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Basic Raster Graphics Algorithms for Drawing 2D Primitives Prof. Lizhuang Ma Shanghai Jiao Tong University

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Contents Architecture of a Raster Display Scan Converting Lines Filling Rectangles Filling Polygons Clipping Lines Clipping Polygons Antialiasing

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Architecture of a Raster Display Video

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Definitions Pixel: a screen consists of N x M pixels Bilevel = monochrome, 1 bit / pixel Color: RGB/CYK/LUV… Bitmap / pixmap Frame buffer: an array of data in memory mapped to screen

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Scan Converting Line A scan-converted line showing intensified pixels as black circles

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The Basic Incremental Algorithm A scan-converted line showing intensified pixels as black circles

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The Basic Incremental Algorithm void Line (intx0, inty0, intx1, inty1, value) { intx; floatdy, dx, y, m; dy=y1-y0; dx=x1-x0; m=dy/dx; y=y0; for(x=x0; x<=x1; x++) { WritePixel(x, (int)floor(y+0.5), value); y+=m; }

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Midpoint Line Algorithm

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void MidpointLine(intx0, inty0, intx1, inty1, value) { int dx, dy, incrE, incrNE, d, x, y; dy=y1-y0;dx=x1-x0;d=dy*2-dx; incrE=dy*2;incrNE=(dy-dx)*2; x=x0;y=y0; WritePixel(x, y, value); while(x

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Filling Rectangles for(y from y min to y max of the rectangle) { for(x from x min to x max ) { WritePixel(x, y, value); }

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Filling Polygons

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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 that lie interior to the polygon

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Special Cases Horizontal Edges Slivers

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Clipping Lines

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The Cohen-Sutherland Algorithm

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Clipping Polygons

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The Sutherland-Hodgman Polygon-Clipping Algorithm

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Antialiasing

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