LÖVE

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 -- gives points on the line every gap; also returns tangents in this points
function get_points_along_line (line, gap)
	local points = {}
	local tangents = {}
	local rest = gap/2 -- rest is gap to start point on this section
	local x1, y1, x2, y2, dx, dy = line[1],line[2]
	for i=3, #line-1, 2 do
		x2, y2 = line[i],line[i+1]
		dx, dy = x2-x1, y2-y1
		local sector_length = (dx*dx+dy*dy)^0.5
		if sector_length > rest then
			-- rest is always shorter than gap; sector is shorter than rest (or gap)
			dx, dy = dx/sector_length, dy/sector_length
			while sector_length > rest do
				local x, y = x1+rest*dx, y1+rest*dy
				table.insert (points, x)
				table.insert (points, y)
				table.insert (tangents, dx)
				table.insert (tangents, dy)
				rest = rest + gap
			end
		else -- no point in this distance
		end
		-- the tail for the next 
		rest = rest-sector_length
		x1, y1 = x2, y2
	end
	return points, tangents
end

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-- Offsets the given polyline (line) by offset (offset)
function get_offset_polyline (line, offset, reversed)
	local offset_polyline = {}
	local loop = is_loop (line)
	if reversed then
		line = reverse_line (line)
	end
	if #line == 4 then
		local x1, y1 = line[1], line[2]
		local x2, y2 = line[3], line[4]
		local px1, py1, px2, py2 = get_parallel_segment (x1, y1, x2, y2, offset)
		offset_polyline = {px1, py1, px2, py2}
	elseif not loop then
		local x1, y1, x2, y2 = line[1], line[2], line[3], line[4]
		local px1, py1, px2, py2 = get_parallel_segment (x1, y1, x2, y2, offset)
		table.insert (offset_polyline, px1)
		table.insert (offset_polyline, py1)
		for i = 5, #line-1, 2 do
			local x3, y3 = line[i], line[i+1]
			local px3, py3, px4, py4 = get_parallel_segment (x2, y2, x3, y3, offset)
			local x, y = get_intersection (px1, py1, px2, py2, px3, py3, px4, py4, false, 4)
			table.insert (offset_polyline, x)
			table.insert (offset_polyline, y)
			x1, y1, x2, y2 = x2, y2, x3, y3
			px1, py1, px2, py2 = px3, py3, px4, py4
		end
		table.insert (offset_polyline, px2)
		table.insert (offset_polyline, py2)
	else -- loop
		local x1, y1, x2, y2 = line[#line-5], line[#line-4], line[#line-3], line[#line-2]
		local px1, py1, px2, py2 = get_parallel_segment (x1, y1, x2, y2, offset)
--		for i = 1, #line-1, 2 do
		for i = 1, #line-1, 2 do
			local x3, y3 = line[i], line[i+1]
			local px3, py3, px4, py4 = get_parallel_segment (x2, y2, x3, y3, offset)
			local x, y = get_intersection (px1, py1, px2, py2, px3, py3, px4, py4, false, 4)
			table.insert (offset_polyline, x)
			table.insert (offset_polyline, y)
			x1, y1, x2, y2 = x2, y2, x3, y3
			px1, py1, px2, py2 = px3, py3, px4, py4
		end
	end
	if #offset_polyline > 3 then
		return offset_polyline
	end
end

Code: Select all

-- new canvas
tmp = love.graphics.newCanvas()
	love.graphics.setCanvas(tmp)
	love.graphics.setColor({0.6,0.8,0.8}) -- rail color
	love.graphics.setLineWidth (rail_width+3) -- three pixels more
	love.graphics.line(line)
	love.graphics.setBlendMode("replace")
	love.graphics.setColor(0,0,0,0) -- black transparent color
	love.graphics.setLineWidth (rail_width-3) -- three pixels less
	love.graphics.line(line) -- each rail is 3 pixels wide now
-- disabling canvas
love.graphics.newCanvas()
love.graphics.setBlendMode("alpha")