Guardband clipping method and apparatus for 3-D graphics display system

• US 4,888,712 A
• Filed: 11/04/1987
• Issued: 12/19/1989
• Est. Priority Date: 11/04/1987
• Status: Expired due to Fees

• Current Assignee: Applicon, Inc. (Perspecta, Inc.)
• Original Assignee: Schlumberger Systems Incorporated
• Inventors: Durant, Thomas L., Barkans, Anthony C., Lach, Jorge, Gordon, Dorothy, Schroeder, Brian D.
• Primary Examiner(s): Harkcom, Gary V.
• Assistant Examiner(s): HERNDON, HEATHER R
• Application Number:
  US07/117,112
• Time in Patent Office: 776 Days
• Field of Search: 364/518, 364/521, 364/522, 340/729, 340/731, 340/734, 340/723, 382/44-48
• US Class Current: 345/627
• CPC Class Codes: G06T 15/30 Clipping

Abstract

A system for clipping polygons representing three-dimensional objects to produce a representation of the portion of the objects in a desired viewing space is disclosed. A guardband space at least partially enclosing the viewing space is defined. The polygons are compared to the guardband space to determine which polygons intersect at least one of the guardband planes defining the guardband space. The intersecting polygons are also compared to the viewing space to determine if they intersect at least one of the viewing planes defining the viewing space. Only polygons intersecting both a viewing plane and a guardband plane are clipped.

Claims

1. A system for clipping polygons representing a three-dimensional objects to produce a representation of the objects in a viewing space defined by a plurality of viewing planes, comprising:

means for determining coordinates of a guardband space defined by a plurality of guardband planes enclosing said viewing space on at least one side;

means for comparing representations of said polygons to said coordinates of said guardband space to determine which polygons intersect at least one of said guardband planes;

means for comparing said representations of said polygons intersecting said guardband planes to coordinates of said viewing space to determine which polygons are two-space polygons which also intersect at least one of said viewing planes; and

means for clipping by generating new representations for only polygons which intersect both at least one of said guardband planes and at least one of said viewing planes wherein said new representations exclude at least portions of said polygons outside said guardband space.

2. The system of claim 1 further comprising means for discarding representations of polygons totally outside said viewing space and means for outputting, without generating new representations, remaining representations of polygons totally inside said guardband space.

3. They system of claim 1 wherein one of said viewing space and said guardband space is designated as a clipping space and further comprising:

means for determining which of said two space polygons are single plane polygons which intersect only one plane of said clipping space; and

means for generating said new representations for said single plane polygons with only said one plane of said clipping space being used to form a new edge for said single plane polygons.

4. The system of claim 1 wherein said polygons are triangles.

5. The system of claim 1 wherein said guardband space is defined by four planes surrounding said viewing space except in front and in back of said viewing space.

6. The system of claim 1 further comprise means for sorting polygons into one of the following classes

(a) all vertices are completely outside at least one of the planes of said viewing space,

(b) all vertices are completely inside all of said guardband planes, but at least one vertex is completely inside said viewing space,

(c) at least one vertex is completely inside said viewing space, but with some vertices outside exactly one of said guardband planes,

(d) at least one vertex is completely inside said viewing space, with some vertices outside more than one of said guardband planes;

and

means for clipping polygons in said class (c) against only one of said viewing planes.

7. The system of claim 1 further comprising means for rasterizing pixels representing said image only up to an end of said viewing space.

8. The system of claim 1 further comprising

means for rasterizing pixels representing said image;

and

means for incrementing scan lines without rasterizing all of the pixels in a scan line for scan lines outside said prior to said viewing space.

9. The system of claim 1 further comprising

means for rasterizing pixels representing said image;

and

means for stopping said rasterizing for scan lines outside and beyond said viewing space.

10. A system for clipping polygons representing three-dimensional objects to produce a representation of the objects in a viewing space defined by a plurality of viewing planes, comprising:

means for determining a guardband space defined by a plurality of guardband planes at least partially enclosing said viewing space;

means for comparing said polygons to said guardband space to determine which polygons intersect at least one of said guardband planes;

means for comparing said polygons intersecting said guardband planes to said viewing space to determine which polygonals are two-space polygons which also intersect at least one of said viewing planes;

means for clipping by generating new representations for only polygons which intersect both at least one of said guardband planes and at least one of said viewing planes, wherein said new representations exclude at least portions of said polygons outside a clipping space, said clipping space being one of said viewing space and said guardband space;

means for discarding polygons totally outside said viewing space and means for outputting without clipping remaining polygons totally inside said guardband space;

means for determining which of said two space polygons are single plane polygons which intersect only one plane of said clipping space;

means for clipping said single plane polygons with only said one plane of said clipping space;

means for rasterizing pixels representing said image only up to an end of said viewing space;

means for incrementing scan lines without rasterizing all of the pixels in the scan line for scan lines outside and prior to said viewing space; and

means for stopping said rasterizing for scan lines outside and beyond said viewing space.

11. A method for clipping polygons representing three-dimensional objects to produce a representation of the objects in a viewing space defined by a plurality of viewing planes, comprising the steps of:

determining coordinates of a guardband space defined by a plurality of guardband planes enclosing said viewing space on at least one side;

comparing representations of said polygons to said coordinates of said guardband space to determine which polygons intersect at least one of said guardband planes;

comparing said representations of said polygons intersecting said guardband planes to coordinates of said viewing space to determine which polygons also intersect at least one of said viewing planes; and

clipping by generating new representations for only polygons which intersect both at least one of said guardband planes and at least one of said viewing planes wherein said new representations exclude at least portions of said polygons outside said guardband space.

12. A method of claim 11 further comprising the steps of discarding representations of polygons totally outside said viewing space and outputting, without generating new representations, remaining representations of polygons totally inside said guardband space.

13. The method of claim 11 wherein one of said viewing space and said guardband space is designated as a clipping space and further comprising the steps of:

determining which of said two space polygons are single plane polygons which intersect only one plane of said clipping space; and

generating said new representations for said single plane polygons with only said one plane of said clipping space being used to form a new edge for said single plane polygons.

14. The method of claim 11 wherein said polygons are triangles.

15. The method of claim 11 further comprising the step of rasterizing pixels representing said image only up to an end of said viewing space.

16. The method of claim 11 further comprising the steps of rasterizing pixels representing said image and incrementing scan lines without rasterizing all of the pixels in a scan line for scan lines outside and prior to said viewing space.

17. The method of claim 11 further comprising the steps of rasterizing pixels representing said image and stopping said rasterizing for scan lines outside and beyond said viewing space.

18. A method for clipping polygons representing three-dimensional objects to produce a representation of the objects, in a viewing space defined by a plurality of viewing planes, comprising the steps of:

determining a guardband space defined by a plurality of guardband planes at least partially enclosing said viewing space;

comparing said polygons to said guardband space to determine which polygons intersect at least one of said guardband planes;

comparing said polygons intersecting said guardband planes to said viewing space to determine which polygons also intersect at least one of said viewing planes;

clipping by generating new representations for only polygons which intersect both at least one of said guardband planes and at least one of said viewing planes, wherein said new representations exclude at least portions of said polygons outside a clipping space, said clipping space being one of said viewing space and said guardband space;

discarding polygons totally outside said viewing space;

outputting without clipping remaining polygons totally inside said guardband space;

determining which of said two space polygons are single plane polygons which intersect only one plane of said clipping space; and

clipping said single plane polygons with only said one plane of said clipping space.

19. A system for clipping polygons representing three-dimensional objects to produce a representation of the objects in a viewing space defined by a plurality of viewing planes, comprising:

a central processing unit programmed to produce, from an input of said viewing space, a representation of a guardband space defined by a plurality of guardband planes enclosing said viewing space on at least one side;

a memory coupled to receive said representation of a guardband space and a representation of said viewing space from said central processing unit;

comparison logic means for comparing representations of said polygons to said representation of a guardband space and said representation of said viewing space in said memory and producing a first comparison output only when a polygon intersects both one of said viewing planes and one of said guardband planes; and

clipping logic means, coupled to said comparison logic means, for clipping, by generating new representations, said polygons only when said first comparison output is received for a particular polygon, said new representations excluding at least portions of said particular polygon outside said guardband space.