Polygon Position - Plugin Cafe Forums

I feel like I have done the rotation matrix correctly here. But my object is not rotating to the normal of the polygon. Does anyone see anything that I am doing wrong creating the rotation matrix?

I would GREATLY, appreciate any help anyone could offer. This one is irritating me because everything I have read about rotation matrices says that I am doing it right. I am using a vector that represents the center of the polygon (polygonLocation) I am using another point in the polygon to determine the plane upon which the polygon rests. I am then using the cross product of those vectors to determine the up vector that is perpendicular to the plane. So, if my understanding of rotation matrices is correct, this should create a rotation matrix that can then be adapted by an object and that object should rotate to the normal of the polygon.

However, that is not happening with what I have pasted above. I am at a loss. Does anyone see anything that I am doing wrong?

Thanks so much for anyone who has the time to help me out.

~Shawn

Thanks Matthias,

I must not fully understand how to get the normal then. I thought that the normal was A - B. How to I get the value for N then?

THanks,

~Shawn

For triangles the normal is the cross product of two edges of the triangle. For quadrangles it's not exactly defined. The easiest way which gives a good result is too use the cross product of the diagonales.

PS. please read up on 3D geometry basics, it will help you in the long run. Most of it can be found through Google and Wikipedia.

cheers,
Matthias

MAXON
developer support

yes and yes :)

source code of CalcFaceNormal()


inline Vector CalcFaceNormal(const Vector *padr, const CPolygon &v)
{
    if (v.c==v.d)
        return !((padr[v.b]-padr[v.a])%(padr[v.c]-padr[v.a]));
    else
        return !((padr[v.b]-padr[v.d])%(padr[v.c]-padr[v.a]));
}

cheers,
Matthias

MAXON
developer support

What are you trying to do?

~Shawn

hmmmm... what are you using for the "padr" and for "v"... ?

in

CalcFaceNormal(padr, v)

are you creating a rotation matrix based on a specific polygon. ? When I get home I'll show you how I am doing it. I'm driving home right now.

Edited by emberintherain - 2010 Mar 15 at 1:14pm

I'm pretty sure that if you are trying to rotate objects based on a polygon then you will need to create a rotation matrix for that polygon and then make the matrix of the object you want to rotate equal to the rotation matrix. That's how I understand it.

Here's how I am doing it.

[CODE]

//ROTATION MATRIX

//GetPoints

Vector c = polyLocation; //midpoint

Vector p = points[lngA]; //polygon point

Vector n = CalcFaceNormal(points, selectedPoly[lngI]); //normal

Vector scale = Vector(Len(opMatrix.v1), Len(opMatrix.v2), Len(opMatrix.v3)); //Get Scale

//Contruct Matrix

Matrix rotMatrix;

rotMatrix.off = p; //The base of the matrix

rotMatrix.v1 = !((p - c)%n); //X axis points toward the second point

rotMatrix.v2 = !(rotMatrix.v1 % n);//Y Axis is perpendicular to the X axis

rotMatrix.v3 = !(n); //Z Axis is along the normal

rotMatrix.v1 = !(rotMatrix.v1 * scale.x);

rotMatrix.v2 = !(rotMatrix.v2 * scale.y);

rotMatrix.v3 = !(rotMatrix.v3 * scale.z);

//place the first object within the rotation matrix

firstSelection->SetMg(rotMatrix);

firstSelection->SetPos((opMatrix * polyLocation));// + firstSelection->GetRad().x);

[/code]

This is working quite well now.. creating a rotation matrix gives me the exact rotation of that specific polygon.

Hope this helps you.

~Shawn

Edited by emberintherain - 2010 Mar 15 at 2:41pm

Great. I'm glad it worked for you. I have got it now so everything is working 100% for me too.... Good day for us both! :)

~Shawn