Efficient Rasterization Techniques for Line and Polygon Filling

The Rasterization Problem: Idealized PrimitivesMap to Discrete Display Space

Solution Involves Selection of DiscreteRepresentation Values

Scan Converting Lines:Characterizing the Problem ideal line i.e. for each x, choose y i.e. for each y, choose x

Scan Converting Lines:The Strategy • Pick pixels closest to endpoints • Select in between pixels “closest” to ideal line • Minimize calculation required!

Calculate ideal line equation Pick endpoints: two dimensional round. Starting at leftmost point: for each xi Calculate Select pixel at Scan Converting Lines:A Basic Algorithm Selected Not Selected

Scan Converting Lines:DDA Algorithm, Incremental Form Therefore, rather than recomputing y each step, simply add m. The Algorithm: void Line(int x0, int y0, int x1, int y1) { int x; float dy, dx, y, m; dy = y1 - y0; dx = x1 - x0; m = dy/dx; y = y0; for (x = x0; x<=x1,x++){ WritePixel(x, round(y)); y += m; } }

Midpoint Algorithm:Allowable Pixel Selections Not Allowed Option NE 0 < Slope < 1 Option E Last Selection

Midpoint Algorithm:Iterating 0 < Slope < 1 Select E Select NE

Midpoint AlgorithmDecision Function: (implicit equation of the line)

Midpoint Algorithm:Calculating the Decision Function

Option NE Option E Midpoint Algorithm:Incremental Calculation of di

Midpoint Algorithm:The Code void MidpointLine(int x0, int y0, int xn, int yn) { int dx,dy,incrE,incrNE,d,x,y; dx=xn-x0; dy=yn-y0; d=2*dy-dx; /* initial value of d */ incrE=2*dy; /* decision funct incr for E */ incrNE=2*dy-2*dx; /* decision funct incr for NE */ x=x0; y=y0; DrawPixel(x,y) /* draw the first pixel */ while (x<xn){ if (d<=0){ /* choose E */ d+=incrE; x++; /* move E */ }else{ /* choose NE */ d+=incrNE; x++; y++; /* move NE */ } DrawPixel (x,y); } }

Midpoint AlgorithmAn example 12 11 10 9 8 7 4 5 6 7 8 9

Other Incremental Algorithms(Wu 1987)

Midpoint Circle Algorithm(Bresenham 1977)

Filling Polygons:Scan Line Algorithm 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14

Filling Polygons:Scan Line Algorithm 1. Find the intersections of current scan line with all edges of the polygon. 2. Sort the intersections by increasing x coordinate. 3. Fill in pixels that lie between pairs of intersections that lie interior to the polygon using the odd/even parity rule. Special Cases: 1. Intersections at integer coordinates 2. Shared vertices 3. Horizontal edges

Filling Polygons:Special Cases Only count ymin, not ymax. Leftmost integer pixel is interior; rightmost exterior. 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14

11 10 9 8 7 6 5 4 3 2 1 0 Scan Line Polygon Fill Algorithm:Exploiting Coherence l l 14 l D 12 l EF DE F 10 -5/2 6/4 l 9 7 7 12 CD l 8 l 12 13 0 E 6 C FA l 4 l 9 2 0 A 2 l AB BC B 0 6/4 l -5/2 7 3 7 5 0 2 4 6 8 10 12 14 l Scan Line

Scan Line Polygon Fill:The Algorithm • Set y to smallest y coordinate that has an entry in ET • Initialize the active edge table (AET) to empty • Repeat until the ET and AET are both empty: • Move from ET bucket at y to AET those edges whose ymin = y. (entering edges) • Remove from AET those edges for which y = ymax. • Sort AET on x • Fill in desired pixels on scan line using even-odd parity from AET • Increment y by one (next scan line) • For each edge in the AET, update x by adding Dx/Dy

Efficient Rasterization Techniques for Line and Polygon Filling