Cross Product

[Samuel]

Ive having trouble getting a collision detection algorithm to work in 3D. When I walk around in the 3D world, it detects collisions all sorts of weird places. I think the problem is comming from the cross product function because everything but this is copied from previous projects. The code is supposed to return a normal for a plane.

Code: Select all

class vec
{
    public:
        float x, y, z;
};

vec crossproduct(float Ax, float Ay, float Az, float Bx, float By, float Bz)
{
    vec vector;
    vector.x = (Ay*Bz)-(By*Az);
    vector.y = -(Ax*Bz)+(Bx*Az);
    vector.z = (Ax*By)-(Ay*Bx);
    return vector;
}


[/code]

[schloob]

cross product:

Code: Select all

| i  j  k  |
| x1 y1 z1 |
| x2 y2 z2 |

figure out if it's right

*edit* blah typos

[Jack Brown]

What is that schloob? That is not correct. Looks like you're taking the cross product of two vectors in R^2 (which yet is the wrong way since the cross product of two 2D vectors is det(u, v)). The correct version should be

|e1 e2 e3|
|x1 y1 z1 |
|x2 y2 z2 |

Edit: Your version would also be correct if we took (x1, y1, x1)x(x2, y2, x2)
But how often does the first element equal the last one?

[schloob]

oh whoops, that second x was supposed to be a z, the characters look so similar i didn't notice it. whoopsies!

[Samuel]

i kind of suck a lot at maths. Is what I posted right or wrong? if its wrong, whats wrong with it?

[Apocalypse]

Cross Product:

vec vector;
vector.x = Ay*Bz - By*Az;
vector.y = Bx*Az - Ax*Bz;
vector.z = Ax*By - Ay*Bx;


... so I think it's pretty correct