Abstract:
Alpha blending and video blending are both provided by a color definition in an RGBAV format allows for an additional component to control video blending. The V value defines blending of an alpha blended color with a video background.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to graphics overlay for video data providing independent blending control for how graphics pixels are blended together from how graphics pixels are blended with the background video.  
         BACKGROUND OF THE INVENTION  
         [0002]    A function of increasing importance is the simultaneous provision of graphics and video on a display. The video and graphics may be provided in a number of different contexts, whether through multimedia distribution systems, combined computer and television functions or video games. Current application program interface (API) definitions for three-dimensional graphics specify a color definition utilizing four components, red, green, blue and alpha. The red, green and blue components represent actual color, and alpha represents a blending factor which indicates a level of transparency. One use of alpha blending, for example, is to represent a vehicle interior as seen through a car windshield. Pixels representing points in areas of polygons modeling a car would have the alpha representing the windshield set to an intermediate value, while polygons representing the vehicles interior would have alpha values set to make them appear opaque. There is also the case of blending with the background color or background image if there are no polygons underneath a particular pixel in the windshield. The case of additional transparent polygons underneath the windshield is also possible.  
           [0003]    Alpha blending is a well-known technique for providing transparency information for transparent or translucent objects. In alpha blending, a resultant color of a pixel is a combination of a foreground color, i.e., the color of the translucent object, and a background color, i.e. the color of what is underneath the blended polygon. Alpha blending has been implemented by rendering polygons through a stipple mask whose on-off density is proportional to the transparency of the object, although this technique is rarely used anymore. The most common implementation is to use the alpha value to blend colors on a pixel-by-pixel basis. For greatest convenience in computation, alpha is given as an unsigned integer value in the range of 0 to 255 for each color pixel. A new pixel equals ((α) (pixel A color component)+(1−α)(pixel B color component))/255 where A and B are the foreground and background color components respectively. This equation is applied to each individual color components, red, green, blue and alpha resulting in new red, green and blue and alpha components. The capital letters RGBA signal is commonly provided by a 32-bit frame buffer with 24 bits of color, 8 each for red, green and blue and 8 bits for the value of alpha. It is also recognized that there are alternate forms of this blending equation such as ((α+1) (pixel A color component)+(1−(α+1)) (pixel B color component))&gt;&gt;7. This form of the equation allows a right shift (&gt;&gt;7) to replace the divide by 255. A shift operation is executed much more quickly than a divide operation. With many pixels to process this makes the overall processing much faster.  
           [0004]    A further graphics capability beyond simple alpha blending may be required in systems where, for example, it is desired to have three-dimensional graphics blended with a video background. The RGBA signal methodology is not robust enough to accommodate both blending of colors of polygons within the rendered 3D image in addition to an independent blending of the information in the rendered image with a video background. The alpha value is used to provide blending for pixel colors and for blending with the background video. There is no way to distinguish whether the blending is to be applied to the pixel color or with the background video. In setting the RGBA values for the above windshield example, the capability is not provided to allow a transparent windshield through which the vehicle interior is seen and in which there is no video blending of the interior with background video. It is not a satisfactory solution simply to add an additional digital word in addition to the digital word representing the color definition RGBA. The common RGBA definition is 32 bits per pixel, or 16 bits per pixel in some systems. These bit lengths already correspond conveniently with current sizes for word-aligned direct memory access (DMA) transfers, word-aligned memory operations and standard bus sizes.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the present invention, color definition is provided which allows for alpha blending and also provides for video blending. Additionally, the ability to maintain a standard color definition length such as 32 bits is maintained. A format called AGBAV is provided which modifies color definition to allow for an additional component to control video blending. A multibit definition is established for pixel color definition including values for red, green, blue, alpha blending and video blending. In one form 8 bits each may be provided for the red, green and blue values with the remaining 8 bits divided between the alpha and video values. Additionally, a specialized processor is provided in which coded RGBAV values are read and in which graphics are processed using the alpha value. The RGBA output is combined with a video or V processing value to provide a composite. The technique used to blend graphics with video is similar to the one explained previously, for example, ((V)graphics+(1−V)video)/maximum_V to generate a display pixel color. The same method can be applied to other color formats beyond RGB. For example, these same principles may be applied to YUV encoding.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The invention, both as to its organization and manner of operation may be understood by reference to the following description taken in connection with the following drawings.  
         [0007]    Of the drawings:  
         [0008]    [0008]FIG. 1 is a block diagram illustrating graphical and video data to be combined polygons and hardware for blending and displaying blended data;  
         [0009]    [0009]FIG. 2 is a block diagram of the information in FIG. 1 illustrating hardware storing the data;  
         [0010]    [0010]FIGS. 3 a  and  3   b  are illustrations of an exemplary multibit word providing RGBA and RGBAV information for a pixel respectively;  
         [0011]    [0011]FIGS. 4 a  and  4   b  are block diagrams of video graphics systems incorporating the present invention;  
         [0012]    [0012]FIG. 5 is a block diagrammatic representation of RGBAV processing in the present invention; and  
         [0013]    [0013]FIG. 6 is a flow chart describing the method and machine-readable medium of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIGS. 1 and 2 are each a block diagram to illustrate alpha blending and blending of graphics with video and display of visual data. The same reference numerals are used to denote corresponding components. In the present description, visual data refers to graphics or video. In FIG. 1, the shapes represented by data are illustrated. In FIG. 2, data storage and combining hardware is illustrated. For purposes of the present description, blending of graphics and video, where the graphics may or may not already be alpha blended, is called video blending. Just as a level of alpha blending is represented by the letter A, in the present description video blending levels are represented by V.  
         [0015]    As seen in FIG. 1, a rectangle  10  represents a graphics overlay plane to be alpha blended with a polygon  12 . The polygon  12  in the present example is a triangle and comprises a component of a three dimensional image rendered for display in two dimensions. The rectangle  10  is unfilled in FIG. 1 to indicate transparency. The polygon  12  is lined for a first color. In prior art graphic displays, the rectangle  10  and polygon  12  can be blended through the use of RGBA signals where the letters stand for red, green, blue and alpha blending respectively. There are applications in which it is desired to superimpose the graphics comprising the rectangle  10  and polygon  12  on a video background  14 . In the present example, the video background  14  is lined for a second color. RGBA encoding of values for pixels does not accommodate independent blending of both the polygon  12  with the rectangle  10  and the blending polygon  12  with the video background  14 . The use of the video blending level V will permit blending of an alpha blended polygon with the video background  14 . Visual data is provided to a compositor  20  to drive a display comprising a monitor  24 .  
         [0016]    In FIG. 2, the function of the compositor  20  is provided by a graphics blending arithmetic unit  30  and a graphics and video blending arithmetic unit  28 . The graphics blending arithmetic unit  30  alpha receives RGBAV data, further described with respect to FIG. 3 b,  from a register  34  indicative of current data representing the color information of the rectangle  10  that will be alpha blended with a signal indicative of the polygon  12 . When the graphics background information for the rectangle  10  changes, update data is provided to the arithmetic unit  30  from an update register  38 . Video data representing the video background  14  is supplied from an RGB video data register  42 . Graphics and video blending based on the V value is performed in the arithmetic unit  28 . Further specific details of graphics and video blending are described with respect to FIG. 5 below.  
         [0017]    In accordance with the present invention, encoding, a process and a processor are provided for providing both blending operations. FIG. 3 a  is a representation of the conventional prior art 32-bit representation of a pixel including RGBA components. FIG. 3 b  represents encoding according to the present invention, providing values for RGBA and V, where V is a video blending value. In the particular example 8 bits are provided for red, green and blue information and 4 bits each are provided for A and V information. However, as is well known in the art, other numbers of bits may be provided. Additionally, further forms of encoding other than RGB may be provided. While most conventional apparatus will utilize equal numbers of bits for the values of R, G and B, this is not a necessity. Other schemes use unequal numbers of bits. Also, color here is denoted by RGB. This term also covers color definitions where RGB may be in a different order or where particular hues are defined by symbols other than R, G and B.  
         [0018]    In accordance with the present invention, a multi-component driving signal is constructed comprising first second and third sections, one section containing color information, another section containing alpha blending information, and another section with V blending information. Normally, the driving signal will take the form RGBAV. However, the driving signal could, for example, take the forms AVRGB, VARGB or ARGBV.  
         [0019]    [0019]FIGS. 4A and 4B are each a block diagram illustrating a system incorporating the present invention in which the same reference numerals are used to denote corresponding components. In the embodiment of FIG. 4A, a graphics central processor unit (CPU)  110  processes data via an input/output (I/O) interface  114  and utilizes an independent CPU memory  118 . A decoder memory  120  contains information indicative of the graphics overlay rectangle  10  and the polygon  12 . Information indicative of the video background  14  is supplied from a video source  124 . The video source  124  may be analog or digital. In this embodiment, processing required to combine the graphics information and video information is performed in a video decoder  128  which interfaces with both the video source  124  and decoder memory  120 . The video decoder  128  provides an output to a display monitor  132 . In one form, first register  34  comprises a memory kept within the video decoder  128 . The memory provides a signal that is equal to a preselected number of scan lines of video data. In this embodiment, the wherein said memory is continuously re-rendered in a manner synchronous with the scan out of a video display monitor  132 .  
         [0020]    In the embodiment of FIG. 4B, a CPU  112  interfaces directly with the video decoder  128 , which also interacts with a unified memory  122 . The unified memory  132  corresponds to the decoder memory  120  and provides memory for the CPU  112 . Many other architectures will also suggest themselves to those skilled in the art to provide the form of processing taught herein. One of the many forms that the video source  124  could comprise is a MPEG-2 source and transport. The CPUs  110  or  112  may be included in a set top box and comprise graphical user interfaces.  
         [0021]    [0021]FIG. 5 is a block diagram of a mixer  150  suitable for performing the alpha blending and the V blending within the video decoder  128 . For purposes of the present description, the various values to be utilized are attributed to the displayed items in FIG. 1. At block  210 , the RGB register for a pixel in the rectangle  10  is illustrated. An intensity output at terminal  214  is provided. The alpha value is provided from a location  212  of the pixel register and supplied to a multiplier  220 . The output of the multiplier indicates the color of the pixel to be provided times the blending factor α. The output is supplied to a summer  230 . The summer  230  preferably includes circuitry, e.g. scaling registers or resistors, to normalize the blending factor α. More specifically, the summed result is divided by a value corresponding to the size of the maximum alpha value. An RGB value for a corresponding pixel in the polygon  12  is stored in a register  240  and provided at an output terminal  244 . For blending purposes the color to be provided for the polygon  12  is multiplied by 1−α, stored in register location  246 . The output from the terminal  244  is multiplied by 1−α in a multiplier  250 . The multiplier  250  provides a second input to the summer  230 .  
         [0022]    After corresponding pixels in the rectangle  10  and polygon  12  have been blended. It is necessary to combine with the corresponding pixel from the video display  14 . A value V in a register  260  represents the degree of blending by which the graphics will be multiplied. The output of the summer  230  is multiplied by V at a multiplier  270 . The video, whose pixel information is stored in a register  280  is to be multiplied by 1−V, which is stored in a location  266  and is the ones complement of the value V. The video pixel value is multiplied by 1−V at a multiplier  290 . The outputs of the multipliers  270  and  290  are combined at a summer  294 . As with the α value, it is preferable to normalize the V value. The summer  294  divides the summed result by a value corresponding to a maximum value of V. The summer  294  produces an output to drive a pixel at pixel driver location  298 .  
         [0023]    [0023]FIG. 6 is a flow diagram illustrating the operation and the method of the present invention. At block  400 , incoming graphics information, such as the new graphics overlay data in register  38  of FIG. 2 is accessed. At block  410 , existing pixel data, such as data in the register  34  of FIG. 2 is accessed. At block  420 , blending of background information, such as at arithmetic unit  30  is performed. New RBGA information is produced which, at block  430  becomes the new currently existing graphics overlay data in register  34 . The new RGBA information is available for access at block  400  of a next operating cycle.  
         [0024]    In preparation for a next blending operation, at block  440 , the video information for the background  14  is accessed, as from the data register  42  and made available to the arithmetic unit  28 . At block  450 , the new overlay value is obtained from the register  38 , and at block  460 , the arithmetic unit  28  blends components according to the value V. At block  470 , the blended result is provided to display drivers. The correct displayed pixel color is thus provided at the desired intensity and blending.  
         [0025]    The specification has been written to enable those skilled in the art to make many departures from the specific embodiments disclosed to produce a method and apparatus in machine-readable medium in accordance with the present invention.