| OCR Text |
Show 41 the list coming from the edges of that particular polygon. Thre must be an even number of these and they should be connected pairwise as the scan beam sweeps across them in the X direction. Since there may be several different polygons in the active list there must be some link between the two edge blocks defining a segment so that when the left end enters the active point list, the right hand edge block may be found. Sometimes this link is represented only list is usually kept sorted in X order. The X and Z implicitly by storing a polygon number in each edge block. This necessitates a search of the active edge list to find the edge block containing the same polygon number as the entering edge. During the X scan a list of intersection points is maintained, consisting of all points formed by intersections of an active curve with the scanning beam. A point is in the active list while the X coordinate of the scan beam lies between the X coordinates of two edges of the polygon. As the beam moves in X, active points are generated from elements in the active edge list. For this reason the edge coordinates at the straddling edge blocks are used to define the line segment to intersect with the scan beam. While a POint is active its Z coordinate is found, just as for the edge list, by linear interpolation between these two endpoints. The point descriptor block then contains the Z cOordinate of the point. As the beam sweeps over one unit in X, the Z field must be updated. This may be done in an |
