Calculation of normals

[Ramesh]

Hi
How can I calculate normal to a polygon?
Assume that I have a polygon of vertices v1,v2,v3,v4.

Can you give me some info.
thanks

[This message has been edited by Ramesh (edited 12-18-2000).]

[Spaceman]

For flat polygons you only need one normale because its everywhere the same.
Take 3 Points of you Surface, for example a,b,c and make two vectors:

/calculate the vector v1
v1x=bx-ax
v1y=by-ay
v1z=bz-az

/calculate the vector v2
v2x=cx-ax
v2y=cy-ay
v2z=cz-az

/do the cross product with v1 and v2
n1x=(v1y*v2z - v1z*v2y);
n1y=(v1z*v2x - v1x*v2z);
n1z=(v1x*v2y - v1y*v2x);

/divide the normal through its length
n1x/=sqrt(n1x*n1x+n1y*n1y+n1z*n1z);
n1y/=sqrt(n1x*n1x+n1y*n1y+n1z*n1z);
n1z/=sqrt(n1x*n1x+n1y*n1y+n1z*n1z);

/...and put the normal in the glNormal function
glNormal3f(n1x,n1y,n1z);


If the polygon isn't flat (with a minimum of 4 points) you need to calculate the normal of every edge with the vector before and after every vertex


[This message has been edited by Spaceman (edited 12-18-2000).]