Graphics display system with unified memory architecture6721837Abstract A graphics display system integrated circuit is used in a set-top box for controlling a television display. The graphics display system processes analog video input, digital video input, and graphics input. The system incorporates a unified memory architecture that is shared by the graphics system, a CPU, and other peripherals. The unified memory architecture uses real time scheduling to service tasks. Critical instant analysis is used to find a schedule for memory usage that does not affect memory requirements of real time tasks while at the same time servicing non-real-time tasks as needed. Claims What is claimed is: Description FIELD OF THE INVENTION
TABLE 1
Graphics Data Formats
win.sub.-- Data
format Format Data Format Description
0000b RGB16 5-BIT RED, 6-BIT GREEN, 5-BIT BLUE
0001b RGB15 + 1 RGB15 plus one bit alpha (keying)
0010b RGBA4444 4-BIT RED, GREEN, BLUE, ALPHA
0100b CLUT2 2-bit CLUT with YUV and alpha in table
0101b CLUT4 4-bit CLUT with YUV and alpha in table
0110b CLUT8 8-bit CLUT with YUV and alpha in table
0111b ACLUT16 8-BIT ALPHA, 8-BIT CLUT INDEX
1000b ALPHA0 Single win_alpha and single RGB win_color
1001b ALPHA2 2-bit alpha with single RGB win_color
1010b ALPHA4 4-bit alpha with single RGB win_color
1011b ALPHA8 8-bit alpha with single RGB win_color
1100b YUV422 U and V are sampled at half the rate of Y
1111b RESERVED Special coding for blank line in new header,
i.e., indicates an empty line
The window memory start address preferably is a 26-bit data field that indicates a starting memory address of the graphics data of the graphics window to be displayed on the screen. The window memory start address points to the first address in the corresponding external SDRAM which is accessed to display data on the graphics window defined by the window descriptor. When the window operation parameter indicates the graphics CLUT reloading operation, the window memory start address indicates a starting memory address of data to be loaded into the graphics CLUT. Word 1 in the window descriptor preferably includes a window layer parameter, a window memory pitch value and a window color value. The window layer parameter is preferably a 4-bit data indicating the order of layers of graphics windows. Some of the graphics windows may be partially or completely stacked on top of each other, and the window layer parameter indicates the stacking order. The window layer parameter preferably indicates where in the stack the graphics window defined by the window descriptor should be placed. In the preferred embodiment, a graphics window with a window layer parameter of 0000b is defined as the bottom most layer, and a graphics window with a window layer parameter of 1111b is defined as the top most layer. Preferably, up to eight graphics windows may be processed in each scan line. The window memory pitch value is preferably a 12-bit data field indicating the pitch of window memory addressing. Pitch refers to the difference in memory address between two pixels that are vertically adjacent within a window. The window color value preferably is a 16-bit RGB color, which is applied as a single color to the entire graphics window when the window format parameter is 1000b, 1001b, 1010b, or 1011b. Every pixel in the window preferably has the color specified by the window color value, while the alpha value is determined per pixel and per window as specified in the window descriptor and the pixel format. The engine preferably uses the window color value to implement a solid surface. Word 2 in the window descriptor preferably includes an alpha type, a widow alpha value, a window y-end value and a window y-start value. The word 2 preferably also includes two bits reserved for future definition, such as high definition television (HD) applications. The alpha type is preferably a 2-bit data field that indicates the method of selecting an alpha value for the graphics window. The alpha type of 00b indicates that the alpha value is to be selected from chroma keying. Chroma keying determines whether each pixel is opaque or transparent based on the color of the pixel. Opaque pixels are preferably considered to have an alpha value of 1.0, and transparent pixels have an alpha value of 0, both on a scale of 0 to 1. Chroma keying compares the color of each pixel to a reference color or to a range of possible colors; if the pixel matches the reference color, or if its color falls within the specified range of colors, then the pixel is determined to be transparent. Otherwise it is determined to be opaque. The alpha type of 01b indicates that the alpha value should be derived from the graphics CLUT, using the alpha value in each entry of the CLUT. The alpha type of 10b indicates that the alpha value is to be derived from the luminance Y. The Y value that results from conversion of the pixel color to the YUV color space, if the pixel color is not already in the YUV color, is used as the alpha value for the pixel. The alpha type of 11b indicates that only a single alpha value is to be applied to the entire graphics window. The single alpha value is preferably included as the window alpha value next. The window alpha value preferably is an 8-bit alpha value applied to the entire graphics window. The effective alpha value for each pixel in the window is the product of the window alpha and the alpha value determined for each pixel. For example, if the window alpha value is 0.5 on a scale of 0 to 1, coded as 0x80, then the effective alpha value of every pixel in the window is one-half of the value encoded in or for the pixel itself. If the window format parameter is 1000b, i.e., a single alpha value is to be applied to the graphics window, then the per-pixel alpha value is treated as if it is 1.0, and the effective alpha value is equal to the window alpha value. The window y-end value preferably is a 10-bit data field that indicates the ending display line of the graphics window on the screen. The graphics window defined by the window descriptor ends at the display line indicated by the window y-end value. The window y-start value preferably is a 10-bit data field that indicates a starting display line of the graphics window on a screen. The graphics window defined by the window descriptor begins at the display line indicated in the window y-start value. Thus, a display of a graphics window can start on any display line on the screen based on the window y-start value. Word 3 in the window descriptor preferably includes a window filter enable parameter, a blank start pixel value, a window x-size value and a window x-start value. In addition, the word 3 includes two bits reserved for future definition, such as HD applications. Five bits of the 32-bit word 3 are not used. The window filter enable parameter is a 1-bit field that indicates whether low pass filtering is to be enabled during YUV 4:4:4 to YUV 4:2:2 conversion. The blank start pixel value preferably is a 4-bit parameter indicating a number of blank pixels at the beginning of each display line. The blank start pixel value preferably signifies the number of pixels of the first word read from memory, at the beginning of the corresponding graphics window, to be discarded. This field indicates the number of pixels in the first word of data read from memory that are not displayed. For example, if memory words are 32 bits wide and the pixels are 4 bits each, there are 8 possible first pixels in the first word. Using this field, 0 to 7 pixels may be skipped, making the 1.sup.st to the 8.sup.th pixel in the word appear as the first pixel, respectively. The blank start pixel value allows graphics windows to have any horizontal starting position on the screen, and may be used during soft horizontal scrolling of a graphics window. The window x-size value preferably is a 10-bit data field that indicates the size of a graphics window in the x direction, i.e., horizontal direction. The window x-size value preferably indicates the number of pixels of a graphics window in a display line. The window x-start value preferably is a 10-bit data field that indicates a starting pixel of the graphics window on a display line. The graphics window defined by the window descriptor preferably begins at the pixel indicated by the window x-start value of each display line. With the window x-start value, any pixel of a given display line can be chosen to start painting the graphics window. Therefore, there is no need to load pixels on the screen prior to the beginning of the graphics window display area with black. III. Graphics Window Control Data Passing Mechanism In one embodiment of the present invention, a FIFO in the graphics display path accepts raw graphics data as the raw graphics data is read from memory, at the full memory data rate using a clock of the memory controller. In this embodiment, the FIFO provides this data, initially stored in an external memory, to subsequent blocks in the graphics pipeline. In systems such as graphics display systems where multiple types of data may be output from one module, such as a memory controller subsystem, and used in another subsystem, such as a graphics processing subsystem, it typically becomes progressively more difficult to support a combination of dynamically varying data types and data transfer rates and FIFO buffers between the producing and consuming modules. The conventional way to address such problems is to design a logic block that understands the varying parameters of the data types in the first module and controls all of the relevant variables in the second module. This may be difficult due to variable delays between the two modules, due to the use of FIFOs between them and varying data rate, and due to the complexity of supporting a large number of data types. The system preferably processes graphics images for display by organizing the graphics images into windows in which the graphics images appear on the screen, obtaining data that describes the windows, sorting the data according to the depth of the window on the display, transferring graphics images from memory, and blending the graphics images using alpha values associated with the graphics images. In the preferred embodiment, a packet of control information called a header packet is passed from the window controller to the display engine. All of the required control information from the window controller preferably is conveyed to the display engine such that all of the relevant variables from the window controller are properly controlled in a timely fashion and such that the control is not dependent on variations in delays or data rates between the window controller and the display engine. A header packet preferably indicates the start of graphics data for one graphics window. The graphics data for that graphics window continues until it is completed without requiring a transfer of another header packet. A new header packet is preferably placed in the FIFO when another window is to start. The header packets may be transferred according to the order of the corresponding window descriptors in the window descriptor lists. In a display engine that operates according to lists of window descriptors, windows may be specified to overlap one another. At the same time, windows may start and end on any line, and there may be many windows visible on any one line. There are a large number of possible combinations of window starting and ending locations along vertical and horizontal axes and depth order locations. The system preferably indicates the depth order of all windows in the window descriptor listand implements the depth ordering correctly while accounting for all windows. Each window descriptor preferably includes a parameter indicating the depth location of the associated window. The range that is allowed for this parameter can be defined to be almost any useful value. In the preferred embodiment there are 16 possible depth values, ranging from 0 to 15, with 0 being the back-most (deepest, or furthest from the viewer), and 15 being the top or front-most depth. The window descriptors are ordered in the window descriptor list in order of the first display scan line where the window appears. For example if window A spans lines 10 to 20, window B spans lines 12 to 18, and window C spans lines 5 to 20, the order of these descriptors in the list would be {C, A, B}. In the hardware, which is a preferably a VLSI device, there is preferably on-chip memory capable of storing a number of window descriptors. In the preferred implementation, this memory can store up to 8 window descriptors on-chip, however the size of this memory may be made larger or smaller without loss of generality. Window descriptors are read from main memory into the on-chip descriptor memory in order from the start of the list, and stopping when the on-chip memory is full or when the most recently read descriptor describes a window that is not yet visible, i.e., its starting line is on a line that has a higher number than the line currently being constructed. Once a window has been displayed and is no longer visible, it may be cast out of the on-chip memory and the next descriptor in the list may read from main memory. At any given display line, the order of the window descriptors in the on-chip memory bears no particular relation to the depth order of the windows on the screen. The hardware that controls the compositing of windows builds up the display in layers, starting from the back-most layer. In the preferred embodiment, the back most layer is layer 0. The hardware performs a quick search of the back-most window descriptor that has not yet been composited, regardless of its location in the on-chip descriptor memory. In the preferred embodiment, this search is performed as follows: All 8 window descriptors are stored on chip in such a way that the depth order numbers of all of them are available simultaneously. While the depth numbers in the window descriptors are 4 bit numbers, representing 0 to 15, the on-chip memory has storage for 5 bits for the depth number. Initially the 5 bit for each descriptor is set to 0. The depth order values are compared in a hierarchy of pair-wise comparisons, and the lower of the two depth numbers in each comparison wins the comparison. That is, at the first stage of the test descriptor pairs {0, 1}, {2, 3}, {4, 5}, and {6, 7} are compared, where {0-7} represent the eight descriptors stored in the on-chip memory. This results in four depth numbers with associated descriptor numbers. At the next stage two pair-wise comparisons compare {(0, 1), (2, 3)} and {(4, 5), (6, 7)}. Each of these results in a depth number of the lower depth order number and the associated descriptor number. At the third stage, one pair-wise comparison finds the smallest depth number of all, and its associated descriptor number. This number points the descriptor in the on-chip memory with the lowest depth number, and therefore the greatest depth, and this descriptor is used first to render the associated window on the screen. Once this window has been rendered onto the screen for the current scan line, the fifth bit of the depth number in the on-chip memory is set to 1, thereby ensuring that the depth value number is greater than 15, and as a result this depth number will preferably never again be found to be the back-most window until all windows have been rendered on this scan line, preventing rendering this window twice. Once all the windows have been rendered for a given scan line, the fifth bits of all the on-chip depth numbers are again set to 0; descriptors that describe windows that are no longer visible on the screen are cast out of the on-chip memory; new descriptors are read from memory as required (that is, if all windows in the on-chip memory are visible, the next descriptor is read from memory, and this repeats until the most recently read descriptor is not yet visible on the screen), and the process of finding the back most descriptor and rendering windows onto the screen repeats. Referring to FIG. 7, window descriptors are preferably sorted by the window controller and used to transfer graphics data to the display engine. Each of window descriptors, including the window descriptor 0 through the window descriptor 7300a-h, preferably contains a window layer parameter. In addition, each window descriptor is preferably associated with a window line done flag indicating that the window descriptor has been processed on a current display line. The window controller preferably performs window sorting at each display line using the window layer parameters and the window line done flags. The window controller preferably places the graphics window that corresponds to the window descriptor with the smallest window layer parameter at the bottom, while placing the graphics window that corresponds to the window descriptor with the largest window layer parameter at the top. The window controller preferably transfers the graphics data for the bottom-most graphics window to be processed first. The window parameters of the bottom-most window are composed into a header packet and written to the graphics FIFO. The DMA engine preferably sends a request to the memory controller to read the corresponding graphics data for this window and send the graphics data to the graphics FIFO. The graphics FIFO is then read by the display engine to compose a display line, which is then written to graphics line buffers. The window line done flag is preferably set true whenever the window surface has been processed on the current display line. The window line done flag and the window layer parameter may be concatenated together for sorting. The window line done flag is added to the window layer parameter as the most significant bit during sorting such that {window line done flag [4], window layer parameter [3:0]} is a five bit binary number, a window layer value, with window line done flag as the most significant bit. The window controller preferably selects a window descriptor with the smallest window layer value to be processed. Since the window line done flag is preferably the most significant bit of the window layer value, any window descriptor with this flag set, i.e., any window that has been processed on the current display line, will have a higher window layer value than any of the other window descriptors that have not yet been processed on the current display line. When a particular window descriptor is processed, the window line done flag associated with that particular window descriptor is preferably set high, signifying that the particular window descriptor has been processed for the current display line. A sorter 304 preferably sorts all eight window descriptors after any window descriptor is processed. The sorting may be implemented using binary tree sorting or any other suitable sorting algorithm. In binary tree sorting for eight window descriptors, the window layer value for four pairs of window descriptors are compared at a first level using four comparators to choose the window descriptor that corresponds to a lower window in each pair. In the second level, two comparators are used to select the window descriptor that corresponds to the bottom most graphics window in each of two pairs. In the third and the last level, the bottom-most graphics windows from each of the two pairs are compared against each other preferably using only one comparator to select the bottom window. A multiplexer 302 preferably multiplexes parameters from the window descriptors. The output of the sorter, i.e., window selected to be the bottom most, is used to select the window parameters to be sent to a direct memory access ("DMA") module 306 to be packaged in a header packet and sent to a graphics FIFO 308. The display engine preferably reads the header packet in the graphics FIFO and processes the raw graphics data based on information contained in the header packet. The header packet preferably includes a first header word and a second header word. Corresponding graphics data is preferably transferred as graphics data words. Each of the first header word, the second header word and the graphics data words preferably includes 32 bits of information plus a data type bit. The first header word preferably includes a 1-bit data type, a 4-bit graphics type, a 1-bit first window parameter, a 1-bit top/bottom parameter, a 2-bit alpha type, an 8-bit window alpha value and a 16-bit window color value. Table 2 shows contents of the first header word.
TABLE 2
First Header Word
Bit
Position 32 31-28 27 26 25-24 23-16 15-0
Data Data graphics First top/ alpha window window
Content type type Win- bottom type alpha color
dow
The 1-bit data type preferably indicates whether a 33-bit word in the FIFO is a header word or a graphics data word. A data type of 1 indicates that the associated 33-bit word is a header word while the data type of 0 indicates that the associated 33-bit word is a graphics data word. The graphics type indicates the data format of the graphics data to be displayed in the graphics window similar to the window format parameter in the word 0 of the window descriptor, which is described in Table 1 above. In the preferred embodiment, when the graphics type is 1111, there is no window on the current display line, indicating that the current display line is empty. The first window parameter of the first header word preferably indicates whether the window associated with that first header word is a first window on a new display line. The top/bottom parameter preferably indicates whether the current display line indicated in the first header word is at the top or the bottom edges of the window. The alpha type preferably indicates a method of selecting an alpha value individually for each pixel in the window similar to the alpha type in the word 2 of the window descriptor. The window alpha value preferably is an alpha value to be applied to the window as a whole and is similar to the window alpha value in the word 2 of the window descriptor. The window color value preferably is the color of the window in 16-bit RGB format and is similar to the window color value in the word 1 of the window descriptor. The second header word preferably includes the 1-bit data type, a 4-bit blank pixel count, a 10-bit left edge value, a 1-bit filter enable parameter and a 10-bit window size value. Table 3 shows contents of the second header word in the preferred embodiment.
TABLE 3
Second Header Word
Bit
Position 32 31-28 25-16 10 9-0
Data data Blank pixel Left edge filter window size
Content type count enabler
Similar to the first header word, the second header word preferably starts with the data type indicating whether the second header word is a header word or a graphics data word. The blank pixel count preferably indicates a number of blank pixels at a left edge of the window and is similar to the blank start pixel value in the word 3 of the window descriptor. The left edge preferably indicates a starting location of the window on a scan line, and is similar to the window x-start value in the word 3 of the window descriptor. The filter enable parameter preferably enables a filter during a conversion of graphics data from a YUV 4:4:4 format to a YUV 4:2:2 format and is similar to the window filter enable parameter in word 3 of the window descriptor. Some YUV 4:4:4 data may contain higher frequency content than others, which may be filtered by enabling a low pass filter during a conversion to the YUV 4:2:2 format. The window size value preferably indicates the actual horizontal size of the window and is similar to the window x-size value in word 3 of the window descriptor. When the composition of the last window of the last display line is completed, an empty-line header is preferably placed into the FIFO so that the display engine may release the display line for display. Packetized data structures have been used primarily in the communication world where large amount of data needs to be transferred between hardware using a physical data link (e.g., wires). The idea is not known to have been used in the graphics world where localized and small data control structures need to be transferred between different design entities without requiring a large off-chip memory as a buffer. In one embodiment of the present system, header packets are used, and a general-purpose FIFO is used for routing. Routing may be accomplished in a relatively simple manner in the preferred embodiment because the write port of the FIFO is the only interface. In the preferred embodiment, the graphics FIFO is a synchronous 32.times.33 FIFO built with a static dual-port RAM with one read port and one write port. The write port preferably is synchronous to a 81 MHz memory clock while the read port may be asynchronous (not synchronized) to the memory clock. The read port is preferably synchronous to a graphics processing clock, which runs preferably at 81 MHz, but not necessarily synchronized to the memory clock. Two graphics FIFO pointers are preferably generated, one for the read port and one for the write port. In this embodiment, each graphics FIFO pointer is a 6-bit binary counter which ranges from 000000b to 111111b, i.e., from 0 to 63. The graphics FIFO is only 32 words deep and requires only 5 bits to represent each 33-bit word in the graphics FIFO. An extra bit is preferably used to distinguish between FIFO full and FIFO empty states. The graphics data words preferably include the 1-bit data type and 32-bit graphics data bits. The data type is 0 for the graphics data words. In order to adhere to a common design practice that generally limits the size of a DMA burst into a FIFO to half the size of the FIFO, the number of graphics data words in one DMA burst preferably does not exceed 16. In an alternate embodiment, a graphics display FIFO is not used. In this embodiment, the graphics converter processes data from memory at the rate that it is read from memory. The memory and conversion functions are in a same clock domain. Other suitable FIFO designs may be used. Referring to FIG. 8, a flow diagram illustrates a process for loading and processing window descriptors. First the system is preferably reset in step 310. Then the system in step 312 preferably checks for a vertical sync ("VSYNC"). When the VSYNC is received, the system in step 314 preferably proceeds to load window descriptors into the window controller from the external SDRAM or other suitable memory over the DMA channel for window descriptors. The window controller may store up to eight window descriptors in one embodiment of the present invention. The step in step 316 preferably sends a new line header indicating the start of a new display line. The system in step 320 preferably sorts the window descriptors in accordance with the process described in reference to FIG. 7. Although sorting is indicated as a step in this flow diagram, sorting actually may be a continuous process of selecting the bottom-most window, i.e., the window to be processed. The system in step 322 preferably checks to determine if a starting display line of the window is greater than the line count of the current display line. If the starting display line of the window is greater than the line count, i.e., if the current display line is above the starting display line of the bottom most window, the current display line is a blank line. Thus, the system in step 318 preferably increments the line count and sends another new line header in step 316. The process of sending a new line header and sorting window descriptor continues as long as the starting display line of the bottom most (in layer order) window is below the current display line. The display engine and the associated graphics filter preferably operate in one of two modes, a field mode and a frame mode. In both modes, raw graphics data associated with graphics windows is preferably stored in frame format, including lines from both interlaced fields in the case of an interlaced display. In the field mode, the display engine preferably skips every other display line during processing. In the field mode, therefore, the system in step 318 preferably increments the line count by two each time to skip every other line. In the frame mode, the display engine processes every display line sequentially. In the frame mode, therefore, the system in step 318 preferably increments the line count by one each time. When the system in step 322 determines that the starting display of the window is greater than the line count, the system in step 324 preferably determines from the header packet whether the window descriptor is for displaying a window or re-loading the CLUT. If the window header indicates that the window descriptor is for re-loading CLUT, the system in step 328 preferably sends the CLUT data to the CLUT and turns on the CLUT write strobe to load CLUT. If the system in step 324 determines that the window descriptor is for displaying a window, the system in step 326 preferably sends a new window header to indicate that graphics data words for a new window on the display line are going to be transferred into the graphics FIFO. Then, the system in step 330 preferably requests the DMA module to send graphics data to the graphics FIFO over the DMA channel for graphics data. In the event the FIFO does not have sufficient space to store graphics data in a new data packet, the system preferably waits until such space is made available. When graphics data for a display line of a current window is transferred to the FIFO, the system in step 332 preferably determines whether the last line of the current window has been transferred. If the last line has been transferred, a window descriptor done flag associated with the current window is preferably set. The window descriptor done flag indicates that the graphics data associated with the current window descriptor has been completely transferred. When the window descriptor done flag is set, i.e., when the current window descriptor is completely processed, the system sets a window descriptor done flag in step 334. Then the system in step 336 preferably sets a new window descriptor update flag and increments a window descriptor update counter to indicate that a new window descriptor is to be copied from the external memory. Regardless of whether the last line of the current window has been processed, the system in step 338 preferably sets the window line done flag for the current window descriptor to signify that processing of this window descriptor on the current display line has been completed. The system in step 340 preferably checks the window line done flags associated with all eight window descriptors to determine whether they are all set, which would indicate that all the windows of the current display line have been processed. If not all window line done flags are set, the system preferably proceeds to step 320 to sort the window descriptors and repeat processing of the new bottom-most window descriptor. If all eight window line done flags are determined to be set in step 340, all window descriptors on the current display line have been processed. In this case, the system in step 342 preferably checks whether an all window descriptor done flag has been set to determine whether all window descriptors have been processed completely. The all window descriptor-done flag is set when processing of all window descriptors in the current frame or field have been processed completely. If the all window descriptor done flag is set, the system preferably returns to step 310 to reset and awaits another VSYNC in step 312. If not all window descriptors have been processed, the system in step 344 preferably determines if the new window descriptor update flag has been set. In the preferred embodiment, this flag would have been set in step 334 if the current window descriptor has been completely processed. When the new window descriptor update flag is set, the system in step 352 preferably sets up the DMA to transfer a new window descriptor from the external memory. Then the system in step 350 preferably clears the new window descriptor update flag. After the system clears the new window descriptor update flag or when the new window descriptor update flag is not set in the first place, the system in step 348 preferably increments a line counter to indicate that the window descriptors for a next display line should be processed. The system in step 346 preferably clears all eight window line done flags to indicate that none of the window descriptors have been processed for the next display line. Then the system in step 316 preferably initiates processing of the new display line by sending a new line header to the FIFO. In the preferred embodiment, the graphics converter in the display engine converts raw graphics data having various different formats into a common format for subsequent compositing with video and for display. The graphics converter preferably includes a state machine that changes state based on the content of the window data packet. Referring to FIG. 9, the state machine in the graphics converter preferably controls unpacking and processing of the header packets. A first header word processing state 354 is preferably entered wherein a first window parameter of the first header word is checked (step 356) to determine if the window data packet is for a first graphics window of a new line. If the header packet is not for a first window of a new line, after the first header word is processed, the state preferably changes to a second header word processing state 362. If the header packet is for a first graphics window of a new line, the state machine preferably enters a clock switch state 358. In the clock switch state, the clock for a graphics line buffer which is going to store the new line switches from a display clock to a memory clock, e.g., from a 13.5 MHz clock to a 81 MHz clock. From the clock switch state, a graphics type in the first header word is preferably checked (step 360) to determine if the header packet represents an empty line. A graphics type of 1111b preferably refers to an empty line. If the graphics type is 1111b, the state machine enters the first header word processing state 354, in which the first header word of the next header packet is processed. If the graphics type is not 1111b, i.e. the display line is not empty, the second header word is processed. Then the state machine preferably enters a graphics content state 364 wherein words from the FIFO are checked (step 366) one at a time to verify that they are data words. The state machine preferably remains in the graphics content state as long as each word read is a data word. While in the graphics content state, if a word received is not a data word, i.e., it is a first or second header word, then the state machine preferably enters a pipeline complete state 368 and then to the first header processing state 354 where reading and processing of the next window data packet is commenced. Referring to FIG. 10, the display engine 58 is preferably coupled to memory over a memory interface 370 and a CLUT over a CLUT interface 372. The display engine preferably includes the graphics FIFO 132 which receives the header packets and the graphics data from the memory controller over the memory interface. The graphics FIFO preferably provides received raw graphics data to the graphics converter 134 which converts the raw graphics data into the common compositing format. During the conversion of graphics format, the RGB to YUV converter 136 and data from the CLUT over the CLUT interface 372 are used to convert RGB formatted data and CLUT formatted data, respectively. The graphics converter preferably processes all of the window layers of each scan line in half the time, or less, of an interlaced display line, due to the need to have lines from both fields available in the SRAM for use by the graphics filter when frame mode filtering is enabled. The graphics converter operates at 81 MHz in one embodiment of the present invention, and the graphics converter is able to process up to eight windows on each scan line and up to three full width windows. For example, with a 13.5 MHz display clock, if the graphics converter processes 81 Mpixels per second, it can convert three windows, each covering the width of the display, in half of the active display time of an interlaced scan line. In one embodiment of the present invention, the graphics converter processes all the window layers of each scan line in half the time of an interlaced display line, due to the need to have lines from both fields available in the SRAM for use by the graphics filter. In practice, there may be some more time available since the active display time leaves out the blanking time, while the graphics converter can operate continuously. Graphics pixels are preferably read from the FIFO in raw graphics format, using one of the multiple formats allowed in the present invention and specified in the window descriptor. Each pixel may occupy as little as two bits or as much as 16 bits in the preferred embodiment. Each pixel is converted to a YUVa24 format (also referred to as aYUV 4:4:2:2), such as two adjacent pixels sharing a UV pair and having unique Y and alpha values, and each of the Y, U, V and alpha components occupying eight bits. The conversion process is generally dependent on the pixel format type and the alpha specification method, both of which are indicated by the window descriptor for the currently active window. Preferably, the graphics converter uses the CLUT memory to convert CLUT format pixels into RGB or YUV pixels. Conversions of RGB pixels may require conversion to YUV, and therefore, the graphics converter preferably includes a color space converter. The color space converter preferably is accurate for all coefficients. If the converter is accurate to eight or nine bits it can be used to accurately convert eight bit per component graphics, such as CLUT entries with this level of accuracy or RGB24 images. The graphics converter preferably produces one converted pixel per clock cycle, even when there are multiple graphics pixels packed into one word of data from the FIFO. Preferably the graphics processing clock, which preferably runs at 81 MHz, is used during the graphics conversion. The graphics converter preferably reads data from the FIFO whenever both conditions are met, including that the converter is ready to receive more data, and the FIFO has data ready. The graphics converter preferably receives an input from a graphics blender, which is the next block in the pipeline, which indicates when the graphics blender is ready to receive more converted graphics data. The graphics converter may stall if the graphics blender is not ready, and as a result, the graphics converter may not be ready to receive graphics data from the FIFO. The graphics converter preferably converts the graphics data into a YUValpha ("YUVa") format. This YUVa format includes YUV 4:2:2 values plus an 8-bit alpha value for every pixel, and as such it occupies 24 bits per pixel; this format is alternately referred to as aYUV 4:4:2:2. The YUV444-to-YUV422 converter 138 converts graphics data with the aYUV 4:4:4:4 format from the graphics converter into graphics data with the aYUV 4:4:2:2 format and provides the data to the graphics blender 140. The YUV444-to-YUV422 converter preferably has a capacity of performing low pass filtering to filter out high frequency components when needed. The graphics converter also sends and receives clock synchronization information to and from the graphics line buffers over a clock control interface 376. When provided with the converted graphics data, the graphics blender 140 preferably composites graphics windows into graphics line buffers over a graphics line buffer interface 374. The graphics windows are alpha blended into blended graphics and preferably stored in graphics line buffers. IV. Color Look-Up Table Loading Mechanism A color look-up table ("CLUT") is preferably used to supply color and alpha values to the raw graphics data formatted to address information contents of the CLUT. For a window surface based display, there may be multiple graphics windows on the same display screen with different graphics formats. For graphics windows using a color look-up table (CLUT) format, it may be necessary to load specific color look-up table entries from external memory to on-chip memory before the graphics window is displayed. The system preferably includes a display engine that processes graphics images formatted in a plurality of formats including a color look up table (CLUT) format. The system provides a data structure that describes the graphics in a window, provides a data structure that provides an indicator to load a CLUT, sorts the data structures into a list according to the location of the window on the display, and loads conversion data into a CLUT for converting the CLUT-formatted data into a different data format according to the sequence of data structures on the list. In the preferred embodiment, each window on the display screen is described with a window descriptor. The same window descriptor is used to control CLUT loading as the window descriptor used to display graphics on screen. The window descriptor preferably defines the memory starting address of the graphics contents, the x position on the display screen, the width of the window, the starting vertical display line and end vertical display line, window layer, etc. The same window structure parameters and corresponding fields may be used to define the CLUT loading. For example, the graphics contents memory starting address may define CLUT memory starting address; the width of graphics window parameter may define the number of CLUT entries to be loaded; the starting vertical display line and ending vertical display line parameters may be used to define when to load the CLUT; and the window layer parameter may be used to define the priority of CLUT loading if several windows are displayed at the same time, i.e., on the same display line. In the preferred embodiment, only one CLUT is used. As such, the contents of the CLUT are preferably updated to display graphics windows with CLUT formatted data that is not supported by the current content of the CLUT. One of ordinary skill in the art would appreciate that it is straightforward to use more than one CLUT and switch back and forth between them for different graphics windows. In the preferred embodiment, the CLUT is closely associated with the graphics converter. In one embodiment of the present invention, the CLUT consists of one SRAM with 256 entries and 32 bits per entry. In other embodiments, the number of entries and bits per entry may vary. Each entry contains three color components; either RGB or YUV format, and an alpha component. For every CLUT-format pixel converted, the pixel data may be used as the address to the CLUT and the resulting value may be used by the converter to produce the YUVa (or alternatively RGBa) pixel value. The CLUT may be re-loaded by retrieving new CLUT data via the direct memory access module when needed. It generally takes longer to re-load the CLUT than the time available in a horizontal blanking interval. Accordingly, in the preferred embodiment, a whole scan line time is allowed to re-load the CLUT. While the CLUT is being reloaded, graphics images in non-CLUT formats may be displayed. The CLUT reloading is preferably initiated by a window descriptor that contains information regarding CLUT reloading rather than a graphics window display information. Referring to FIG. 11, the graphics CLUT 146 preferably includes a graphics CLUT controller 400 and a static dual-port RAM (SRAM) 402. The SRAM preferably has a size of 256.times.32 which corresponds to 256 entries in the graphics CLUT. Each entry in the graphics CLUT preferably has 32 bits composed of Y+U+V+alpha from the most significant bit to the least significant bit. The size of each field, including Y, U, V, and alpha, is preferably eight bits. The graphics CLUT preferably has a write port that is synchronized to a 81 MHz memory clock and a read port that may be asynchronous to the memory clock. The read port is preferably synchronous to the graphics processing clock, which runs preferably at 81 MHz, but not necessarily synchronized to the memory clock. During a read operation, the static dual-port RAM ("SRAM") is preferably addressed by a read address which is provided by graphics data in the CLUT images. During the read operation, the graphics data is preferably output as read data 414 when a memory address in the CLUT containing that graphics data is addressed by a read address 412. During write operations, the window controller preferably controls the write port with a CLUT memory request signal 404 and a CLUT memory write signal 408. CLUT memory data 410 is also preferably provided to the graphics CLUT via the direct memory access module from the external memory. The graphics CLUT controller preferably receives the CLUT memory data and provides the received CLUT memory data to the SRAM for writing. Referring to FIG. 12, an exemplary timing diagram shows different signals involved during a writing operation of the CLUT. The CLUT memory request signal 418 is asserted when the CLUT is to be re-loaded. A rising edge of the CLUT memory request signal 418 is used to reset a write pointer associated with the write port. Then the CLUT memory write signal 420 is asserted to indicate the beginning of a CLUT re-loading operation. The CLUT memory data 422 is provided synchronously to the 81 MHz memory clock 416 to be written to the SRAM. The write pointer associated with the write port is updated each time the CLUT is loaded with CLUT memory data. In the preferred embodiment, the process of reloading a CLUT is associated with the process of processing window descriptors illustrated in FIG. 8 since CLUT re-loading is initiated by a window descriptor. As shown in steps 324 and 328 of FIG. 8, if the window descriptor is determined to be for reloading CLUT in step 324, the system in step 328 sends the CLUT data to the CLUT. The window descriptor for the CLUT reloading may appear anywhere in the window descriptor list. Accordingly, the CLUT reloading may take place at any time whenever CLUT data is to be updated. Using the CLUT loading mechanism in one embodiment of the present invention, more than one window with different CLUT tables may be displayed on the same display line. In this embodiment, only the minimum required entries are preferably loaded into the CLUT, instead of loading all the entries every time. The loading of only the minimum required entries may save memory bandwidth and enables more functionality. The CLUT loading mechanism is preferably relatively flexible and easy to control, making it suitable for various applications. The CLUT loading mechanism of the present invention may also simplify hardware design, as the same state machine for the window controller may be used for CLUT loading. The CLUT preferably also shares the same DMA logic and layer/priority control logic as the window controller. V. Graphics Line Buffer Control Scheme In the preferred embodiment of the present invention, the system preferably blends a plurality of graphics images using line buffers. The system initializes a line buffer by loading the line buffer with data that represents transparent black, obtains control of a line buffer for a compositing operation, composites graphics contents into the line buffer by blending the graphics contents with the existing contents of the line buffer, and repeats the step of compositing graphics contents into the line buffer until all of the graphics surfaces for the particular line have been composited. The graphics line buffer temporarily stores composited graphics images (blended graphics). A graphics filter preferably uses blended graphics in line buffers to perform vertical filtering and scaling operations to generate output graphics images. In the preferred embodiment, the display engine composites graphics images line by line using a clock rate that is faster than the pixel display rate, and graphics filters run at the pixel display rate. In other embodiments, multiple lines of graphics images may be composited in parallel. In still other embodiments, the line buffers may not be needed. Where line buffers are used, the system may incorporate an innovative control scheme for providing the line buffers containing blended graphics to the graphics filter and releasing the line buffers that are used up by the graphics filter. The line buffers are preferably built with synchronous static dual-port random access memory ("SRAM") and dynamically switch their clocks between a memory clock and a display clock. Each line buffer is preferably loaded with graphics data using the memory clock and the contents of the line buffer is preferably provided to the graphics filter synchronously to the display clock. In one embodiment of the present invention, the memory clock is an 81 MHz clock used by the graphics converter to process graphics data while the display clock is a 13.5 MHz clock used to display graphics and video signals on a television screen. Other embodiments may use other clock speeds. Referring to FIG. 13, the graphics line buffer preferably includes a graphics line buffer controller 500 and line buffers 504. The graphics line buffer controller 500 preferably receives memory clock buffer control signals 508 as well as display clock buffer control signals 510. The memory clock control signals and the display clock control signals are used to synchronize the graphics line buffers to the memory clock and the display clock, respectively. The graphics line buffer controller receives a clock selection vector 514 from the display engine to control which graphics line buffers are to operate in which clock domain. The graphics line buffer controller returns a clock enable vector to the display engine to indicate clock synchronization settings in accordance with the clock selection vector. In the preferred embodiment, the line buffers 504 include seven line buffers 506a-g. The line buffers temporarily store lines of YUVa24 graphics pixels that are used by a subsequent graphics filter. This allows for four line buffers to be used for filtering and scaling, two are available for progressing by one or two lines at the end of every line, and one for the current compositing operation. Each line buffer may store an entire display line. Therefore, in this embodiment, the total size of the line buffers is (720 pixels/display line)*(3 bytes/pixel)*(7 lines)=15,120 bytes. Each of the ports to the SRAM including line buffers is 24 bits wide to accommodate graphics data in YUVa24 format in this embodiment of the present invention. The SRAM has one read port and one write port. One read port and one write port are used for the graphics blender interface, which performs a read-modify-write typically once per clock cycle. In another embodiment of the present invention, an SRAM with only one port is used. In yet another embodiment, the data stored in the line buffers may be YUVa32 (4:4:4:4), RGBa32, or other formats. Those skilled in the art would appreciate that it is straightforward to vary the number of graphics line buffers, e.g., to use different number of taps for filter, the format of graphics data or the number of read and write ports for the SRAM. The line buffers are preferably controlled by the graphics line buffer controller over a line buffer control interface 502. Over this interface, the graphics line buffer controller transfers graphics data to be loaded to the line buffers. The graphics filter reads contents of the line buffers over a graphics line buffer interface 516 and clears the line buffers by loading them with transparent black pixels prior to releasing them to be loaded with more graphics data for display. Referring FIG. 14, a flow diagram of a process of using line buffers to provide composited graphics data from a display engine to a graphics filter is illustrated. After the graphics display system is reset in step 520, the system in step 522 receives a vertical sync (VSYNC) indicating a field start. Initially, all line buffers preferably operate in the memory clock domain. Accordingly, the line buffers are synchronized to the 81 MHz memory clock in one embodiment of the present invention. In other embodiments, the speed of the memory clock may be different from 81 MHz, or the line buffers may not operate in the clock domain of the main memory. The system in step 524 preferably resets all line buffers by loading them with transparent black pixels. The system in step 526 preferably stores composited graphics data in the line buffers. Since all buffers are cleared at every field start by the display engine to the equivalent of transparent black pixels, the graphics data may be blended the same way for any graphics window, including the first graphics window to be blended. Regardless of how many windows are composited into a line buffer, including zero windows, the result is preferably always the correct pixel data. The system in step 528 preferably detects a horizontal sync (HSYNC) which signifies a new display line. At the start of each display line, the graphics blender preferably receives a line buffer release signal from the graphics filter when one or more line buffers are no longer needed by the graphics filter. Since four line buffers are used with the four-tap graphics filter at any given time, one to three line buffers are preferably made available for use by the graphics blender to begin constructing new display lines in them. Once a line buffer release signal is recognized, an internal buffer usage register is updated and then clock switching is performed to enable the display engine to work on the newly released one to three line buffers. In other embodiments, the number of line buffers may be more or less than seven, and more or less than three line buffers may be released at a time. The system in step 534 preferably performs clock switching. Clock switching is preferably done in the memory clock domain by the display engine using a clock selection vector. Each bit of the clock selection vector preferably corresponds to one of the graphics line buffers. Therefore, in one embodiment of the present invention with seven graphics line buffers, there are seven bits in the clock selection vector. For example, a corresponding bit of logic 1 in the clock selection vector indicates that the line buffer operates in the memory clock domain while a corresponding bit of logic 0 indicates that the line buffer operates in the display clock domain. Other embodiments may have different numbers of line buffers and the number of bits in the clock selection vector may vary accordingly. Clock switching logic preferably switches between the memory clock and the display clock in accordance with the clock selection vector. The clock selection vector is preferably also used to multiplex the memory clock buffer control signals and the display clock buffer control signals. Since there is preferably no active graphics data at field and line starts, clock switching preferably is done at the field start and the line start to accommodate the graphics filter to access graphics data in real-time. At the field and line starts, clock switching may be done without causing glitches on the display side. Clock switching typically requires a dead cycle time. A clock enable vector indicates that the graphics line buffers are ready to synchronize to the clocks again. The clock enable vector is preferably the same size at the clock selection vector. The clock enable vector is returned to the display engine to be compared with the clock selection vector. During clock switching, the clock selection vector is sent by the display engine to the graphics line buffer block. The clocks are preferably disabled to ensure a glitch-free clock switching. The graphics line buffers send the clock enable vector to the display engine with the clock synchronization settings requested in the clock selection vector. The display engine compares contents of the clock selection vector and the clock enable vector. When the contents match, the clock synchronization is preferably turned on again. After the completion of clock switching during the video inactive region, the system in step 536 preferably provides the graphics data in the line buffers to the graphics filter for anti-flutter filtering, sample rate conversion (SRC) and display. At the end of the current display line, the system looks for a VSYNC in step 538. If the VSYNC is detected, the current field has been completed, and therefore, the system in step 530 preferably switches clocks for all line buffers to the memory clock and resets the line buffers in step 524 for display of another field. If the VSYNC is not detected in step 538, the current display line is not the last display line of the current field. The system continues to step 528 to detect another HSYNC for processing and displaying of the next display line of the current field. VI. Window Soft Horizontal Scrolling Mechanism Sometimes it is desirable to scroll a graphics window softly, e.g., display text that moves from left to right or from right to left smoothly on a television screen. There are some difficulties that may be encountered in conventional methods that seek to implement horizontal soft scrolling. Graphics memory buffers are conventionally implemented using low-cost DRAM, SDRAM, for example. Such memory devices are typically slow and may require each burst transfer to be within a page. Smooth (or soft) horizontal scrolling, however, preferably enables the starting address to be set to any arbitrary pixel. This may conflict with the transfer of data in bursts within the well-defined pages of DRAM. In addition, complex control logic may be required to monitor if page boundaries are to be crossed during the transfer of pixel maps for each step during soft horizontal scrolling. In the preferred embodiment, an implementation of a soft horizontal scrolling mechanism is achieved by incrementally modifying the content of a window descriptor for a particular graphics window. The window soft horizontal scrolling mechanism preferably enables positioning the contents of graphics windows on arbitrary positions on a display line. In an embodiment of the present invention, the soft horizontal scrolling of graphics windows is implemented based on an architecture in which each graphics window is independently stored in a normal graphics buffer memory device (SDRAM, EDO-DRAM, DRAM) as a separate object. Windows are composed on top of each other in real time as required. To scroll a window to the left or right, a special field is defined in the window descriptor that tells how many pixels are to be shifted to the left or right. The system according to the present invention provides a method of horizontally scrolling a display window to the left, which includes the steps of blanking out one or more pixels at a beginning of a portion of graphics data, the portion being aligned with a start address; and displaying the graphics data starting at the first non-blanked out pixel in the portion of the graphics data aligned with the start address. The system according to the present invention also provides a method of horizontally scrolling a display window to the right which includes the steps of moving a read pointer to a new start address that is immediately prior to a current start address, blanking out one or more pixels at a beginning of a portion of graphics data, the portion being aligned to the new start address, and displaying the graphics data starting at the first non-blanked out pixel in the portion of the graphics data aligned with the new start address. In practice, each graphics window is preferably addressed using an integer word address. For example, if the memory system uses 32 bit words, then the address of the start of a window is defined to be aligned to a multiple of 32 bits, even if the first pixel that is desired to be displayed is not so aligned. Each graphics window also preferably has associated with it a horizontal offset parameter, in units of pixels, that indicates a number of pixels to be ignored, starting at the indicated starting address, before the active display of the window starts. In the preferred embodiment, the horizontal offset parameter is the blank start pixel value in the word 3 of the window descriptor. For example, if the memory system uses 32-bit words and the graphics format of a window uses 8 bits per pixel, each 32-bit word contains four pixels. In this case, the display of the window may ignore one, two or three pixels (8, 16, or 24 bits), causing an effective left shift of one, two, or three pixels. In the embodiment illustrated by the above example, the memory system uses 32-bit words. In other embodiments, the memory system may use more or less number of bits per word, such as 16 bits per word or 64 bits per word. In addition, pixels in other embodiments may have various different number of bits per pixel, such as 1, 2, 4, 8, 16, 24 and 32. Referring to FIG. 15, in the preferred embodiment, a first pixel (e.g., the first 8 bits) 604 of a 32-bit word 600, which is aligned to the start address, is blanked out. The remaining three 8-bit pixels, other than the blanked out first pixel, are effectively shifted to the left by one pixel. Prior to blanking out, a read pointer 602 points to the first bit of the 32-bit word. After blanking out, the read pointer 602 points to the ninth bit of the 32-bit word. Further, a shift of four pixels is implemented by changing the start address by one to the next 32-bit word. Shifts of any number of pixels are thereby implemented by a combination of adjusting the starting word address and adjusting the pixel shift amount. The same mechanism may be used for any number of bits per pixel (1, 2, 4, etc.) and any memory word size. To shift a pixel or pixels to the right, the shifting cannot be achieved simply by blanking some of the bits at the start address since any blanking at the start will simply have an effect of shifting pixels to the left. Further, the shifting to the right cannot be achieved by blanking some of the bits at the end of the last data word of a display line since display of a window starts at the start address regardless of the position of the last pixel to be displayed. Therefore, in one embodiment of the present invention, when the graphics display is to be shifted to the right, a read pointer pointing at the start address is preferably moved to an address that is just before the start address, thereby making that address the new start address. Then, a portion of the data word aligned with the new start address is blanked out. This provides the effect of shifting the graphics display to the right. For example, a memory system may use 32-bit words and the graphics format of a window may use 2 bits per pixel, e.g., a CLUT 2 format. If the graphics display is to be shifted by a pixel to the right, the read pointer is moved to an address that is just before the start address, and that address becomes a new start address. Then, the first 30 bits of the 32-bit word that is aligned with the new start address are blanked out. In this case, blanking out of a portion of the 32-bit word that is aligned with the new start address has the effect of shifting the graphics display to the right. Referring to FIG. 16, a 32-bit word 610 that is aligned with the starting address is shifted to the right by one pixel. The 32-bit word 610 has a CLUT 2 format, and therefore contains 16 pixels. A read pointer 612 points at the beginning of the 32-bit word 610. To shift the pixels in the 32-bit word 610 to the right, an address that is just before the start address is made a new start address. A 32-bit data word 618 is aligned with the new start address. Then, the first 30 bits (15 pixels) 616 of the 32-bit data word 618 aligned with the new start address are blanked out. The read pointer 612 points at a new location, which is the 31.sup.st bit of the new start address. The 31.sup.st bit and the 32.sup.nd bit of the new start address may constitute a pixel 618. Insertion of the pixel 618 in front of 16 pixels of the 32-bit data word 610 effectively shifts those 16 pixels to the right by one pixel. VII. Anti-Aliased Text and Graphics TV-based applications, such as interactive program guides, enhanced TV, TV navigators, and web browsing on TV frequently require the display of text and line-oriented graphics on the display. A graphical element or glyph generally represents an image of text or graphics. Graphical element may refer to text glyphs or graphics. In conventional methods of displaying text on TV or computer displays, graphical elements are rendered as arrays of pixels (picture elements) with two states for every pixel, i.e. the foreground and background colors. In some cases the background color is transparent, allowing video or other graphics to show through. Due to the relatively low resolution of most present day TVs, diagonal and round edges of graphical elements generally show a stair-stepped appearance which may be undesirable; and fine details are constrained to appear as one or more complete pixels (dots), which may not correspond well to the desired appearance. The interlaced nature of TV displays causes horizontal edges of graphical elements, or any portion of graphical elements with a significant vertical gradient, to show a "fluttering" appearance with conventional methods. Some conventional methods blend the edges of graphical elements with background colors in a frame buffer, by first reading the color in the frame buffer at every pixel where the graphical element will be written, combining that value with the foreground color of the graphical element, and writing the result back to the frame buffer memory. This method requires there to be a frame buffer; it requires the frame buffer to use a color format that supports such blending operations, such as RGB24 or RGB16, and it does not generally support the combination of graphical elements over full motion video, as such functionality may require repeating the read, combine and write back function of all pixels of all graphical elements for every frame or field of the video in a timely manner. The system preferably displays a graphical element by filtering the graphical element with a low pass filter to generate a multi-level value per pixel at an intended final display resolution and uses the multi-level values as alpha blend values for the graphical element in the subsequent compositing stage. In one embodiment of the present invention, a method of displaying graphical elements on televisions and other displays is used. A deep color frame buffer with, for example, 16, 24, or 32 bits per pixel, is not required to implement this method since this method is effective with as few as two bits per pixel. Thus, this method may result in a significant reduction in both the memory space and the memory bandwidth required to display text and graphics. The method preferably provides high quality when compared with conventional methods of anti-aliased text, and produces higher display quality than is available with conventional methods that do not support anti-aliased text. Referring to FIG. 17, a flow diagram illustrates a process of providing very high quality display of graphical elements in one embodiment of the present invention. First, the bi-level graphical elements are filtered by the system in step 652. The graphical elements are preferably initially rendered by the system in step 650 at a significantly higher resolution than the intended final display resolution, for example, four times the final resolution in both horizontal and vertical axes. The filter may be any suitable low pass filter, such as a "box" filter. The result of the filtering operation is a multi-level value per pixel at the intended display resolution. The number of levels may be reduced to fit the number of bits used in the succeeding steps. The system in step 654 determines whether the number of levels are to be reduced by reducing the number of bits used. If the system determines that the number of levels are to be reduced, the system in step 656 preferably reduces the number of bits. For example, the result of box-filtering 4.times.4 super-sampled graphical elements normally results in 17 possible levels; these may be converted through truncation or other means to 16 levels to match a 4 bit representation, or eight levels to match a 3 bit representation, or four levels to match a 2 bit representation. The filter may provide a required vertical axis low pass filter function to provide anti-flutter filter effect for interlaced display. In step 658, the system preferably uses the resulting multi-level values, either with or without reduction in the number of bits, as alpha blend values, which are preferably pixel alpha component values, for the graphical elements in a subsequent compositing stage. The multi-level graphical element pixels are preferably written into a graphics display buffer where the values are used as alpha blend values when the display buffer is composited with other graphics and video images. In an alternate embodiment, the display buffer is defined to have a constant foreground color consistent with the desired foreground color of the text or graphics, and the value of every pixel in the display buffer is defined to be the alpha blend value for that pixel. For example, an Alpha-4 format specifies four bits per pixel of alpha blend value in a graphics window, where the 4 bits define alpha blend values of 0/16, 1/16, 2/16, . . . , 13/16, 14/16, and 16/16. The value 15/16 is skipped in this example in order to obtain the endpoint values of 0 and 16/16 (1) without requiring the use of an additional bit. In this example format, the display window has a constant foreground color which is specified in the window descriptor. In another alternate embodiment, the alpha blend value per pixel is specified for every pixel in the graphical element by choosing a CLUT index for every pixel, where the CLUT entry associated with every index contains the desired alpha blend value as part of the CLUT contents. For example, a graphical element with a constant foreground color and 4 bits of alpha per pixel can be encoded in a CLUT 4 format such that every pixel of the display buffer is defined to be a 4 bit CLUT index, and each of the associated 16 CLUT entries has the appropriate alpha blend value (0/16, 1/16, 2/16, . . . , 14/16, 16/16) as well as the (same) constant foreground color in the color portion of the CLUT entries. In yet another alternate embodiment, the alpha per pixel values are used to form the alpha portion of color+alpha pixels in the display buffer, such as alphaRGB (4,4,4,4) with 4 bits for each of alpha, Red, Green, and Blue, or alphaRGB32 with 8 bits for each component. This format does not require the use of a CLUT. In still another alternate embodiment, the graphical element may or may not have a constant foreground color. The various foreground colors are processed using a low-pass filter as described earlier, and the outline of the entire graphical element (including all colors other than the background) is separately filtered also using a low pass filter as described. The filtered foreground color is used as either the direct color value in, e.g., an alphaRGB format (or other color space, such as alphaYUV) or as the color choice in a CLUT format, and the result of filtering the outline is used as the alpha per pixel value in either a direct color format such as alphaRGB or as the choice of alpha value per CLUT entry in a CLUT format. The graphical elements are displayed on the TV screen by compositing the display buffer containing the graphical elements with optionally other graphics and video contents while blending the subject display buffer with all layers behind it using the alpha per pixel values created in the preceding steps. Additionally, the translucency or opacity of the entire graphical element may be varied by specifying the alpha value of the display buffer via such means as the window alpha value that may be specified in a window descriptor. VIII. Video Synchronization When a composite video signal (analog video) is received into the system, it is preferably digitized and separated into YUV (luma and chroma) components for processing. Samples taken for YUV are preferably synchronized to a display clock for compositing with graphics data at the video compositor. Mixing or overlaying of graphics with decoded analog video may require synchronizing the two image sources exactly. Undesirable artifacts such as jitter may be visible on the display unless a synchronization mechanism is implemented to correctly synchronize the samples from the analog video to the display clock. In addition, analog video often does not adhere strictly to the television standards such as NTSC and PAL. For example, analog video which originates in VCRs may have synchronization signals that are not aligned with chroma reference signals and also may have inconsistent line periods. Thus, the synchronization mechanism preferably should correctly synchronize samples from non-standard analog videos as well. The system, therefore, preferably includes a video synchronizing mechanism that includes a first sample rate converter for converting a sampling rate of a stream of video samples to a first converted rate, a filter for processing at least some of the video samples with the first converted rate, and a second sample rate converter for converting the first converted rate to a second converted rate. Referring to FIG. 18, the video decoder 50 preferably samples and synchronizes the analog video input. The video receiver preferably receives an analog video signal 706 into an analog-to-digital converter (ADC) 700 where the analog video is digitized. The digitized analog video 708 is preferably sub-sampled by a chroma-locked sample rate converter (SRC) 708. A sampled video signal 710 is provided to an adaptive 2H comb filter/chroma demodulator/luma processor 702 to be separated into YUV (luma and chroma) components. In the 2H comb filter/chroma demodulator/luma processor 702, the chroma components are demodulated. In addition, the luma component is preferably processed by noise reduction, coring and detail enhancement operations. The adaptive 2H comb filter provides the sampled video 712, which has been separated into luma and chroma components and processed, to a line-locked SRC 704. The luma and chroma components of the sample video is preferably sub-sampled once again by the line-locked SRC and the sub-sampled video 714 is provided to a time base corrector (TBC) 72. The time base corrector preferably provides an output video signal 716 that is synchronized to a display clock of the graphics display system. In one embodiment of the present invention, the display clock runs at a nominal 13.5 MHz. The synchronization mechanism preferably includes the chroma-locked SRC 70, the line-locked SRC 704 and the TBC 72. The chroma-locked SRC outputs samples that are locked to chroma subcarrier and its reference bursts while the line-locked SRC outputs samples that are locked to horizontal syncs. In the preferred embodiment, samples of analog video are over-sampled by the ADC 700 and then down-sampled by the chroma-locked SRC to four times the chroma sub-carrier frequency (Fsc). The down-sampled samples are down-sampled once again by the line-locked SRC to line-locked samples with an effective sample rate of nominally 13.5 MHz. The time base corrector is used to align these samples to the display clock, which runs nominally at 13.5 MHz. Analog composite video has a chroma signal frequency interleaved in frequency with the luma signal. In an NTSC standard video, this chroma signal is modulated on to the Fsc of approximately 3.579545 MHz, or exactly 227.5 times the horizontal line rate. The luma signal covers a frequency span of zero to approximately 4.2 MHz. One method for separating the luma from the chroma is to sample the video at a rate that is a multiple of the chroma sub-carrier frequency, and use a comb filter on the sampled data. This method generally imposes a limitation that the sampling frequency is a multiple of the chroma sub-carrier frequency (Fsc). Using such a chroma-locked sampling frequency generally imposes significant costs and complications on the implementation, as it may require the creation of a sample clock of the correct frequency, which itself | ||||||