue can be defined in a twenty-four bit per pixel system. When displaying the pixel at X 0 , Y 0 , the bit values at memory location  150  are provided to video driver  170 . The bits corresponding to the red (R) component are provided to the red driver, the bits representing the green (G) component are provided to the green driver, and the bits representing the blue (B) component are provided to the blue driver. These drivers activate the red, green and blue phosphors at the pixel location  190 . The bit values for each color, red, green and blue, determine the intensity of that color in the display pixel. By varying the intensities of the red, green and blue components, different colors may be produced at that pixel location. 
   Color information may be represented by color models other than RGB. One such color model is known as the YUV (or Y′CbCr as specified in ITU.BT-601) color space which is used in the commercial color TV broadcasting system. The YUV color space is a recoding of the RGB color space, and can be mapped into the RGB color cube. The RGB to YUV conversion that performs the mapping may be defined, for example, by the following matrix equation: 
   
     
       
         
           
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                       0.299 
                     
                     
                       0.587 
                     
                     
                       0.144 
                     
                   
                   
                     
                       
                         - 
                         0.169 
                       
                     
                     
                       
                         - 
                         0.331 
                       
                     
                     
                       0.500 
                     
                   
                   
                     
                       0.500 
                     
                     
                       
                         - 
                         0.419 
                       
                     
                     
                       
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                         0.081 
                       
                     
                   
                 
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   The inverse of the matrix is used for the reverse conversion. The Y axis of the YUV color model represents the luminance of the display pixel, and matches the luminosity response curve for the human eye. U and V are chrominance values. In a monochrome receiver, only the Y value is used. In a color receiver, all three axes are used to provide display information. 
   In operation, an image may be recorded with a color camera, which may be an RGB system, and converted to YUV for transmission. At the receiver, the YUV information is then retransformed into RGB information to drive the color display. 
   Many other color models are also used to represent video data. For example, CMY (cyan, magenta, yellow) is a color model based on the complements of the RGB components. There are also a variety of color models, similar to YUV, which specify a luminance value and multiple chrominance values, such as the YIQ color model. Each color model has its own color transformation for converting to a common displayable video format such as RGB. Most transformations may be defined with a transform matrix similar to that of the YIQ color space. 
   Image data is often provided as output of an application executing on a computer system. More than one such application may output image data to the same display device. For example, in a windowing display environment, a window manager may be implemented to manage the display of data from multiple applications into multiple windows on the display device. 
     FIG. 2  is a block diagram of a video display system comprising multiple applications ( 200  and  201 ) writing to a single frame buffer ( 140 ) under control of a window manager ( 202 ). As illustrated, applications  200  and  201  are coupled to transmission medium  203 , as are window manager  202  and frame buffer  140 . Frame buffer  140  drives display  160  as described with respect to  FIG. 1 . Transmission medium  203  may be a high-bandwidth bus in a personal computer system. In a network environment, such as one implementing thin clients or terminals, transmission medium  203  may comprise a lower bandwidth network that is shared with other applications executing on one or more servers, as well as frame buffers supporting other client displays. 
   In  FIG. 2 , window manager  202  manages the way in which video output of applications  200  and  201  (and the video output of any other application operating under the given windowing environment) is displayed on display  160 . Applications  200  and  201  register with window manager  203  when those applications have video output to be displayed. In accordance with one frame buffer access scheme, when an application wishes to write to the frame buffer  140 , the application may transmit a write request to window manager  202 . Window manager  202  then writes the specified image data to the frame buffer on behalf of the application. However, in a direct graphics access (DGA) scheme, the applications may write directly to the frame buffer, bypassing window manager  202 . 
   A mechanism by which window manager  202  manages video output is via a clip-list associated with frame buffer  140 . A clip-list provides information about which portions of a frame buffer, and hence the display, may be written by a given application. The clip-list may, for example, take the form of a bit mask or a list of rectangles that defines those portions of an application display window that are not overlapped by another window and are, therefore, visible to the user. When a user alters a window on the display (e.g., by closing, opening, dragging or resizing the window, or reordering layered windows), window manager  202  modifies the clip-list accordingly. When an application attempts to write to frame buffer  140 , the application determines that the clip-list has changed, and modifies its frame buffer writing operations appropriately. 
   In some systems, frame buffer  140  has an associated lock that must be obtained by an application before access to the buffer is granted. For example, if application  200  wishes to draw to display  160  by writing to frame buffer  140 , application  200  must first obtain the frame buffer lock. If another application currently has the lock, application  200  must wait until the lock is released by that other application. In similar fashion, window manager  202  must obtain the frame buffer lock to access the associated clip-list. 
     FIG. 3  is a block diagram illustrating an example of a display image  300  that could be displayed on display  160 . Display image  300  comprises a desktop window (window D) and application windows A, B and C. Application windows A–C may represent the video output of one or more applications, such as applications  200  and  201  of  FIG. 2 . The desktop window (D) may represent output of window manager  202 , or an associated windowing application. As shown, window A is partially occluded by windows B and C. Window B exists on top of, and within the borders of, window A, whereas window C overlaps the lower right corner of window A. 
     FIG. 3  also comprises clip-list  301 , containing a list of rectangles associated with each application. The display region assigned to an application includes a patchwork of rectangles representing those portions of the display containing the application window, excluding those regions occluded by another overlying window. The visible (and thus writable) portions of desktop window (D) include rectangles D 1 , D 2 , D 3 , D 4 , D 5  and D 6 . The visible portions of window A include rectangles A 1 , A 2 , A 3 , A 4  and A 5 . Windows B and C, being on the top layer, are unoccluded. Windows B and C include rectangles B 1  and C 1 , respectively. 
   Assuming that window A corresponds to application  200 , when application  200  writes to frame buffer  140 , application  200  will typically write only to those portions of the frame buffer corresponding to rectangles A 1 –A 5 . If the alignment state of windows A–C is modified, the clip-list will change accordingly. For example, if window B is closed or shifted behind window A, then rectangle B 1  is reassigned to window A and application  200 . Application  200  will recognize the change to the clip-list upon its next frame buffer access. 
     FIG. 4  is a flow diagram illustrating a process by which application  200  might display image data in window A. In step  400 , application  200  obtains image data, such as an M×N block of YUV image data ( 404 ). This data may be obtained from another video source, from an image file, or from a rendered image, for example. In step  401 , the image data is color converted into image data supported by the display, e.g., M×N RGB color data  405  suitable for driving a CRT display. 
   Based on the resolution of window A, RGB image data  405  is scaled to fill the window in step  402 . Assuming horizontal and vertical resolution scale factors α and β, the resulting image data is αM×βN RGB data  406 , containing (α×β) times as much image data as original image data  404 . For example, assuming doubled resolution (e.g., from 320×240 to 640×480) with α=β=2, the scaled image data is four times as large as the original image data. In step  403 , the scaled image data  406  is clipped in accordance with the clip-list function F C ( ) which extracts only those regions that are viewable. The viewable regions represented by clipped image  407  are written to the frame buffer for display. 
   For a single computer system in which an executing application is separated from the frame buffer only by a high-bandwidth processor bus, the video display process of  FIG. 4  may be adequate. However, in a network system, particularly one implementing thin clients, it may be desired to have the video application execute on a server separated from the frame buffer (and clip-list) by a shared, lower-bandwidth network. It is also possible that the applications and the window manager are executed on separate servers. Under these conditions, the transmission of image data, in the form of scaled image data  406  or clipped image data  407 , for every video application on the network may be prohibitive. In a network, transmission efficiency is important to the satisfactory performance of not only the video application being displayed, but other applications sharing the network. A more efficient video display process for networks is desired. 
   SUMMARY OF THE INVENTION 
   A method and apparatus for clipping video information before scaling is described. In an embodiment of the invention, a transmitter obtains video information in the form of image data, as well as clipping information defining one or more display regions in which the image data is to be displayed. In accordance with the clipping information, the transmitter performs clipping operations on the image data, and transmits the clipped image data to a receiver. Prior to displaying the clipped image data, the receiver performs any needed scaling of the clipped image data to conform to the dimensions of the display regions. By performing clipping operations prior to transmission, and scaling operations subsequent to transmission, unnecessary image data is omitted and greater transmission efficiency is achieved. 
   In one embodiment of the invention, the clipping information specifies rectangular display regions within a larger display boundary assigned to the image data. When clipping, the transmitter maps the rectangular display regions to corresponding rectangular data regions of the image data. The rectangular data regions are individually transmitted to the receiver where those data regions are scaled to meet the dimensions of the corresponding display regions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a video display apparatus. 
       FIG. 2  is a block diagram of a video display system having multiple applications. 
       FIG. 3  illustrates a display having multiple overlapping windows, and an associated clip-list. 
       FIG. 4  is a flow diagram of a video display process. 
       FIG. 5  is a flow diagram of a video display process in accordance with an embodiment of the invention. 
       FIGS. 6A–6C  illustrate the mapping of a clip rectangle to image data in accordance with an embodiment of the invention. 
       FIG. 7  is a flow diagram of a process for mapping a clip rectangle to image data in accordance with an embodiment of the invention. 
       FIG. 8  is a block diagram of a computer execution environment. 
       FIG. 9  is a block diagram of a human interface device computer system. 
       FIG. 10  is a block diagram of an embodiment of a human interface device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is a method and apparatus for clipping video information before scaling. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the invention. 
   Implementation of Network Video Display Process 
   In an embodiment of the invention, applications are executed on a server separated from a frame buffer on a thin client by a network. In reference to  FIG. 2 , the network acts as transmission medium  203 . The server can be considered as a video transmitter with the thin client acting as the receiver. By performing clipping of image data prior to transmission and scaling, the amount of image data that is transmitted over the network is minimized for greater efficiency. 
   To facilitate clipping at the server, each application is provided with a clip-list upon which to base its respective clipping operations. An application writes to the frame buffer in asynchronous packets of image data that are queued by the client. As changes are made to windows, the window manager application updates the status of each registered application&#39;s clip-list. 
     FIG. 5  is a flow diagram of a video display process in accordance with an embodiment of the invention. In step  500 , application  200  (executing on a server) obtains M×N image data  506  (e.g., in YUV or another known color scheme) to be output to window A of the display. This image data may be obtained from another video source, from an image file, or from a rendered image, for example. In step  501 , based on a resident copy of the clip-list identifying those portions of the frame buffer representing visible regions of window A, application  200  applies a clipping function F C ( ) to the image data to obtain clipped image date  507 . Clipped image data  507  is transmitted from the server to the client in step  502 . In one embodiment, the transmitted image data  508  is transmitted in blocks corresponding to rectangles in the clip-list (or sub-rectangles of the clip-list rectangles). 
   In step  503 , a receiver process executing at the client receives the transmitted image data  508  and converts, if necessary, the transmitted image data  508  into image data  509  of a displayable color format such as RGB. Image data  509  is scaled in step  504  to generate image data  510  having the resolution of display window A. Alternatively, steps  503  and  504  may be reversed such that color conversion is carried out on scaled image data. In step  505 , the scaled and color converted image data is written to the frame buffer to provide displayed image  511  filling all unoccluded portions of window A. 
   In  FIG. 5 , the bandwidth requirements for transmission of the image data at step  502  are reduced due to the extraction of occluded data by the clipping process. A further reduction in bandwidth requirements is achieved by performing upscaling of image data after transmission, in step  504 . 
     FIGS. 6A–6C  illustrate the mapping of a clip rectangle (such as one of rectangles A 1 –A 5 ) to image data in accordance with an embodiment of the invention.  FIG. 6A  comprises an array of image data ( 600 ) and clip rectangle  601 . Clip rectangle  601  is defined by an upper left corner location, C UL , and a lower right corner location, C LR . As shown, because the respective resolutions for the image data and the display data (and hence the clip rectangle) may differ, the corners of the clip rectangle may not correspond to pixels of the image data. Rather, the corner locations of the clip rectangle will map into a neighborhood of four image data pixels in this example. 
     FIGS. 6A and 6B  show enlarged portions of image data  600  comprising the four nearest pixel locations to the upper left and lower right corners, respectively, of the clip rectangle  601 . Upper left corner location C UL  of the clip rectangle is mapped to the nearest image data pixel P UL , whereas, the lower right corner location C LR  of the clip rectangle is mapped to the nearest image data pixel P LR . The nearest pixel may be determined, for example, based on the smallest Euclidean distance from the corner location. The portion of image data  600  that is mapped to clip rectangle  601  is the rectangle of image data (indicated with filled-in ovals) defined by pixels P UL  and P LR . It will be obvious that other locations within the clip rectangle (such as the upper right and lower left corners) may be similarly used to determine a rectangle of image data. 
   In one embodiment of the invention, where the original image data comprises more than a single resolution (such as YUV with subsampled chroma values), the pixel array is defined using the lowest resolution to ensure that the mapped rectangle of image data comprises complete pixel data. For example, using YUV data with chroma values subsampled by two, each pixel contains a luma (Y) value, but chroma values (U,V) are skipped every other pixel location along each axis. This embodiment ensures that pixels PUL and PLR are the nearest pixels that contain both luma and chroma values. 
   As illustrated in  FIG. 6A , mapping clip rectangle  601  onto image data  600  results in a rectangle of image data that is slightly larger than the clip rectangle in the horizontal axis, and slightly smaller along the vertical axis. When the rectangle of image data is scaled up at the client (receiver), this results in slightly reduced scaling along the horizontal axis, and slightly increased scaling along the vertical axis. There may be a slight horizontal and/or vertical shift of the image data as well. For windows that are partially occluded, multiple clip rectangles will be mapped to the image data, with each resulting rectangle of image data possibly having slightly different scale and shift factors from the other rectangles. However, those slight variations produce substantially negligible visual distortion. 
     FIG. 7  is a flow diagram of a clipping process in accordance with an embodiment of the invention. In step  701 , the first clip rectangle from the clip-list is obtained. The clip rectangle is effectively scaled down to the resolution of the image data, though the location values within the clip rectangle (e.g., C UL  and C LR ) reside at fractional pixel locations with respect to the image data. In step  702 , the upper left corner (C UL ) of the clip rectangle is identified, and, in step  703 , the closest image data pixel (P UL ) is determined. In step  704 , the lower right corner (C LR ) of the clip rectangle is identified, and, in step  705 , the closest image data pixel (P LR ) is determined. In step  706 , those image data pixels in the rectangle defined by pixels P UL  and P LR  are selected for display. The selected rectangle of image data pixels are transmitted to the receiver in step  707  to fill the display region defined by the respective clip rectangle. If no more clip rectangles exist in step  708  (meaning that all portions of the display window are either filled or occluded by other windows), display of the current frame of image data is complete. If clip rectangles in the clip-list remain to be processed in step  708 , the next clip rectangle is obtained at step  709 , and the process continues at step  702 . 
   As described in  FIG. 7 , in one embodiment, clip rectangles are processed, and the resulting image data rectangles are transmitted for display, individually. This method conforms well to a packet delivery format. One such format suitable for implementing an embodiment of the invention is described below. 
   Video Protocol Embodiment 
   In an embodiment of the invention, a video protocol is implemented in which data packets are used to transmit variably sized blocks of video data between a transmitter and a receiver using a connectionless datagram scheme. A connectionless scheme means that each packet of video data, i.e., each video block, is processed as an independent unit, and the loss of a data packet does not affect the processing of other data packets. This independence provides for robust video processing even on unreliable networks where packet loss may be commonplace. 
   Some networks are prone to periodic packet loss, i.e., packet loss at regular intervals. This periodic behavior can result in the stagnation of portions of the video display as the same video blocks are repeatedly lost. To prevent video block stagnation, the spatial order in which video blocks are sent to the receiver for display may be pseudo-randomly determined to disrupt any periodicity in packet performance. 
   In one embodiment, the data packets containing video data are provided with a sequence number. By tracking the sequence numbers, the receiver can note when a sequence number is skipped, indicating that the packet was lost during transmission. The receiver can then return to the transmitter a list or range of sequence numbers identifying the lost packet or packets. When the transmitter receives the list or range of sequences, the transmitter can decide whether to ignore the missed packets, resend the missed packets (such as for still images), or send updated packets (such as for streaming video that may have changed since the packet was lost). 
   In one embodiment of the invention, the video data packet comprises the following information:
         Sequence number—A video stream is processed as a series of blocks of video data. The sequence number provides a mechanism for the receiver to tell the transmitter what sequence numbers have been missed (e.g., due to packet loss), so that the transmitter may determine whether to resend, update or ignore the associated video block.   X—The X field designates the x-coordinate of the receiver&#39;s display device where the video block is to be displayed, e.g., the x-coordinate of the first pixel (i.e., the upper-left corner) of the video block.   Y—The Y field designates the y-coordinate of the receiver&#39;s display device where the video block is to be displayed, e.g., the y-coordinate of the first pixel (i.e., the upper-left corner) of the video block.   Width—The width field specifies the width of the destination rectangle on the receiver&#39;s display device wherein the video block is to be displayed.   Height—The height field specifies the height of the destination rectangle on the receiver&#39;s display device wherein the video block is to be displayed.   Source_w—The source width specifies the width of the video block in pixels. Note that the source width may be smaller than the width of the destination rectangle on the receiver&#39;s display device. If this is so, the receiver will upscale the video block horizontally to fill the width of the destination rectangle. The source width should not be larger than the width of the destination rectangle as this implies downscaling, which should be performed by the transmitter for efficiency.   Source_h—The source height specifies the height of the video block in pixels. Note that, as with source_w, the source height may be smaller than the height of the destination rectangle on the receiver&#39;s display device. As above, the receiver will upscale the video block vertically to fill the height of the destination rectangle. The source height should not be larger than the height of the destination rectangle as this implies downscaling, which should be performed by the transmitter for efficiency.   Luma encoding—The luma encoding field allows the transmitter to designate a particular luma encoding scheme from a set of specified luma encoding schemes.   Chroma_sub_X—This field allows the transmitter to designate the degree of horizontal subsampling performed on the video data chroma values.   Chroma_sub_Y—This field allows the transmitter to designate the degree of vertical subsampling performed on the video data chroma values.   Video data—The video data includes (source_w*source_h) pixel luma values (Y), and ((source_w/chroma_sub_x)* (source_h/chroma_sub_y)) signed chroma values (U, V or Cb, Cr).       

   YUV (Y′CbCr) Color 
   In an embodiment of the invention, the color model of the video protocol is specified by the International Telecommunications Union in ITU.BT-601 referring to an international standard for digital coding of television pictures using video data components Y′CbCr, where Y′ is a luminance or “luma” value, Cb (or U′) is a first chromaticity or “chroma” value represented as a blue color difference proportional to (B′-Y′) and Cr (or V′) is a second chroma value represented as a red color difference proportional to (R′-Y′) (Note that primed values such as Y′ indicate a gamma corrected value). This ITU specification is independent of any scanning standard and makes no assumptions regarding the “white” point or CRT gamma. For 0&lt;=(R,G,B)&lt;=1, the range for Y′ is 0&lt;=Y′&lt;=1 and the range for Cb and Cr is −0.5&lt;=(Cb, Cr)&lt;=0.5. 
   The R′G′B′&lt;—&gt; Y′CbCr color transforms are as follows: 
   
     
       
         
           
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                         - 
                         0.169 
                       
                     
                     
                       
                         - 
                         0.331 
                       
                     
                     
                       0.500 
                     
                   
                   
                     
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   Under the specified protocol, the transmitter performs any transformations required to convert the video data into the YUV format. This may include performing the RGB to YUV matrix conversion shown above to convert RGB data. Transformations may also include decompression from other color formats (e.g., H.261, MPEG1, etc.). The receiver can drive an RGB display device by performing the above matrix operation to convert incoming YUV (Y′CbCr) data received from a transmitter into RGB data for display at the display rectangle identified in the data packet. No other color transformations are necessary at the receiver. The receiver is also able to accept RGB data in the same video block format because RGB data is directly supported in the receiver. For transmission efficiency, however, any sizable video data transfers between a transmitter and receiver should be performed in the YUV color format to take advantage of the compression schemes described below. 
   Luma Compression 
   In each data packet containing a video block, there are (source_w* source_h) luma values—one for each pixel. If the luma encoding field indicates that no encoding is being performed, the luma values are unsigned eight-bit values. If, however, luma encoding is indicated in the luma encoding field, the luma values are encoded to achieve a compression ratio of 2:1. In an embodiment of the invention, the luma value “Y” is compressed using a quantized differential coding (QDC) scheme. In other embodiments, other compression schemes may be specified in the luma encoding field. 
   Luma values do not tend to vary significantly from one pixel to another. It is therefore possible to transmit the difference value between luma values for consecutive pixels rather than the luma values themselves. Further, the luma difference values can be satisfactorily quantized to one of sixteen quantization levels, each of which is identified by a four-bit code word. The quantization is non-linear, with more quantization levels near zero where luma differences between consecutive pixels are more likely to occur. 
   In one embodiment, the luma difference quantization is performed according to the following table: 
   
     
       
         
             
             
             
           
             
                 
             
             
               Difference Range 
               Code (Binary) 
               Quantized Difference Level 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               −255 to −91  
               0 (0000) 
               −100 
             
             
               −90 to −71 
               1 (0001) 
               −80 
             
             
               −70 to −51 
               2 (0010) 
               −60 
             
             
               −50 to −31 
               3 (0011) 
               −40 
             
             
               −30 to −16 
               4 (0100) 
               −20 
             
             
               −15 to −8  
               5 (0101) 
               −10 
             
             
               −7 to −3 
               6 (0110) 
               −4 
             
             
               −2 to 0  
               7 (0111) 
               −1 
             
             
               1 to 2 
               8 (1000) 
               1 
             
             
               3 to 7 
               9 (1001) 
               4 
             
             
                8 to 15 
               10 (1010)  
               10 
             
             
               16 to 30 
               11 (1011)  
               20 
             
             
               31 to 50 
               12 (1100)  
               40 
             
             
               51 to 70 
               13 (1101)  
               60 
             
             
               71 to 90 
               14 (1110)  
               80 
             
             
                91 to 255 
               15 (1111)  
               100 
             
             
                 
             
          
         
       
     
   
   The luma compression scheme is based on a subtraction of a “last_value” from the current pixel luma value to generate the luma difference. “Last_value” is used to model the luma value of the preceding pixel. To prevent divergence of the compression and decompression processes, the “last_value” is modeled to account for the previous quantized luma difference rather than to match the actual luma value of the last pixel. The modeled “last_value” in the compression process therefore matches the corresponding modeled “last_value” extracted in the decompression process. 
   Chroma Compression 
   The human eye is less sensitive to chroma information than to luma information, particularly in a spatial sense. For example, if, in generating an image, some of the chroma information is spread beyond the actual edges of an object in the image, the human eye will typically pick up on the edge cues provided by the luma information and overlook the inaccuracies in the spatial location of the chroma information. For this reason, some latitude can be taken with the manner in which chroma information is provided. Specifically, subsampling may be performed without significantly degrading visual quality. 
   In accordance with an embodiment of the invention, the amount of chroma information, and hence the amount of chroma compression, is specified by the chroma_sub_X and chroma_sub_Y fields in the video block data packet. If the values for both of those fields are zero, then there is no chroma information and the video block is monochrome, i.e., luma only. One possible specification for chroma subsampling is: 
   0—No chroma values; monochrome image 
   1—Subsample by one (i.e., no subsampling) 
   2—Subsample by two 
   3—Subsample by four 
   Further subsample arrangements may be provided by extending the above specification. Chroma_sub_X and chroma_sub_Y independently specify subsampling along respective axes. Several subsampling arrangements achieved by different combinations of chroma_sub_X and chroma_sub_Y, as defined above, are: 
   
     
       
         
             
             
             
             
           
             
                 
             
             
               chroma sub X 
               chroma sub Y 
               one chroma value per 
               compression 
             
             
                 
             
           
          
             
               0 
               0 
               0, no chroma data 
               — 
             
             
               0 
               1 
               not permitted 
               — 
             
             
               1 
               0 
               not permitted 
               — 
             
             
               1 
               1 
               pixel 
               1:1 
             
             
               2 
               1 
               1 × 2 pixel array 
               2:1 
             
             
               1 
               2 
               2 × 1 pixel array 
               2:1 
             
             
               3 
               1 
               1 × 4 pixel array 
               4:1 
             
             
               1 
               3 
               4 × 1 pixel array 
               4:1 
             
             
               3 
               2 
               2 × 4 pixel array 
               8:1 
             
             
               2 
               3 
               4 × 2 pixel array 
               8:1 
             
             
               3 
               3 
               4 × 4 pixel array 
               16:1  
             
             
                 
             
          
         
       
     
   
   Subsampling may be performed when packing data into a video block data packet by taking data only at the specified intervals in the specified directions. For example, for encoded value (chroma_sub_X, chroma_sub_Y)=(3, 2), chroma data would be taken at every fourth pixel along each row, and every other row would be skipped. Other schemes may be used to select a single pixel from the subsampling matrix, such as pseudo-random assignments. Further, the chroma values from each pixel in a subsampling matrix may be used to calculate a single set of average chroma values (U, V) for each subsampling matrix. 
   Subsampling is performed as the video block data packet is being packed and may occur before or after luma compression as luma and chroma compression are substantially independent. When the video block data packet reaches the receiver, the chroma subsamples are upsampled prior to being converted to RGB. Upsampling may be accomplished by taking the subsampled chroma information and duplicating or interpolating the chroma values for each pixel in the associated subsampling matrix. 
   Upscaling and Downscaling of Video Data 
   In an embodiment of the invention, the pixel array size of the source video block may differ from the size of the destination rectangle on the receiver&#39;s display. This size variation allows for a receiver with a large display to “blow up” or upscale a small video scene to make better use of the display resources. For example, a receiver may wish to upscale a 640×480 video stream to fill a 1024×1024 area on a large display device. Also, a receiver may have a smaller display than the size of a video stream. For this case, the video stream should be scaled down to be fully visible on the small display. 
   In accordance with an embodiment of the invention, upscaling is performed by the receiver, whereas downscaling is performed by the transmitter. One reason for this segregation of scaling duties is that scaled down video data requires lower network bandwidth to transmit. By downscaling video data on its own, the transmitter avoids sending video data that would be later discarded by the receiver. This also permits some simplification of the receiver in that resources, such as software code for downscaling video data, are not needed at the receiver. 
   Upscaling typically involves duplication of video data. It would be inefficient to send duplicated video data over a network. Therefore, the receiver performs all upscaling operations after receipt of the video data in its smaller form. Upscaling of video data is supported in the fields associated with the video data packet. Specifically, the video protocol provides separate fields for specifying the video source pixel array size and the destination display rectangle size. The amount of horizontal scaling is (width/source_w), and the amount of vertical scaling is (height/source_h). 
   Upscaling is performed after the video data has been decompressed and transformed into RGB format, though in certain embodiments upscaling may precede, or be combined with, the decompression steps. The receiver expands the video data vertically, horizontally or both as need to make the video data fill the designated display rectangle. Expanding video data may be performed as simply as doubling pixel values, but more advanced image filtering techniques may be used to affect re-sampling of the image for better display quality. 
   Embodiment of Computer Execution Environment (Hardware) 
   An embodiment of the invention can be implemented as computer software in the form of computer readable code executed on a general purpose computer such as computer  800  illustrated in  FIG. 8 , or in the form of bytecode class files executable within a runtime environment (e.g., the Java runtime environment) running on such a computer. Optionally, a keyboard  810  and mouse (or other pointing device)  811  are coupled directly or indirectly to system bus  818 . The keyboard and mouse are for introducing user input to the computer system and communicating that user input to processor  813 . Other suitable input devices may be used in addition to, or in place of, the mouse  811  and keyboard  810 . I/O (input/output) unit  819  coupled to system bus  818  represents such I/O elements as a printer, A/V (audio/video) I/O, etc. 
   Computer  800  includes a video memory  814 , main memory  815  and, optionally, mass storage  812 , all coupled to system bus  818  along with keyboard  810 , mouse  811  and processor  813 . The mass storage  812  may include both fixed and removable media, such as magnetic, optical or magnetic optical storage systems or any other available mass storage technology. Bus  818  may contain, for example, thirty-two address lines for addressing video memory  814  or main memory  815 . The system bus  818  also includes, for example, a 64-bit data bus for transferring data between and among the components, such as processor  813 , main memory  815 , video memory  814  and mass storage  812 . Alternatively, multiplex data/address lines may be used instead of separate data and address lines. 
   In one embodiment of the invention, the processor  813  is a SPARC™ microprocessor from Sun Microsystems™, Inc., or a microprocessor manufactured by Motorola, such as the 680×0 processor or a microprocessor manufactured by Intel, such as the 80×86, or Pentium processor. However, any other suitable microprocessor or microcomputer may be utilized. Main memory  815  is comprised of dynamic random access memory (DRAM). Video memory  814  acts as the frame buffer, and is a dual-ported video random access memory. One port of the video memory  814  is coupled to video amplifier  816 . The video amplifier  816  is used to drive the cathode ray tube (CRT) raster monitor  817 . Video amplifier  816  is well known in the art and may be implemented by any suitable apparatus. This circuitry converts pixel data stored in video memory  814  to a raster signal suitable for use by monitor  817 . Monitor  817  is a type of monitor suitable for displaying graphic images. Alternatively, the video memory could be used to drive a flat panel or liquid crystal display (LCD), or any other suitable data presentation device. 
   Computer  800  may also include a communication interface  820  coupled to bus  818 . Communication interface  820  provides a two-way data communication coupling via a network link  821  to a local network  822 . For example, if communication interface  820  is an integrated services digital network (ISDN) card or a modem, communication interface  820  provides a data communication connection to the corresponding type of telephone line, which comprises part of network link  821 . If communication interface  820  is a local area network (LAN) card, communication interface  820  provides a data communication connection via network link  821  to a compatible LAN. Communication interface  820  could also be a cable modem or wireless interface. In any such implementation, communication interface  820  sends and receives electrical, electromagnetic or optical signals which carry digital data streams representing various types of information. 
   Network link  821  typically provides data communication through one or more networks to other data devices. For example, network link  821  may provide a connection through local network  822  to local server computer  823  or to data equipment operated by an Internet Service Provider (ISP)  824 . ISP  824  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  825 . Local network  822  and Internet  825  both use electrical, electromagnetic or optical signals which carry digital data streams. The signals through the various networks and the signals on network link  821  and through communication interface  820 , which carry the digital data to and from computer  800 , are exemplary forms of carrier waves transporting the information. 
   Computer  800  can send messages and receive data, including program code, through the network(s), network link  821 , and communication interface  820 . In the Internet example, remote server computer  826  might transmit a requested code for an application program through Internet  825 , ISP  824 , local network  822  and communication interface  820 . 
   The received code may be executed by processor  813  as it is received, and/or stored in mass storage  812 , or other non-volatile storage for later execution. In this manner, computer  800  may obtain application code in the form of a carrier wave. 
   Application code may be embodied in any form of computer program product. A computer program product comprises a medium configured to store or transport computer readable code or data, or in which computer readable code or data may be embedded. Some examples of computer program products are CD-ROM disks, ROM cards, floppy disks, magnetic tapes, computer hard drives, and servers on a network. 
   Human Interface Device Computer System 
   The invention has application to computer systems where the data is provided through a network. The network can be a local area network, a wide area network, the internet, world wide web, or any other suitable network configuration. One embodiment of the invention is used in a computer system configuration referred to herein as a human interface device computer system. 
   In this system the functionality of the system is partitioned between a display and input device, and data sources or services. The display and input device is a human interface device (HID). The partitioning of this system is such that state and computation functions have been removed from the HID and reside on data sources or services. In one embodiment of the invention, one or more services communicate with one or more HIDs through some interconnect fabric, such as a network. An example of such a system is illustrated in  FIG. 9 . Referring to  FIG. 9 , the system consists of computational service providers  900  communicating data through interconnect fabric  901  to HIDs  902 . 
   Computational Service Providers—In the HID system, the computational power and state maintenance is found in the service providers, or services. The services are not tied to a specific computer, but may be distributed over one or more traditional desktop systems such as described in connection with  FIG. 8 , or with traditional servers. One computer may have one or more services, or a service may be implemented by one or more computers. The service provides computation, state, and data to the HIDs and the service is under the control of a common authority or manager. In  FIG. 9 , the services are found on computers  910 ,  911 ,  912 ,  913 , and  914 . In an embodiment of the invention, any of computers  910 – 914  could be implemented as a transmitter. 
   Examples of services include X11/Unix services, archived video services, Windows NT service, Java™ program execution service, and others. A service herein is a process that provides output data and responds to user requests and input. 
   Interconnection Fabric—The interconnection fabric is any of multiple suitable communication paths for carrying data between the services and the HIDs. In one embodiment the interconnect fabric is a local area network implemented as an Ethernet network. Any other local network may also be utilized. The invention also contemplates the use of wide area networks, the internet, the world wide web, and others. The interconnect fabric may be implemented with a physical medium such as a wire or fiber optic cable, or it may be implemented in a wireless environment. 
   HIDs—The HID is the means by which users access the computational services provided by the services.  FIG. 9  illustrates HIDs  921 ,  922 , and  923 . A HID consists of a display  926 , a keyboard  924 , mouse  925 , and audio speakers  927 . The HID includes the electronics need to interface these devices to the interconnection fabric and to transmit to and receive data from the services. In an embodiment of the invention, an HID is implemented as a receiver. 
   A block diagram of the HID is illustrated in  FIG. 10 . The components of the HID are coupled internally to a PCI bus  1012 . A network control block  1002  communicates to the interconnect fabric, such as an ethernet, through line  1014 . An audio codec  1003  receives audio data on interface  1016  and is coupled to block  1002 . USB data communication is provided on lines  1013  to USB controller  1001 . 
   The USB controller  1001 , network controller  1002  and an embedded processor  1004  are all coupled to the PCI bus  1012 . Embedded processor  1004  may be, for example, a Sparc2ep with coupled flash memory  1005  and DRAM  1006 . Also coupled to PCI bus  1012  is the video controller  1009  that provides SVGA output on line  1015 . The video controller  1009  may be, for example, an ATI RAGE128 frame buffer controller with coupled SGRAM  1017 . NTSC or PAL data is provided into the video controller through video decoder  1010 . A smartcard interface  1008  may also be coupled to the video controller  1009 . 
   The computer systems described above are for purposes of example only. An embodiment of the invention may be implemented in any type of computer system or programming or processing environment. 
   Thus, a method and apparatus for clipping video information before scaling have been described in conjunction with one or more specific embodiments. The invention is defined by the claims and their full scope of equivalents.