Having a bit of trouble with point in polygon collision detection using the ray method
i.e. http://en.wikipedia.org/wiki/Point_in_polygon
My problem is I need to give an end to the infinite line created.
As with this infinite line I always get an even number of intersections and hence an invalid result.
i.e. ignore or intersection to the right of the point being checked
what I have

what I want

My current code based of Mecki awesome response

for (int side = 0; side < vertices.Length; side++)
        {
            // Test if current side intersects with ray.

            // create infinite line 
            // See: http://en.wikipedia.org/wiki/Linear_equation
            a = end_point.Y - start_point.Y;
            b = start_point.X - end_point.X;
            c = end_point.X * start_point.Y - start_point.X * end_point.Y;

        //insert points of vector
            d2 = a * vertices[side].Position.X + b * vertices[side].Position.Y + c;
            if (side - 1 < 0)
                d1 = a * vertices[vertices.Length - 1].Position.X + b * vertices[vertices.Length - 1].Position.Y + c;
            else
                 d1 = a * vertices[side-1].Position.X + b * vertices[side-1].Position.Y + c;

            // If points have opposite sides, intersections++;
            if (d1 > 0 && d2 < 0 )
                intersections++;
            if (d1 < 0 && d2 > 0 )
                intersections++;

        }
//if intersections odd inside = true
    if ((intersections % 2) == 1)
        inside = true;
    else
        inside = false;