Rotate points openGL - c++

Question

I've trying to generate figures by revolution reading the profile of figure from a ply. I follow the steps from here and other similar questioons, but my problem persist. When I try to rotate a Ply with only 2 points 36 stesps I get this if I puth the camara on the top the cilinder:
My code after a revision of the method rotate is:
void Figura::rotateY(int ngiros){
//Variables de rotacion.
//double alfa = 2*M_PI/ngiros;
int long_perfil = vertices.size();
vector<_vertex3f> new_vertices;
cout << long_perfil << " vertices" << endl;
_vertex3f aux1, aux2;
for(int i=0; i < ngiros; i++){
double alfa = (2*M_PI/ngiros)*i;
for(int j=0; j < long_perfil; j++){
aux1 = vertices.at(j);
aux1._0 = (cos(alfa) * aux1._0) + (sin(alfa) * aux1._2);
aux1._2 = (cos(alfa) * aux1._2) - (sin(alfa) * aux1._0);
vertices.push_back(aux1);
}
}
//vertices.clear();
//vertices = new_vertices;
//caras
for(int i=0; i < vertices.size(); i++){
_vertex3i aux(i, i+1, i+long_perfil);
_vertex3i aux2(i, i+long_perfil+1, i+1);
caras.push_back(aux);
caras.push_back(aux2);
}
}
}
I can't find my error. Some help will be welcome.

Answer 1

It looks like you're unclear over what coordinate system your original curve is in, and how you're applying a rotation to it. With your current code, you're just rotating the points by a variable amount, but keep them all within the same plane. You can tell from just looking at the code superficially: You never set a value for the y-coordinate of any of the points, so the whole result is not a 3D shape, but entirely in the y=0 plane. As flat as a pancake...
Another thing you need to be careful about is that you don't modify values while you're still using the old value:
aux1._0 = (cos(alfa) * aux1._0) + (sin(alfa) * aux1._2);
aux1._2 = (cos(alfa) * aux1._2) - (sin(alfa) * aux1._0);
Here, you're modifying the value of aux1._0 in the first statement, while the second statement should in fact still use the old value.
Let's say your original curve is in the x/y-plane, and you want to rotate around the y-axis. For this to result in a nice 3D shape, all the x-coordinates of the curve should be positive:
^ y
|
|--
| \
| \_
| | x
--------------->
| |
| /
| /
| _/
|/
Picture the z-axis pointing out of the screen.
Now, to rotate this curve by a given angle alpha around the y-axis, we leave the y-coordinate unchanged, and rotate the point (x, 0) by alpha within the xz-plane to get the new values for x and z. The new coordinates (x', y', z') for input point (x, y) of the shape are then:
x' = x * cos(alpha)
y' = y
z' = x * sin(alpha)
As a modified version of your code:
for(int i=0; i < ngiros; i++){
double alfa = (2*M_PI/ngiros)*i;
for(int j=0; j < long_perfil; j++){
aux1 = vertices.at(j);
aux2._0 = cos(alfa) * aux1._0;
aux2._1 = aux1._1;
aux2._2 = sin(alfa) * aux1._0;
vertices.push_back(aux2);
}
}