Thanks, guys. I appreciate the help. I figured I was overthinking,
that's why I asked. It's amazing sometimes how difficult I can make
things. :>
~Scott
Marten (dev) Svanfeldt wrote:
> Hi,
>
> You are overthinking. As the others have said the trick is to use the
> dot product with the normal.
>
> Now in terms of CS code it is even simpler:
>
> csPlane3 myPlane(point1,point2,point3);
>
> if (myPlane.Classify(anotherPoint) < 0)
> { //then }
> else
> { //otherwise }
>
> -Marten
>
> Quoting Scott Johnson <[EMAIL PROTECTED]>:
>
>
>> Hi Everyone!
>>
>> This may seem like a basic question, but in the interest of time, I am
>> going to ask it anyway rather than struggle with it.
>>
>> All of the sideness tests I have seen in 2D (i.e. the Area() function in
>> triangulate.cpp in libcrystalspace, which is essentially a copy of
>> AreaSide() in
>> http://orion.math.iastate.edu/burkardt/c_src/orourke/tri.c) uses a
>> matrix determinant to find whether a point is on one side or another of
>> a line, (essentially uses cross products). The problem I have is that
>> these are all assuming that the planes in which the vectors lie are
>> parallel to the XY plane. That is, the Z component can essentially be
>> disregarded. Of course, when I take cross products of 3D vectors, I
>> obtain another 3D vector. This happens in 2D of course, but there is a
>> matrix which can be used to obtain a determinant telling the side on
>> which a point lies, given a vector.
>>
>> Now, my question is this: Given two vectors (three points), we know they
>> are all coplanar, so is there a way to utilize a matrix determinant
>> trick similar to in 2D which will give us the side of the line the point
>> lies on (i.e. negative or positive - it doesn't really mean 'right' or
>> 'left', as these don't have meanings in arbitrary 3-dimesions).
>>
>> My initial intuition was to determine the angle the vector makes with
>> the XY plane, in X and Y dimensions, then rotate all points by this, to
>> ensure that we are parallel to the XY plane, then utilize the same
>> method. Would this be too much work? I am wondering if I am
>> overthinking the problem.
>>
>> Any suggestions would be helpful. Thanks!
>>
>> ~Scott
>>
>>
>>
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>
>
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