From Vertices to Fragments II Software College, Shandong University Instructor: Zhou Yuanfeng

Review Cyrus-Beck clipping algorithm; Liang-Barsky clipping algorithm; Sutherland-Hodgman polygon clipping; DDA line algorithm; Bresenham’s line algorithm;

3 Objectives Rasterization: Polygon scan conversion algorithm; Hidden surface removal Aliasing

4 Polygon Scan Conversion Scan Conversion = Fill How to tell inside from outside ­Convex easy ­Nonsimple difficult ­Odd even test Count edge crossings ­Winding number odd-even fill

5 Winding Number Count clockwise encirclements of point Alternate definition of inside: inside if winding number  0 winding number = 2 winding number = 1

OpenGL & Concave Polygon OpenGL can only fill convex polygon correctly 6

OpenGL & Concave Polygon // create tessellator GLUtesselator *tess = gluNewTess(); // describe non-convex polygon gluTessBeginPolygon(tess, user_data); // first contour gluTessBeginContour(tess); gluTessVertex(tess, coords[0], vertex_data);... gluTessEndContour(tess);... gluTessEndPolygon(tess); // delete tessellator after processing gluDeleteTess(tess); 7

Constrained Delaunay Triangulation 8

9 Filling in the Frame Buffer Fill at end of pipeline ­Convex Polygons only ­Nonconvex polygons assumed to have been tessellated ­Shades (colors) have been computed for vertices (Gouraud shading) ­Combine with z-buffer algorithm March across scan lines interpolating shades Incremental work small

10 Using Interpolation span C1C1 C3C3 C2C2 C5C5 C4C4 scan line C 1 C 2 C 3 specified by glColor or by vertex shading C 4 determined by interpolating between C 1 and C 2 C 5 determined by interpolating between C 2 and C 3 interpolate between C 4 and C 5 along span

11 Flood Fill Fill can be done recursively if we know a seed point located inside (WHITE) Scan convert edges into buffer in edge/inside color (BLACK) flood_fill(int x, int y) { if(read_pixel(x,y)= = WHITE) { write_pixel(x,y,BLACK); flood_fill(x-1, y); flood_fill(x+1, y); flood_fill(x, y+1); flood_fill(x, y-1); } } //4 directions

Flood Fill 12 //8 directions

Flood Fill with Scan Line Stack

14 Scan Line Fill Can also fill by maintaining a data structure of all intersections of polygons with scan lines ­Sort by scan line ­Fill each span vertex order generated by vertex list desired order

Singularities 15

16 Data Structure

Scan line filling For each scan line: 1.Find the intersections of the scan line with all edges of the polygon; 2.Sort the intersections by increasing x- coordinate; 3.Fill in all pixels between pairs of intersections. Problem: Calculating intersections is slow. Solution: Incremental computation / coherence 17

Coherence of Region 18 Trapezoid: inside and outside

Coherence of Scan Line e is an integer; ≥e ≥ Intersections number is even; are inside; 19 inside

Coherence of Edge Intersection points of d is l e and l d The number of l e = l d and are on the same edge 20 e d y 63 y 24

Coherence of Edge Observation: Not all edges intersect each scanline. Many edges intersected by scanline i will also be intersected by scanline i+1 Formula for scanline s is y = s, for an edge is y = mx + b Their intersection is s = mx s + b --> x s = (s-b)/m For scanline s + 1, x s+1 = (s+1 - b)/m = x s + 1/m Incremental calculation: x s+1 = x s + 1/m 21

Data Structure 22 Edge index table ET Active Edge List AEL Node: y max The max coord y; x The bottom x coord of edge in ET, the intersection x of edge and scan line in AEL. Δx 1/m next the next node Where is the end of one edge while scanning Initial x Current scan line and edge intersection When y=y+1, x=x+1/m Next edge

Data Structure ET 23 y

Data Structure AEL 24 AEL is the edges list which intersect with current scan line, the next intersection can be computed by: Active Edge List -5/3 5 7 AEL e0e0 e5e AEL at y=2 scan line -5/3 5 5 AEL e0e0 e5e AEL at y=3 scan line 2 y max x 1/m Edge list is ordered according x increasing

Algorithm Construct the Edge Table (ET); Active Edge List (AEL) = null; for y = Ymin to Ymax Merge-sort ET[y] into AEL by x value Fill between pairs of x in AEL for each edge in AEL if edge.ymax = y remove edge from AEL else edge.x = edge.x + dx/dy sort AEL by x value end scan_fill 25

26 Hidden Surface Removal Object-space approach: use pairwise testing between polygons (objects space) Worst case complexity O(n 2 ) for n polygons partially obscuringcan draw independently

27 Image Space Approach Look at each projector ( nm for an n x m frame buffer) and find closest of k polygons Complexity O (nmk) Ray tracing z-buffer Fast but with low paint quality

28 Painter’s Algorithm Render polygons a back to front order so that polygons behind others are simply painted over B behind A as seen by viewer Fill B then A

29 Depth Sort Requires ordering of polygons first ­O(n log n) calculation for ordering ­Not every polygon is either in front or behind all other polygons Order polygons and deal with easy cases first, harder later Polygons sorted by distance from COP

30 Easy Cases (1) A lies behind all other polygons ­Can render (2) Polygons overlap in z but not in either x or y ­Can render independently

31 Hard Cases (3) Overlap in all directions but can one is fully on one side of the other cyclic overlap penetration (4)

32 Back-Face Removal (Culling)  face is visible iff 90    -90 equivalently cos   0 or v n  0 plane of face has form ax + by +cz +d =0 but after normalization n = ( ) T need only test the sign of c In OpenGL we can simply enable culling but may not work correctly if we have nonconvex objects

33 z-Buffer Algorithm Use a buffer called the z or depth buffer to store the depth of the closest object at each pixel found so far As we render each polygon, compare the depth of each pixel to depth in z buffer If less, place shade of pixel in color buffer and update z buffer

z-Buffer Algorithm 34

z-Buffer Algorithm 35

Example 36

Example 37

Example 38

39 Efficiency If we work scan line by scan line as we move across a scan line, the depth changes satisfy a  x+b  y+c  z=0 Along scan line  y = 0  z = -  x In screen space  x = 1

40 Scan-Line Algorithm Can combine shading and hsr through scan line algorithm scan line i: no need for depth information, can only be in no or one polygon scan line j: need depth information only when in more than one polygon

z-Buffer Scan-Line 41

42 Implementation Need a data structure to store ­Flag for each polygon (inside/outside) ­Incremental structure for scan lines that stores which edges are encountered ­Parameters for planes for intersections

43 Visibility Testing In many real-time applications, such as games, we want to eliminate as many objects as possible within the application ­Reduce burden on pipeline ­Reduce traffic on bus Partition space with Binary Spatial Partition (BSP) Tree

44 Simple Example consider 6 parallel polygons top view The plane of A separates B and C from D, E and F

45 BSP Tree Can continue recursively ­Plane of C separates B from A ­Plane of D separates E and F Can put this information in a BSP tree ­Use for visibility and occlusion testing

BSP Tree 46

BSP Algorithm Choose a polygon P from the list. Make a node N in the BSP tree, and add P to the list of polygons at that node. For each other polygon in the list: If that polygon is wholly in front of the plane containing P, move that polygon to the list of nodes in front of P. If that polygon is wholly behind the plane containing P, move that polygon to the list of nodes behind P. If that polygon is intersected by the plane containing P, split it into two polygons and move them to the respective lists of polygons behind and in front of P. If that polygon lies in the plane containing P, add it to the list of polygons at node N. Apply this algorithm to the list of polygons in front of P. Apply this algorithm to the list of polygons behind P. 47

48 BSP display Type Tree Tree* front; Face face; Tree *back; End Algorithm DrawBSP(Tree T; point: w) //w 为视点 If T is null then return; endif If w is in front of T.face then DrawBSP(T.back,w); Draw(T.face,w); DrawBSP(T.front,w); Else // w is behind or on T.face DrawBSP(T.front,w); Draw(T.face,w); DrawBSP(T. back,w); Endif end

49 Aliasing Ideal rasterized line should be 1 pixel wide Choosing best y for each x (or visa versa) produces aliased raster lines

50 Antialiasing by Area Averaging Color multiple pixels for each x depending on coverage by ideal line original antialiased magnified

51 Polygon Aliasing Aliasing problems can be serious for polygons ­Jaggedness of edges ­Small polygons neglected ­Need compositing so color of one polygon does not totally determine color of pixel All three polygons should contribute to color

Time-domain aliasing Adding ray tracing line from one pixel; 52