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Adjustment of Z-buffer values for lines on the surface of a polygon
| Details |
Inventors: Fossum, Gordon C.;
Assignee: International Business Machines Corporation (Armonk, NY)
Primary Examiner: Harkcom; Gary V.
Assistant Examiner: Jankus; Almis
Attorney, Agent or Firm: McBurney; Mark E.
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. |
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DETAILED DESCRIPTION In contrast to the prior art, the present invention provides a method of setting the relationship between lines and the polygon surface such that the surface of the polygon is viewed as being deeper, thereby allowing the lines to be visible (i.
e.
painted).
A standard Z-buffer generally has a possible range of depth values from -2**23 to 2**23-1 which are characterized such that the larger values are deeper into the screen, or display.
Therefore, to ensure that the lines are painted rather than the polygon surface, when both have an identical mathematical depth, the polygon must be considered to be deeper than the line and thereby overwritten by the graphics adapter, when a line on that polygon is considered.
The present invention achieves this goal by always characterizing the depth values associated with a polygon surface as larger, or deeper than the line depth values.
This is accomplished by computing an offset, or shift amount once for each polygon, as a function of the normal to the polygon surface, and adjusting the adapter's output Z-buffer range for each polygon painted relative to the output range used for all lines.
For example, if a polygon is oriented such that its normal is pointing directly out of the screen, then the shift will be minimal.
However, if the polygon is oriented virtually on its side, or edge-on, then the offset will be large to compensate for the larger errors that a occur in the depth estimating process, contained in the line drawing portion of the present invention.
It should be noted that the effect of the Z-buffer adjustment in drawing the polygon will not change the appearance of the polygon.
The possible exceptions being insignificant changes in color based upon depth cueing (for example, one level of color intensity out of a total of 256, which is not discernible to the human eye), or an isolated pixel error at the point of intersection between the polygon surface and any lines non-planar with respect thereto.
For these reasons, it is important to keep the Z-buffer adjustment as small as possible
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