# Help understanding formula (3 plane intersection)

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dawidjoubert    161
hi there, so the unique intersection point of a plane is given by [pic]http://softsurfer.com/Archive/algorithm_0104/Eqn_3planes.gif[\pic] So the way i understand it is that we have 3 planes plane 1 ,plane 2,plane 3 n1,n2,n3 are vectors and they store A,B,C in the correct place like this -- Plane-1 = Ax + By + Cz = -D n1.x = A n1.y = B n1.z = C d1 = D okay and then it can be all worked out except how do you multiply n2 by n3 now you could use the dot product but as the picture shows the dot product is shown by a . Here is the formula in text:
                   -d1(n2*n3) -d2(n3*n1) -d3(n1*n2)
Point (Vector) = ------------------------------------
n1.(n2*n3)
[\source]


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the little 'x' operator among the normal vectors denotes the cross product, which is one of the most fundamental operation you can perform on 3d vectors. Unlike the dot product which yields a real number, this one yields a vector which is normal to the plane of the operands.

It's formula is v1Xv2 = |v1|*|v2|*sin(phi)*n, where n is a vector normal to the plane of v1,v2 and points to the direction necessary, in order for {v1,v2,n} to form a right-handed system (if you are in a right-handed reference system) or vice-versa.

This formula isn't the most useful form. There is another one involving the actual components of the vectors.

Let v1=(x1,y1,z1), v2=(x2,y2,z2), then their cross product is
V = (v1)x(v2) = {y1*z2 -y2*z1, x2*z1 - x1*z2, x1*y2 -x2*y1}

Do you know what type of 3d coordinates system you're using? left handed/right handed? You'll probably need that sooner or later.

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Btw, some common properties of the cross product that you may need:

1) v1 x v2 = - v2 x v1

2) If c is a scalar number
(c*v1)x(v2) = (v1)x(c*v2) = c*(v1 x v2)

3) v1 x (v2 + v3) = (v1 x v2) + (v1 x v3)
therefore also: (v2 + v3) x v1 = (v2 x v1) + (v3 x v1)

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This uses a slightly different formulation of the intersection formula you posted a link to, but because you seem to be looking for code, here's one implementation:

// Compute the point p at which the three planes p1, p2, and p3 intersect (if at all)int IntersectPlanes(Plane p1, Plane p2, Plane p3, Point &p){    Vector u = Cross(p2.n, p3.n);    float denom = Dot(p1.n, u);    if (Abs(denom) < EPSILON) return 0; // Planes do not intersect in a point    p = (p1.d * u + Cross(p1.n, p3.d * p2.n - p2.d * p3.n)) / denom;    return 1;}

Cross() and Dot() are the cross product and dot product, respectively, of two vectors. If you are not familiar with them (which you really need to be if you are going to be working with planes and vectors) you can google for the terms (or better, get an introductory textbook on "Elementary Linear Algebra"; a used one you can get for just a few bucks, if that).