-- public-domain code by Darel Rex Finley, 2007
-- from http://alienryderflex.com/polygon_fill/
-- converted to Lua by Stefan Reich

w, h = 400, 400

polyX = {10, 100, 50}
polyY = {20, 20, 60}
polyCorners = #polyY

-- int  nodes, nodeX[MAX_POLY_CORNERS], pixelX, pixelY, i, j, swap ;
nodeX = {}
pixels = {}
for i=1, w*h do pixels[i] = -1 end

-- Loop through the rows of the image.
for pixelY = 0, h-1 do
  -- Build a list of nodes.
  nodes = 0
  j = polyCorners-1
  for i=0, polyCorners-1 do
    if polyY[i+1] < pixelY and polyY[j+1] >= pixelY
      or polyY[j+1] < pixelY and polyY[i+1] >= pixelY then
      nodes = nodes+1
      nodeX[nodes] = math.ceil(0.5+(polyX[i+1]+(pixelY-polyY[i+1])/(polyY[j+1]-polyY[i+1])
      *(polyX[j+1]-polyX[i+1])))
    end
    j = i
  end

  -- Sort the nodes, via a simple “Bubble” sort.
  i = 1
  while i < nodes do
    if nodeX[i] > nodeX[i+1] then
      nodeX[i], nodeX[i+1] = nodeX[i+1], nodeX[i]
      if i > 1 then i = i-1 end
    else
      i = i+1
    end
  end

  -- Fill the pixels between node pairs.
  for i=1, nodes-1, 2 do
    if nodeX[i  ] >= w then goto raus end
    if nodeX[i+1] > 0 then
      if nodeX[i  ] < 0 then nodeX[i  ] = 0 end
      if nodeX[i+1] > w then nodeX[i+1] = w end
      for pixelX = nodeX[i], nodeX[i+1]-1 do
        pixels[pixelX+pixelY*w+1] = 0
      end
    end
  end
  ::raus::
end