diff --git a/src/r_main.cpp b/src/r_main.cpp
index c7ea2f29d..02577f964 100644
--- a/src/r_main.cpp
+++ b/src/r_main.cpp
@@ -1095,6 +1095,51 @@ void R_Init(void)
 	framecount = 0;
 }
 
+//
+// R_IsPointInSector
+//
+boolean R_IsPointInSector(sector_t *sector, fixed_t x, fixed_t y)
+{
+	size_t i;
+	size_t passes = 0;
+
+	for (i = 0; i < sector->linecount; i++)
+	{
+		line_t *line = sector->lines[i];
+		vertex_t *v1, *v2;
+
+		if (line->frontsector == line->backsector)
+			continue;
+
+		v1 = line->v1;
+		v2 = line->v2;
+
+		// make sure v1 is below v2
+		if (v1->y > v2->y)
+		{
+			vertex_t *tmp = v1;
+			v1 = v2;
+			v2 = tmp;
+		}
+		else if (v1->y == v2->y)
+			// horizontal line, we can't match this
+			continue;
+
+		if (v1->y < y && y <= v2->y)
+		{
+			// if the y axis in inside the line, find the point where we intersect on the x axis...
+			fixed_t vx = v1->x + (INT64)(v2->x - v1->x) * (y - v1->y) / (v2->y - v1->y);
+
+			// ...and if that point is to the left of the point, count it as inside.
+			if (vx < x)
+				passes++;
+		}
+	}
+
+	// and odd number of passes means we're inside the polygon.
+	return passes % 2;
+}
+
 //
 // R_PointInSubsector
 //