sorry for posting again today, but ive been racking my brain and scanning through this for the last few hours, and changing it but i cant fix it. it takes 4 points, the first two are the points of one line, and the second two are the points of the second line. the x and y points at which it thinks the intersection works at are always off, and x is always in a minus figure, which cant be true as its on a pc screen, where the lowest numbers in use are 00. the alkward if statement is just to check the intersection point is within the range of both line segments. i know there are many little stupid flaws, but what im looking for is actual algorithm errors. im hoping fresh eyes might make a diffenenc , thanks.

	private Vector IntersectionTest(Vector p1,Vector p2, Vector p3, Vector p4)//unsure of whats wrong with this function
		{
			double ad,bc;
			double c;
			double xIntersect=0, yIntersect=0;
			Vector intersectionPoint;
						
			ad = p1.Yval - p2.Yval;
			bc = p2.Xval - p1.Xval;

			c = ad* p1.Xval + bc*p1.Yval;

			//line2
			double ad2,bc2;
			double c2;
			
			ad2 = p3.Yval - p4.Yval;
			bc2 = p4.Xval - p3.Xval;

			c2 = ad* p3.Xval + bc*p3.Yval;
			//where two lines intersect
			double det = ad * bc2 - ad2 * bc;
			if( p2.Xval >500)
			{
				int i = 0;
			}

			if(det == 0)// i know this is inefficient, but it should work
			{
				//lines are paralell. nothing to do.
				intersectionPoint = new Vector(0,0);
							}
			else
			{
				xIntersect = ((bc2*c) - (bc*c2))/det;
				yIntersect = ((ad*c2) - (ad2*c))/det;
				
				
				#region //just makes sure intersection is on line segments, and not along the infinite line
				if (xIntersect >= p1.Xval && xIntersect <= p2.Xval && xIntersect >=p3.Xval && xIntersect <= p4.Xval)
				{
					if(yIntersect >= p1.Yval && yIntersect <= p2.Yval && yIntersect >=p3.Yval && yIntersect <= p4.Yval)
					{
						velocity.Xval = 0;
						velocity.Yval = 4;
					}
					
				}
					else{velocity = velocity;}
				#endregion
	
			}
			intersectionPoint = new Vector((float)xIntersect,(float)yIntersect);

			return intersectionPoint;
		}

im sorry for posting again and wasting your time. i had one more look at my code and found the error.

the line c2 = ad* p3.Xval + bc*p3.Yval;

should be ce = ad2 * p3.Xval + bc2*p3.Yval;

just staring at it so long i couldnt see the errors. thanks and again, sorry