Adjustment of Z-buffer values for lines on the surface of a polygon

• US 5,245,700 A
• Filed: 11/21/1989
• Issued: 09/14/1993
• Est. Priority Date: 11/21/1989
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Fossum, Gordon C.
• Primary Examiner(s): Harkcom, Gary V.
• Assistant Examiner(s): JANKUS, ALMIS R
• Application Number:
  US07/439,902
• Time in Patent Office: 1,393 Days
• Field of Search: 364/518-522, 350/717, 350/729, 395/119-122, 395/133-135, 395/125
• US Class Current: 345/422
• CPC Class Codes: G06T 15/405 using Z-buffer

Abstract

A system and method in a graphics environment ensures that a line drawn upon the surface of a polygon is always visible to a user of the graphics system. In a Z-buffered environment the range of depth values contained therein are set to the 75% of the total capacity which is considered closest to the surface of a display screen. The line is then drawn into this 75% set of Z-buffer depth values. A shift is then computed, based upon the normal to the polygon in device coordinates. This shift is then added to the previously set 75% range of Z-buffer depth values. The polygon is then drawn at a depth equal to the previously set range plus the shift. Thus, the line will always be visible since, the polygon surface will always be considered by a graphics adapter to be deeper than the line contained on the surface thereof.

Claims

1. A method of three dimensionally displaying on a computer graphics system having a Z-buffer and a display, a polygon having a line contained on a surface of the polygon, said Z-buffer for storing depth values associated with said polygon and said line, said method comprising the steps of:

setting a range of depth values corresponding to a percentage of all of said depth values in said Z-buffer;

drawing said line on said display wherein the depth value for said line falls within said range;

shifting said range of said depth values within said Z-buffer by a shift amount dependent upon an angle between a normal vector to said polygon surface and a vector perpendicular to said display; and

drawing said polygon on said display at a depth within said shifted range, said polygon surface having a greater depth within said Z-buffer than said line.

2. A method according to claim 1 wherein said step of shifting comprises the steps of:

computing a normal to said polygon surface;

mapping coordinates of said computed normal into coordinates dependent upon a specific display device;

computing said shift amount by calculating the tangent of said angle between the normal vector to the polygon surface and the vector perpendicular to said display;

limiting said shift amount to a predetermined value; and

adding said shift amount to each depth value within said range of depth values.

3. A method according to claim 2 wherein said step of computing a shift amount comprises the step of:

multiplying said calculated tangent by a non-zero positive number to obtain said shift amount.

4. A method according to claim 2 wherein said step of limiting said shift amount comprises the steps of:

determining whether said shift amount corresponds to a predetermined number of all said depth values of said Z-buffer; and

limiting said shift amount to said predetermined number of said depth values.

5. A computer graphics system, for three dimensionally displaying a polygon having a line contained on a surface of the polygon, comprising:

a Z-buffer for storing depth values associated with said polygon and said line;

means for setting a range of depth values corresponding to a percentage of all said depth values within said Z-buffer;

means for drawing said line on a display at a depth value falling within said range of depth values;

means for shifting said range of said depth values in said Z-buffer by a shift amount dependent upon an angle between a normal vector to said polygon surface and a vector perpendicular to said display; and

means for drawing said polygon at a depth within said range, shifted by said means for shifting, said polygon surface having a greater depth within said Z-buffer than said line.

6. A system according to claim 5 wherein said means for shifting comprises:

means for computing a normal to said polygon surface;

means for mapping coordinates of a normal, computed by said means for computing, into coordinates dependent upon a specific display device;

means for computing a shift amount by calculating the tangent of said angle between the normal vector to the polygon surface and the vector perpendicular to the display;

means for limiting said shift amount to a predetermined value; and

means for adding said shift amount to each depth value within said range of depth values.

7. A system according to claim 6 wherein said means for computing a shift amount comprises:

means for multiplying said tangent, calculated by said means for, computing a shift amount by a non-zero, positive number to obtain said shift amount.

8. A system according to claim 6 wherein said means for limiting said shift amount comprises:

means for determining whether said shift amount corresponds to a predetermined number of all of said depth values of said Z-buffer; and

means for limiting said shift amount to said predetermined number of said depth values.