Q: For the following polygon, prepare an initial sorted edge list and then make the active edge list for scan lines y = 5,20,30,35 Coordinates of the vertices are as shown in Figure.

Answer: First label the vertices and edges as shown in the Figure

Start with y = 0 and continue till y = 40. For y = 0 there are four edges, namely E₀, E₁, E₂ and E₃.  The sorted edge table is created as follows:

Active edge lists (AEL) for scan lines y = 5, 20, 30, 35 are as follows.

Y = 5, AEL = { E₀, E₁, E₂, E₃}

Y = 20, AEL = { E₆, E₃}

Y = 30, AEL = { E₆, E₅, E₄, E₃}

Y = 35, AEL = { E₆, E₅, E₄, E₃}