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# Defining an affine axis from a point or points

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2 replies to this topic

### #1erm1  Members

Posted 19 February 2013 - 01:33 AM

I have become kind of stuck on this problem and was wondering if you might be able to help but it is probably best if I  include an image first to illustrate what  have been doing  :

What I need to figure out is how can I calculate an arbitrary axis that is aligned to the  selected quad.

as the equation wX cross product n to give ax and ax cross product wY is not going to align my 'sub planes' with the selected quad.

So, do I have to create a D3DXPlane by passing some of the points of the quad and if so how would I then create an axis that would align to the orientation of the selected quad?

Regards,

### #2TheChubu  Members

Posted 19 February 2013 - 02:02 AM

I don't quite understand what you're asking for. But you're trying to find the plane in which the quad is in right? For a plane you need two vectors or three points if you don't have defined vectors (a triangle).

Say that you have p1 (0,2,0) p2 (1,0,0) p3 (0,1,0) that you know they're in your quad and that at least two of them aren't multiples of each other. You'd make two vectors from them by selecting one point as the begining of the two vectors, and the other points as the end of those vectors.

Say p1 is the beggining. p2-p1 = v1 = (1,-2,0). p3-p1 = v2 = (0,-1,0). v1 and v2 represent your plane. Make the cross product between them and you have your orthogonal vector that represents your axis.

You can normalize it and displace it by the same amount that the quad is displaced if you need it centered on your quad.

There is more to it (like finding the equation of the plane for example) but I don't remember much more of the algebra classes i took

Edited by TheChubu, 19 February 2013 - 02:04 AM.

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### #3erm1  Members

Posted 20 February 2013 - 03:05 AM

Thanks Chubu,

I have to say that is the best reply I have ever received on any forum and thank you so much.

I may pop back and ask another question sometime, thanks again !.

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