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Scan line method
- 1. Prof. Neeraj Bhargava Pooja Dixit Department of Computer Science School of Engineering & System Sciences MDS, University Ajmer, Rajasthan, India 1
- 2. It is an image-space method to identify visible surface. This method has a depth information for only single scan- line. In order to require one scan-line of depth values, we must group and process all polygons intersecting a given scan-line at the same time before processing the next scan-line. Two important tables, edge table and polygon table, are maintained for this. The Edge Table − It contains:- ◦ coordinate endpoints of each line in the scene ◦ the inverse slope of each line ◦ and pointers into the polygon table to connect edges to surfaces. The Polygon Table − It contains:- ◦ the plane coefficients ◦ surface material properties ◦ other surface data ◦ may be pointers to the edge table. 2
- 3. Each of the scan line is processes from left to right. The surface flag is turned on at left intersection and at right intersection it is turned off. In this method, as each scan line is processed, all polygon surfaces intersecting that line are examined to determine which are visible. A cross each scan line, depth calculations are made for each overlapping surface to determine which is nearest to the view plane. When the visible surface has been determined, the intensity value for that position is entered into the image buffer. The search for the surfaces that cross a given scan line can be facilitated by an active list of edges. Only the edges that cross the scan line is stored by the active list. For indicating whether the position along a scan line is inside or outside the surface, a flag is set. Each of the scan line is processes from left to right. The surface flag is turned on at left intersection and at right intersection it is turned off. 3
- 4. 4 Scan lines corssing the projection of two furfaces S1, S2 in the view plane. Dashed lines indicate the boundaries of hidden surfaces.
- 5. Figure illustrates the scan-line method for locating visible portions of surfaces for pixel positions along the line. The active list for line 1 contains information from the edge table for edges AB, BC, EH, and FG. For positions along this scan line between edges AB and BC, only the flag for surface S1 is on. Therefore no depth calculations are necessary, and intensity information for surface S1, is entered from the polygon table into the refresh buffer. Similarly, between edges EH and FG, only the flag for surface S2 is on. NO other positions along scan line 1 intersect surfaces, so the intensity values in the other areas are set to the background intensity. The background intensity can be loaded throughout the buffer in an initialization routine. For scan lines 2 and 3 in Fig. , the active edge list contains edges AD, EH, BC, and FG. Along scan line 2 from edge AD to edge EH, only the flag for surface S1 is on. But between edges EH and BC, the flags for both surfaces are on. In this interval, depth calculations must be made using the plane coefficients for the two surfaces. For this example, the depth of surface S1 is assumed to be less than that of S2, so intensities for surface S1 are loaded into the refresh buffer until boundary BC is encountered. Then the flag for surface S1 goes off, and intensities for surface S2 are stored until edge FG is passed. We can take advantage of-coherence along the scan lines as we pass from one scan line to the next. In Fig. 4 , scan line 3 has the same active list of edges as scan line 2. Since no changes have occurred in line intersections, it is unnecessary again to make depth calculations between edges EH and BC. The two surfaces must be in the same orientation as determined on scan line 2, so the intensities for surface S1 can be entered without further calculations. 5