# determinants + solving linear equations

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im finding the intersection point of two lines using determinants. the algorithm seems to work ok most of the time, but not always. im wondering if there has to be any exceptions made when dealing with horizontal and/or vertical lines? ( the original way i was trying to do this was with y = mx + c, but i learned here that way is very alkward and i was told to avoid it) [Edited by - fguihen on February 11, 2005 2:42:01 PM]

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No, no exceptions for horizontal or vertical lines. However, parallel and/or colinear lines may need to be special-cased.

Anyway, what sort of problems are you having? You could also post your line intersection code - it should be pretty easy to figure out what's going wrong.

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your prob right. it seems to work for sloped lines, but in my app, i have a bot and his "sight" is a line out in front of him. when this intersects any lines, a collision has occoured, but there are many lines that are paralell with his sight line

___________________________    OO____________  |    OO              |                    |                    |___________________________

as you can see here, my bot ( the four 0,s) is going through a corridor. his sight line is in front of him and will detect when he will colide with the vertical wall in front of him, but tests against the paralell walls fail.

im not sure enough of determinants to know how to solve this.

the function is here:
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(det == 0)			{				//lines are paralell. nothing to do.				intersectionPoint = new Vector(0,0);				velocity =velocity;			}			else			{				xIntersect = ((bc2*c) - (bc*c2))/det;				yIntersect = ((ad*c2) - (ad2*c))/det;				velocity.Xval = 0;				velocity.Yval = 4;			}			intersectionPoint = new Vector((float)xIntersect,(float)yIntersect);			return intersectionPoint;

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That isn't c++ is it? (C#?) In any case there are a couple of things I'm not sure about. What is velocity = velocity supposed to do? Also, why do you set velocity to 0, 4 in the other case?

It looks like your function is responding to the parallel case by returning a zero vector. I'm not sure what your intention is with this, but in any case it's not a very good way of informing the caller of the parallel case. (For one thing, what if the intersection point is actually 0, 0? Highly unlikely, but not impossible.)

Even if you just return 'false' for the parallel case, you'll be ignoring colinear but intersecting lines. For your purposes though this is probably ok.

Also, it doesn't look like you're checking to make sure that the intersection point is actually contained in both line segments. Although again perhaps in your particular application that's ok.

I would start by having this function return a boolean result telling whether or not the segments intersect, and have the intersection point be a pointer or reference argument.

Lastly, there are (IMO) better methods for segment intersection than the one you're using. I'd try to get your current method working first, though.

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i have added a few if statements to check if the point is on both lines. in the suituation im using this, there is absolutely zero chance of intersection being on 0,0. there is a chance though of colinear. il bear in mind what you said. thanks

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