Abstract:
A system, method, and computer program product are provided for antialiasing during rendering in a graphics pipeline. Initially, a primitive of vertex data is received in a graphics pipeline. Next, a super-sampling operation is performed on the primitive of vertex data ufilizing the graphics pipeline. Further, a multi-sampling operation is performed on the primitive of vertex data utilizing the graphics pipeline.

Description:
FIELD OF THE INVENTION 
     The present invention relates to computer graphics, and more particularly to antialiasing in a computer graphics processing pipeline. 
     BACKGROUND OF THE INVENTION 
     Images to be drawn on a raster-scan display, having a two dimensional array of pixel locations, must first be quantized to discrete memory locations, such as within a frame buffer, which correspond to the pixel locations of the array. This process limits the resolution of such displays to the physical characteristics of the array (i.e., 1024.times.1280 pixel locations) and often produces visual artifacts of the images when a human viewer&#39;s ability to resolve the displayed image exceeds the limited resolution of the display on which the image is drawn. 
     This effect is referred to as “aliasing” because visual artifacts are created by differences in the spatial frequency characteristics of the display and the spatial frequency characteristics of the viewer&#39;s eyes. A familiar example of such an artifact is the jagged appearance of an image, such as a straight line or primitive edge, that results when the image is not aligned along a horizontal or vertical line of pixel locations. Prior Art FIG. 1 illustrates two versions of a line: one perfect line and one heavily aliased. 
     Techniques for removing or reducing artifacts by smoothing the appearance of the displayed image and/or increasing the spatial frequency characteristics of the display are known as “antialiasing” techniques. 
     Super-sampling is an antialiasing technique that increases the spatial quantization of the display&#39;s array by increasing the number of data samples that are taken at or around each pixel location corresponding to a portion of the image to be displayed, and then combining the resulting values of these multiple data samples to obtain a final display value for each pixel location. In prior art systems, each super-sample typically includes both a color-value (for either color or gray-scale systems) and a Z-value (a representation of the depth of a pixel which is used to perform hidden surface removal) relating to a reference point (one reference point per super-sample) within the pixel. 
     Another well known technique includes multi-sampling. Unlike super-sampling, multi-sampling uses the color-values from the original sample for all sub-samples, and relies on the sub-samples positions to achieve its effect. The number of samples per pixel may vary from application to application. 
     While super-sampling exhibits output of a higher quality, it offers no speed advantages over multi-sampling. On the other hand, multi-sampling exhibits less quality and higher speed. 
     There is thus a need for achieving a compromise between quality and speed when antialiasing. 
     DISCLOSURE OF THE INVENTION 
     A system, method, and computer program product are provided for antialiasing during rendering in a graphics pipeline. Initially, a primitive of vertex data is received in a graphics pipeline. Next, a super-sampling operation is performed on the primitive of vertex data utilizing the graphics pipeline. Further, a multi-sampling operation is performed on the primitive of vertex data utilizing the graphics pipeline. 
     These and other advantages of the present invention will become apparent upon reading the following detailed description and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages are better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
     Prior Art FIG. 1 illustrates two versions of a line: one perfect line and one heavily aliased. 
     FIG. 1A shows a block diagram of a graphics computer, in accordance with one embodiment. 
     FIG. 2 illustrates a more detailed diagram showing the internal structure of the graphics subsystem of FIG.  1 A. 
     FIGS. 2A-2C illustrate more information on super-sampling, in accordance with one embodiment. 
     FIG. 3 illustrates more information on multi-sampling, in accordance with one embodiment. 
     FIG. 4 illustrates the manner in which texture samples retrieved by the shader of FIG. 2 are sampled. 
     FIG. 5 illustrates the manner in which the present embodiment achieves horizontally neighboring pixel information. 
     FIG. 6 illustrates the manner in which the edge quality of the present embodiment is improved. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A shows a block diagram of a graphics computer, in accordance with one embodiment. As shown, a host processor  101  stores and executes an application program. Such application program issues graphics commands over a connection  102  to a graphics subsystem  103 . The interconnect  102  can either be a general purpose bus that is shared with other I/O devices, e.g. PCI bus, or a dedicated graphics bus, e.g. AGP. Of course, any type of interconnect  102  may be utilized. In use, the graphics subsystem  103  may receive graphics commands and, in response, either update its internal state or renders primitives described by the host processor  101  to a display device  104 . 
     FIG. 2 illustrates a more detailed diagram showing the internal structure of the graphics subsystem  103  of FIG.  1 A. As shown, a geometry stage  201  is provided which transforms primitives into a screen-aligned coordinate system. The geometry stage  201  also removes portions of the primitives that fall outside the rendering area (clipping). Further, various computations are performed by the geometry stage  201  such as lighting to determine the color of each vertex describing the primitives. Such vertex data may thus be processed in any desired manner. 
     Such vertex data is then received by the scan conversion subsystem  200  which breaks the vertex data down into pixel data, such as pixel screen coordinates and parameter values for each of the individual pixels of the display  104 . Each pixel is thereby assigned at least an X and Y screen coordinate, plus a number of parameters, such as red (R), green (G) and blue (B) color values, and alpha (A) values if alpha blending is supported. Alpha values are used to represent the opacity of a particular pixel. 
     An additional pixel parameter is the super-sample coverage mask, which is based on the size and geometry of a predefined super-sample size, and which includes an indication of those super-samples which are covered or uncovered by the particular primitive to be drawn. More information on the super-sampling techniques of the present embodiment will be set forth in greater detail during reference to FIGS. 2A-2C. 
     The transformed, clipped and lit vertices form the input for a rasterizer  202 . The rasterizer  202  compares new pixel data to old pixel data stored in a memory, and updates old pixel data as necessary on a pixel-by-pixel basis with new pixel data. 
     Also included is a shader  203  that computes the final fragment, e.g. by applying texture maps or shader programs to the fragment. A sampling stage  204  then generates multiple samples for each fragment via multi-sampling. More information on the multi-sampling of the present embodiment will be set forth in greater detail during reference to FIG.  4 . 
     After multi-sampling, the individual samples are sent to a raster-processor  205  as if they were regular fragments. The raster-processor  205  performs various operations on the fragments, including z/stencil testing and color or alpha blending. This may require the raster-processor  205  to read a frame buffer memory  206  in order to retrieve the destination Z or the destination color. To this end, the final pixel color and Z are written back to the frame buffer memory  206 . 
     When all primitives in the scene have been rendered in this manner, the contents of the frame buffer memory  206  are scanned out by a video refresh unit  207  and sent to the display  104 . In the case of a multi-sampled color buffer, the (high-resolution) multi-sample image is downfiltered to display resolution. Downfiltering can be done as a separate operation prior to display. Alternatively, downfiltering can be done during the process of scanout and display. 
     As an option, each of the foregoing components of the graphics subsystem  103  may be situated on a single semiconductor platform. In the present description, the single semiconductor platform may refer to a sole unitary semiconductor-based integrated circuit or chip. It should be noted that the term single semiconductor platform may also refer to multi-chip modules with increased connectivity which simulate on-chip operation, and make substantial improvements over utilizing a conventional CPU and bus implementation. Of course, the various modules may also be situated separately or in various combinations of semiconductor platforms per the desires of the user. 
     In the context of the foregoing processing, tile rending may be carried out. Typically, the frame buffer memory  206  takes the form of direct random access memory (DRAM). Such DRAM is organized as a matrix of storage bits. Read or write accesses to bits in the same row can be performed more efficiently than to bits in different rows. The present embodiment thus organizes the storage of pixels in the frame buffer such that neighboring pixels are stored in a single row in a DRAM bank (one or more associated DRAM chips). Such neighborhoods or pixels are often referred to as “tiles.” Since the present frame buffer memory  206  is organized to store pixels in a tile in one page of memory, it may be referred to as a “tiled buffer.” 
     To fully exploit the faster access time to pixels in a tile, processing of fragments is organized such that all fragments for pixels in a particular tile are grouped together. In one embodiment, it may be assumed that tiles are 4×4 pixel neighborhoods, i.e. fragments are processed in groups of up to 16. It should be noted, however, that tile sizes larger and smaller are possible and may be preferable in certain circumstances. 
     FIG. 2A illustrates how a 16-bit super-sample coverage mask is derived by the scan conversion subsystem  200  of FIG.  2 . It should be noted that the super-sampling may take place in any other component of the graphics subsystem  103 , and further in any desired order with respect to the multi-sampling. 
     Super-sampling is accomplished by sampling a primitive  230  of the vertex data at any number [i.e. sixteen (0-15)] of discrete points  232 . Each point  232  is aligned to a geometrically predefined (i.e. 8-by-8) grid  234  over the current pixel, which has an origin  236 . With respect to the super-sample coverage mask of the preferred embodiment, the predetermined method for choosing such super-samples is that of point sampling. It should be noted, however, that the present invention is independent of the method by which samples are chosen and would work equally well with an area sampling technique. 
     If a sample point  232  intersects the primitive, the corresponding bit in the super-sample coverage mask  240  of FIG. 2B is set to “1.” In the present example, bits four to seven, nine to eleven and fifteen are set to. “1.” If the sample point  232  lies outside the primitive  230 , then the corresponding bits of the super-sample coverage mask  240  are set to “0.” Although hardware constraints of the scan conversion subsystem of the preferred embodiment of the present invention dictate that sample patterns must be constant from pixel to pixel and that each sample must fall on the grid covering the pixel, a system excluding these constraints could be readily envisioned by a person of ordinary skill in the art in view of the present disclosure. 
     FIG. 2C illustrates how color values for a pixel  242  are first determined by the scan conversion subsystem  200  computing R, G and B (and A, if alpha blending is enabled) equations for the primitive  244 . at the centroid  246  of the pixel fragment  248 . The pixel fragment  248  is formed by the intersection of the pixel  242  and the primitive  244 . Since the centroid  246  is guaranteed to lie inside both the pixel  242  and the primitive  244 , in contrast to the pixel&#39;s origin which can lie outside of the primitive  244 , the centroid  246  is used to compute the color values, thereby preventing illegal color values from being generated for the different pixels. 
     FIG. 3 illustrates the effect  300  of multi-sampling, as carried out by the sampling stage  204  of FIG.  2 . It should be noted that the multi-sampling may take place in any other component of the graphics subsystem  103 , and further in any desired order with respect to the super-sampling. 
     The process of multi-sampling adjusts the fragment depth value to a sampling location in the pixel. All samples within a fragment carry the same color. As shown, sample locations  302  may be jittered, rotated, or skewed within a pixel  304 . In other words, the sample locations  302  may be diagonally positioned within the pixel  304 . Of course, these options may be varied per the desires of the user. 
     In another embodiment, either 2-sample multi-sampling  306 , 4-sample multi-sampling  308 , or any other #-multi-sampling may be employed per the desires of the user. 
     FIG. 4 illustrates the manner in which texture samples  400  generated by the shader  203  of FIG. 2 are sampled. As shown, two separate texture samples  400  may be mapped to two separate pixels  402  which are, in turn, multi-sampled in the aforementioned manner. Also shown is the manner in which the multi-sampled pixels  402  are downfiltered to a display resolution. As mentioned earlier, downfiltering can be done as a separate operation prior to display. Alternatively, downfiltering can be done during the process of scanout and display. In any case, the blended result is shown with downfiltered pixel  404 . By this feature duplicate texture information is used across pixels, which reduces texture aliasing. 
     FIG. 5 illustrates the manner  500  in which the present embodiment achieves horizontally neighboring pixel information. As shown, two bilinear samples are taken, but the sample positions are placed very close to the middle 2×2 true samples, rather than exactly in the middle of the 2×2 area to be sampled, which would give an even weighting for all 8 samples. To this end, the 4 true samples are weighted more (i.e. say 75%) and each set of 2 neighbors only 12.5% each. 
     FIG. 6 illustrates the manner  600  in which the edge quality of the present embodiment is improved. There are three factors associated with perceived anti-alias quality. See Table 1. 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 1. 
                 The number of possible coverage values for nearly horizontal and 
               
               
                   
                 vertical lines 
               
             
          
           
               
                   
                 Due to the eye having special sensitivity to these lines 
               
               
                   
                 Also due to undersampling of shallow and steep slopes due to 
               
               
                   
                 the raster pattern on the screen 
               
               
                   
                 This is an advantage for irregular sample patterns 
               
             
          
           
               
                   
                 It is not beneficial to have samples line up vertically 
               
               
                   
                 or horizontally 
               
             
          
           
               
                 2. 
                 The number of actual coverage samples 
               
             
          
           
               
                   
                 Controls how many coverage values available 
               
               
                   
                 More samples reduce pixel popping 
               
             
          
           
               
                 3 
                 The number of texture samples 
               
             
          
           
               
                   
                 More samples produces results like anisotropic filtering 
               
               
                   
                 Makes things clearer, especially poorly mip-mapped games 
               
               
                   
                   
               
             
          
         
       
     
     As shown in FIG. 6, the benefits of the skewed multi-sampling is shown by a comparison between a first pixel  602  with aligned samples and a second pixel  604  with vertically jittered samples. While vertically jittered samples are set forth herein, it should be noted that horizontally jittered samples may also be employed. As shown, coverage mask boundaries  606  are more likely to overlap the samples of the second pixel  602  since they are dispersed more evenly within the pixel. By increasing the chance of overlap, more accurate blending is afforded. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.