Opengl: earth texturing

Question

i'm trying to use a texture to draw the Earth over a sphere. My sphere is composed by by quads, each quad is composed by two triangles:

void Sphere(float radius, int n_lat, int n_lon)
{
    glEnable(GL_LIGHTING);
glEnable(GL_TEXTURE_2D); 

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); 
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

    TextureManager::Inst()->BindTexture(1);

    glBegin(GL_TRIANGLES);
for (float i = 0; i < n_lat; i += 1.f)
    for (float j = 0; j < n_lon; j += 1.f)
    {
        Vector3 p;
        GLfloat u;
        GLfloat v;
        // -- first triangle
        p = SphereCoord(i, j, n_lat, n_lon);
                glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);

        p = SphereCoord(i + 1, j, n_lat, n_lon);
        glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);


        p = SphereCoord(i + 1, j + 1, n_lat, n_lon);
        glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);



        // -- second triangle
        p = SphereCoord(i, j, n_lat, n_lon);
                glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);



        p = SphereCoord(i + 1, j + 1, n_lat, n_lon);
        glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);



        p = SphereCoord(i, j + 1, n_lat, n_lon);
        glNormal3(p);

        u=atan2f(p.y,p.x)/2*PI+0.5;
        v=0.5-asinf(p.z)/PI;
        glTexCoord2f(u,v);

        glVertex3(p * radius);
}
glEnd();
}

With this code don't obtain a correct texture mapping. Does anyone can give me some tips?

It looks like the texture is in someway repeated on the longitude axis:

Answer 1

The problem is in your the calculation of your u:

u=atan2f(p.y,p.x)/2*PI+0.5;

You are diving it by 2 then multiplying the result by PI. You should be diving by 2 and PI

u=atan2f(p.y,p.x)/(2*PI)+0.5;