http://www.packtpub.com/sites/default/files/9781849518048_Chapter_07.pdf

setNormals.cpp

//Universal function which sets normals for the triangle mesh
void setNormals( ofMesh &mesh ){
 //The number of the vertices
 int nV = mesh.getNumVertices();
 //The number of the triangles
 int nT = mesh.getNumIndices() / 3;
 vector<ofPoint> norm( nV ); //Array for the normals
 //Scan all the triangles. For each triangle add its
 //normal to norm's vectors of triangle's vertices
 for (int t=0; t<nT; t++) {
 //Get indices of the triangle t
 int i1 = mesh.getIndex( 3 * t );
 int i2 = mesh.getIndex( 3 * t + 1 );
 int i3 = mesh.getIndex( 3 * t + 2 );
 //Get vertices of the triangle
 const ofPoint &v1 = mesh.getVertex( i1 );
 const ofPoint &v2 = mesh.getVertex( i2 );
 const ofPoint &v3 = mesh.getVertex( i3 );
 //Compute the triangle's normal
 ofPoint dir = ( (v2 - v1).crossed( v3 - v1 ) ).normalized();
 //Accumulate it to norm array for i1, i2, i3
 norm[ i1 ] += dir;
 norm[ i2 ] += dir;
 norm[ i3 ] += dir;
 }
 //Normalize the normal's length
 for (int i=0; i<nV; i++) {
 norm[i].normalize();
 }
 //Set the normals to mesh
 mesh.clearNormals();
 mesh.addNormals( norm );
}