main.cpp

// https://stackoverflow.com/questions/785097/how-do-i-implement-a-bézier-curve-in-c
vector_3 getBezierPoint(vector<vector_3> points, float t )
{
    int i = points.size() - 1;
    
    while (i > 0)
    {
        for (int k = 0; k < i; k++)
        {
            points[k].x = points[k].x + t * ( points[k+1].x - points[k].x );
            points[k].y = points[k].y + t * ( points[k+1].y - points[k].y );
            points[k].z = points[k].z + t * ( points[k+1].z - points[k].z );
        }
        
        i--;
    }
    
    return points[0];
}

// ...
    
    vector<vector_3> pos;
    
    vector_3 v0(-0.5, 0.5, 0);
    vector_3 v1( 0.5, 0.5, 0);
    vector_3 v2( 0.5, -0.5, 0);
    vector_3 v3( -1.0, -1.0, 0);
    
    pos.push_back(v0);
    pos.push_back(v1);
    pos.push_back(v2);
    pos.push_back(v3);

    vector<vector_3> v;
    vector<vector_3> deriv;
    
    
    for(float t = 0; t <= 1; t += 0.1)
    {
        vector_3 vline = getBezierPoint(pos, t);
        v.push_back(vline);
        
        if(t == 1)
            deriv.push_back(getBezierPoint(pos, t - 0.001));
        else
            deriv.push_back(getBezierPoint(pos, t + 0.001));
    }
    
    
    glLineWidth(2.0f);
    
    glBegin(GL_LINE_STRIP);

    for(size_t i = 0; i < v.size(); i++)
        glVertex3f(v[i].x, v[i].y, v[i].z);

    glEnd();

    glColor3f(1, 0.5, 0);
    
    glBegin(GL_LINES);
    
    for(size_t i = 0; i < deriv.size(); i++)
    {
        vector_3 line = deriv[i] - v[i];
        line.normalize();
        
        glVertex3f(v[i].x, v[i].y, v[i].z);
        glVertex3f(v[i].x + line.x, v[i].y + line.y, v[i].z + line.z);
    }
    
    glEnd();