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I'm developing an application in Unity3D where a user can define 4 pixel points on screen, and what I would like to do is fill this shape with pixels using Unity's SetPixel.

While I can do this easily for square or rectangular shapes, I was wondering if there is any efficient code or algorithm to do this for more irregular four-sided shapes, such as trapezoids etc.

Note : Filling the shape with a mesh is not an option.

So far the best option I can think of is to draw the outline, then draw a line from each pixel point of every outline, to every other outline point, but I'm not sure that's very efficient.

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You can use the even-odd algorithm. In summary it does the following.

  1. Loop thru each scan line
  2. Intersect the scan line with each polygon edge
  3. Sort the intersection points by x value
  4. Fill interior pixels between intersection points using the even-odd rule to determine interior points

Note, you have to guard against degenerate cases like intersecting with vertices and horizontal lines.

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