love2d polygon substraction

main.lua

local G = love.graphics

function splitPoly(poly, mx, my, dx, dy)
	local poly_a = {}
	local poly_b = {}

	local ax = poly[#poly-1]
	local ay = poly[#poly  ]
	for i = 1, #poly, 2 do
		local bx = poly[i  ]
		local by = poly[i+1]
		local abx = bx - ax
		local aby = by - ay
		local max = ax - mx
		local may = ay - my

		local f = max * dy - may * dx
		if f >= 0 then
			poly_a[#poly_a+1] = ax
			poly_a[#poly_a+1] = ay
		end
		if f <= 0 then
			poly_b[#poly_b+1] = ax
			poly_b[#poly_b+1] = ay
		end

		-- intersect
		local det = dx*aby - dy*abx
		if math.abs(det) > 0.0001 then -- not parallel
			local d = (max*dy - may*dx) / det
			if d and d > 0 and d < 1 then
				local sx = ax + (bx - ax) * d
				local sy = ay + (by - ay) * d
				poly_a[#poly_a+1] = sx
				poly_a[#poly_a+1] = sy
				poly_b[#poly_b+1] = sx
				poly_b[#poly_b+1] = sy
			end
		end
		ax = bx
		ay = by
	end
	if #poly_a < 6 then poly_a = nil end
	if #poly_b < 6 then poly_b = nil end
	return poly_a, poly_b
end


-- input: two convex polygons
-- output: list of convex polygons making up the difference
function subtractPoly(minuend, subtrahend)
	local result = {}
	local bar = minuend
	local foo
	local ax = subtrahend[#subtrahend-1]
	local ay = subtrahend[#subtrahend  ]
	for i = 1, #subtrahend, 2 do
		local bx = subtrahend[i  ]
		local by = subtrahend[i+1]
		foo, bar = splitPoly(bar, ax, ay, bx - ax, by - ay)
		if foo then result[#result+1] = foo end
		if not bar then return { minuend } end
		ax = bx
		ay = by
	end
	return result
end



poly = {}
brush = {}
for i = 1, 16 do
	poly[i*2-1] = 200 + math.floor(math.cos(i / 8 * math.pi) * 100)
	poly[i*2  ] = 200 + math.floor(math.sin(i / 8 * math.pi) * 100)

	brush[i*2-1] = math.floor(math.cos(i / 8 * math.pi) * 80)
	brush[i*2  ] = math.floor(math.sin(i / 8 * math.pi) * 80)
end



function love.draw()
	local mx, my = love.mouse.getPosition()

	local subtr = {}
	for i = 1, #brush, 2 do
		subtr[i  ] = brush[i  ] + mx
		subtr[i+1] = brush[i+1] + my
	end

	G.setLineWidth(3)
	G.setColor(255, 255, 255, 30)
	G.polygon("line", subtr)

	local ps = subtractPoly(poly, subtr)
	for _, p in ipairs(ps) do

		G.setColor(255, 255, 255, 30)
		G.polygon("fill", p)
		G.setColor(255, 255, 255)
		G.setLineWidth(1)
		G.polygon("line", p)
	end
end