Computer graphics processing and selective visual display system – Computer graphics display memory system – For storing condition code – flag or status
Reexamination Certificate
1998-10-14
2001-07-10
Tung, Kee M. (Department: 2671)
Computer graphics processing and selective visual display system
Computer graphics display memory system
For storing condition code, flag or status
C345S506000
Reexamination Certificate
active
06259461
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to computer graphics systems and, more particularly, to a computer graphics system that accelerates the rendering of graphics in a multi-pass rendering environment.
DISCUSSION OF THE RELATED ART
Computer graphics systems are commonly used for displaying graphical representations of objects on a two-dimensional video display screen. Current computer graphics display systems provide highly detailed representations and are used in a variety of applications. A computer graphics display system generally comprises a central processing unit (CPU), system memory, a graphics machine and a video display screen.
In typical computer graphics display systems, an object to be presented on the display screen is broken down into graphics primitives. Primitives are basic components (at the API—application program interface—level) of a graphics display and usually are simple structures such as points, lines, vectors and polygons (e.g., triangles and quadrilaterals). Typically, a hardware/software scheme is implemented to render, or draw, the graphics primitives that represent a view of one or more objects being represented on the display screen.
As is known, traditional computer graphic displays rendered wire frame images of objects, since wire frame images possess a relatively small amount of data to process. However, with ever increasing processing capabilities and operating speeds of computer graphic systems, such systems now frequently render shaded images of the surface of objects. In this fashion, smooth shading provides increased realism and the ability to visualize more complex models. However, in such displays, some of the image information may be lost. Accordingly, and to benefit from the best of both methods, graphics applications often combine wire frame (or edging) and shaded primitives in the same image, by first rendering the shaded surface, and then rendering the edges of the shaded surface to represent the wire frame. In this regard, a wire frame can overlay a shaded surface to provide structural information, such as the original data point that generated the model. When a model is constructed using splines, the wire frame can show spline patch boundaries, or the control points used to calculate the spline surface. Another benefit of wire frame modeling is selective highlighting of a subset of a model, such as a subassembly with a complex mechanical part.
For example, consider a CAD (computer aided design) station where a designer is designing or modeling a mechanical part for some engineering design application. The mechanical part, which may be rather complex, may be displayed on a computer graphics station using shading of the object surface. If, however, there is a particular portion of the object that the designer is particularly interested in, a user interface may be provided to allow the designer to select (e.g., using a mouse) a portion of the graphic image, and the system may respond by displaying further information (e.g., edging information) for that selected portion.
Applications use a variety of methods to simultaneously display wire frame and shaded objects. Some graphics libraries provide polygon edging support directly, which means a polygon can be rendered with a solid fill and an edge in a distinct color with a single call to the library. If no such library support exists, the application must make two separate graphics calls: a first call to display a filled polygon, followed by a second call to display the polygon edges. Some applications prefer to display edge vectors after rendering all polygons, to progressively highlight different features of a model.
As is known, one shortcoming in displaying both shaded information and edging information relates to the processing time required to format and display the image. In this regard, such displays usually require multiple passes, wherein a first pass is made that renders the surface shading information, and a second pass is made to render object edging information. Conventionally, for a given object or segment of graphic data that is to be displayed, an application program provides all the shading information to a rendering pipeline, which performs various graphic computations required for the display. In a second pass, the application program provides the rendering pipeline with edging information, which is then formulated within the rendering pipeline for display.
One shortcoming known to exist in prior art systems relates to the computations required in the rendering pipeline for information that ultimately does not get displayed on the display screen to the user. Returning to the previous example, suppose a CAD designer is modeling a mechanical device that has many internal parts self-contained within an outer shell. If the graphic display is displaying a pictorial representation of the exterior of the object, the internal component objects will not be visible to the developer. Therefore, the graphic system need not process this information for display.
In conventional graphic systems, a well known mechanism, commonly called a z-buffer, is used in graphic display systems. As is known, the z-buffer maintains depth information for objects in a graphic display. For example, as a graphic system is processing and displaying a number of objects that collectively make up a graphic scene, each object may comprise a number of primitives, which in turn comprise a number of display pixels. The z-buffer maintains depth information for each display pixel on the graphic display. As any object is rendered for display, a number of pixels are to be illuminated in a certain defined manner in order to display the object. A depth value is associated for each point on an object that corresponds to a pixel on the display. This depth information is maintained in a z-buffer, for the shallowest (nearest) point depths. For example, for a given pixel on a display screen, when a first object is to be rendered in a graphic image, there may be no initial value in the z-buffer. Therefore, that pixel may be illuminated in accordance with the graphics rendering hardware. When another object is to be displayed that has a point that corresponds to the given pixel, the depth of that point is compared with the stored depth in the z-buffer. If the later rendered point has a depth value that indicates it is closer to the viewer (screen) than the previously rendered object, then that point is rendered and its depth value recorded in the z-buffer. Otherwise, the point is not rendered and its value is not recorded in the z-buffer. As will be appreciated by those skilled in the art, the foregoing example is simplified in that it ignores certain contingencies, such as the rendering of transparent object surfaces.
For purposes of the invention that will be described hereinafter, a shortcoming with prior art systems is that a significant amount of processing time is often expended and lost processing graphic images (or portions thereof) that ultimately will not be rendered on a graphics display. Accordingly, there is a desire to improve the performance of graphic systems by eliminating processing associated with objects or other portions of graphic images that will not be displayed because they may be hidden by other objects on the display or otherwise.
SUMMARY OF THE INVENTION
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the advantages and novel features, the present invention is generally directed to a computer graphics system and method that has increased performance and throughput. Broadly, the increased throughput is achieved by a novel system and method that eliminates unnecessary processing
Hewlett -Packard Company
Tung Kee M.
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