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Filling a Polygon using Scanline polygon filling algorithm

Basic algorithm

 – Assume scan line start from the left and is outside the polygon.

 – When intersect an edge of polygon, start to color each pixel (because now we’re inside the

 polygon), when intersect another edge, stop coloring …

 – dd number of edges! inside

 – "#en number of edges! outside

What happens when a scan line intersects an edge endpoint, it intersects two edges.

$ %wo cases!

 – &ase A! edges are monotonically increasing or decreasing

 – &ase '! edges re#erse direction at endpoint

$ n &ase A, we should consider this as only " edge intersection

$ n &ase ', we should consider this as %W edge intersections

$ ncremental scan line method!

m * (y+-/ – y+/) 0 (x+-/ – x+/)

y+-/ – y+/ *

x[k+1] = x[k] + 1/m x[k]

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Active Edge List

• 1tart with the sorted edge table.

 – n the abo#e diagram, there is an entry for each scan2line.

 – Add only the non2hori3ontal edges into the table.

$ 4or each edge, we add it to the scan2line that it begins with (that is, the scan2line e5ual to its

lowest y2#alue).

$ 4or each edge entry, store

() %he x2intercept with the scan2line

  (6) %he largest y#alue of the edge, and

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  (7) %he in#erse of the slope.

 – "ach scan2line entry thus contains a sorted list of edges. %he edges are sorted left to right. (%o

maintain sorted order, 8ust use insertion based on their x #alue.)

$ ext, we process the scan lines from bottom to top.

 – We maintain an acti#e edge list for the current scan2line.

 – %he acti#e edge list contains all the edges crossed by that scan line. As we mo#e up,

update the acti#e edge list by the sorted edge table if necessary.