From da6e772be72d1bba23f96a0813455acf6ea1596a Mon Sep 17 00:00:00 2001
From: Matthias Richter <matthias@dsboard.de>
Date: Mon, 7 Feb 2011 00:08:02 +0100
Subject: [PATCH] add ray intersection

---
 polygon.lua | 33 +++++++++++++++++++++++++++++++++
 shapes.lua  | 19 +++++++++++++++++--
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/polygon.lua b/polygon.lua
index 39bec54..9c4c447 100644
--- a/polygon.lua
+++ b/polygon.lua
@@ -352,6 +352,39 @@ function Polygon:contains(x,y)
 	return in_polygon
 end
 
+function Polygon:intersectsRay(x,y, dx,dy)
+	local p = vector(x,y)
+	local v = vector(dx,dy):normalize_inplace()
+	local n = v:perpendicular()
+
+	local vertices = self.vertices
+	for i = 1, #vertices do
+		local q1, q2 = vertices[i], vertices[ (i % #vertices) + 1 ]
+		local w = q2 - q1
+		local det = v:cross(w)
+
+		if det ~= 0 then
+			-- there is an intersection point. check if it lies on both
+			-- the ray and the segment.
+			local r = q2 - p
+			local l = r:cross(w)
+			local m = v:cross(r)
+			if l >= 0 and m >= 0 and m <= det then return true end
+		else
+			-- lines parralel or incident. get distance of line to
+			-- anchor point. if they are incident, check if an endpoint
+			-- lies on the ray
+			local dist = (q1 - p) * n
+			if dist == 0 then
+				if n:cross(q1) > 0 or n:cross(q2) > 0 then
+					return true
+				end
+			end
+		end
+	end
+	return false
+end
+
 
 -- module() as shortcut to module.Polygon()
 do
diff --git a/shapes.lua b/shapes.lua
index 66d320a..312c449 100644
--- a/shapes.lua
+++ b/shapes.lua
@@ -216,6 +216,23 @@ function CircleShape:contains(x,y)
 	return (x - self._center):len2() < self._radius * self._radius
 end
 
+
+function ConcavePolygonShape:intersectsRay(x,y, dx,dy)
+	return self._polygon:intersectsRay(x,y, dx,dy)
+end
+
+function ConvexPolygonShape:intersectsRay(x,y, dx,dy)
+	return self._polygon:intersectsRay(x,y, dx,dy)
+end
+
+-- circle intersection if distance of ray/center is smaller
+-- than radius
+function CircleShape:intersectsRay(x,y, dx,dy)
+	local n = vector(-dy,dx):normalize_inplace() -- normal vector
+	local d = vector(x,y) - self._center
+	return d * vector(dx,dy) >= 0 and (d * n) < self._radius
+end
+
 --
 -- auxiliary
 --
@@ -311,5 +328,3 @@ function CircleShape:draw(mode, segments)
 	love.graphics.circle(mode, self._center.x, self._center.y, self._radius, segments)
 end
 
-
-