OpenGL: Help to draw sphere/circle using gl function

Question

In my project I have a requirement to draw a sphere (may be semi-sphere will be ok). For this purpose I used sin() and cos() methods to calculate the angle for (x,y) co-ordinates. But following this method I feel it is hitting performance of my project. Can anyone help me for drawing a sphere with simple gl functions.

Like below I defined a structure:

typedef struct
{
    float x;
    float y;
}Semi_Sphere;

Semi_Sphere Sky_SemiSphere[180000], Grnd_SemiSphere[180000];

In the below method I create an array to store the (x,y) co-ordinates. This method I call in my main() function.

void createSemi_Sphere_Table (void)
{
float angle_d = 1.1f, angle_r=0.0;
float const Gl_Pi = 3.14;
int i = 0;

while ( angle_d < 11.0 )
{
    angle_r = Gl_Pi/angle_d;
    Sky_SemiSphere[i].y = 1.0f + (((3.50)*sin(angle_r)));
    Sky_SemiSphere[i].x = ((3.7)*cos(angle_r)) - 0.52f;

    angle_d = angle_d + 0.001;
    i = i+1;
}

} 

Then, I use those (x,y) co-ordinates in the below method to draw my sphere. I call drawSemi_Sphere_Grnd() method in my drawScene() method.

void drawSemi_Sphere_Grnd (void)
{
int L_Index = 0;

glPushMatrix();
for (L_Index = 0; L_Index < 9750; L_Index++)
{
    glBegin(GL_LINES);
        glVertex2f(Grnd_SemiSphere[L_Index].x, Grnd_SemiSphere[L_Index].y);
        glVertex2f(-1.0f, -2.1f);
    glEnd();
}
glPopMatrix();
}

By the above procedure i get sphere. But the performance is slow.

Answer 1

There are many things you can do to speed this up, from using vertex arrays to buffer objects to display lists. But the simplest is this:

glBegin(GL_LINES);
for (L_Index = 0; L_Index < 9750; L_Index++)
{
    glVertex2f(Grnd_SemiSphere[L_Index].x, Grnd_SemiSphere[L_Index].y);
    glVertex2f(-1.0f, -2.1f);
}
glEnd();

Rather than issuing 9750 separate GL_LINES primitives, you draw one list of lines that is 9750 lines long.