Hi! you could get the normal vector of AB and AC for the 3 coplanar points then given that the halfway vector from A to between BC is 0 degrees, see which direction the point you're testing lies on.
If > pi then its on left, otherwise on right. Don't know if i make any assumptions here though, and I think it should work in most cases (best to test though). On Sunday 08 July 2007 22:12, Scott Johnson wrote: > 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 > > > > ------------------------------------------------------------------------- > This SF.net email is sponsored by DB2 Express > Download DB2 Express C - the FREE version of DB2 express and take > control of your XML. No limits. Just data. Click to get it now. > http://sourceforge.net/powerbar/db2/ > _______________________________________________ > Crystal-main mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/crystal-main > Unsubscribe: > mailto:[EMAIL PROTECTED] ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Crystal-main mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/crystal-main Unsubscribe: mailto:[EMAIL PROTECTED]
