Polygon Scan Conversion

• Once we have mapped the vertices of polygon to device coordinates, we want to scan convert it.
• Scan converting a general polygon is complicated.
• Here we will look at scan conversion of a triangle.
• Look at y value of vertices. Split triangle along horizontal line at middle y value.

  \

• Step along L1 and L2 together along the scan lines from A to C and from B to C respectively.
• Scan convert each horizontal line.

Code for triangle scan conversion

 

• Assume that triangle has been split and that A, B, C are in device coordinates, that A.x<B.x, and that .

  y = A.y;
  d0 = (C.x-A.x)/(C.y-A.y);
  d1 = (C.x-B.x)/(C.y-B.y);
  x0 = A.x;
  x1 = B.x;
  while ( y <= C.y ) do
     for ( x = x0 to x1 ) do
         WritePixel(x,y);
     end
     x0 += d0;
     x1 += d1;
     y++;
  end

• Note: This is a floating point algorithm

Readings: Hearn and Baker, Chapter 3-11; Red book, 3.5 (more general than above); White book, 3.6 (more general than above).