Abstract: A multiple-pass system for determining the primitives that are visible in a predetermined pick aperture for a “visible pick” operation. On the first pass, the primitives contained within the pick aperture and thus potentially visible are selected, and each selected primitive is assigned a pick index as an identifier. On the second pass, the pick indices of the selected primitives are rendered to a temporary frame buffer area, using a Z-buffer for hidden surface removal so that only the pick indices of the primitives visible within the pick aperture are stored in the corresponding frame buffer portion. On the third pass, the frame buffer portion corresponding to the pick aperture is read to determine the pick indices of the visible primitives. This information is then used to report back to the host the picked primitives.

Abstract: Objects are represented in a coordinate space which is twice scanned by scan lines, once in one direction and then once in another direction. At each scan line, a list of line segments is generated, where each line segment represents the intersection of the projection of a polygon and the scan line. The segments are then examined to determine which are visible segments, using the z values of the end points of the segments. The visible segments of a scan line are compared to the visible segments of a previous scan line or an accumulation of visible lines, and if the end points of a visible segment taken from a polygon differ by less than a threshold amount, the lines formed between end points in one scan line and the adjacent scan line are considered to be visible lines. Where the two scans are a horizontal scan and a vertical scan, the threshold amount for vertical scans is the distance between horizontal scans and the threshold amount for horizontal scans is the distance between vertical scans.