How to draw an approximate circle in OpenGL?

[LiLo]

Hi,

I am doing an exercise from the book "Beginning OpenGL Game Programming" by Dave Astle and Kevin Hawkins.

The question asked to write a function to draw a 2D circle approximation. The origin starts at 0,0,0. The function has to accept the radius and the number of edges.

Below is how I wrote the function. But when I ran it, there's only a straight white line appearing across the window, instead of an approximate circle.

void DrawCircleApproximation(float radius, int numberOfSides, bool edgeOnly)
{
int i;
double angle;

if(edgeOnly)
glBegin(GL_LINE_STRIP);
else
glBegin(GL_POLYGON);

for(i = 0;i < numberOfSides;i++)
{
angle = (i * 2 * 3.142) / numberOfSides;

glVertex3f(cosf(angle)*radius,0.0, sinf(angle)*radius);
}

if(edgeOnly)
glVertex3f(radius,0.0,0.0);

glEnd();

}

What is wrong with the code above? I've checked with the book's answer and they are similar.

[fartman_900]

glVertex3f(cosf(angle)*radius,0.0, sinf(angle)*radius);

That line is drawing all the points into the screen (on the XZ plane). Try this:

glVertex3f(cosf(angle)*radius, sinf(angle)*radius, 0.0);

[LiLo]

Hi,

I used glVertex3f(cosf(angle)*radius,sinf(angle)*radius,0.0); but nothing appears on the openGL window now.

I also tried casting the integer variables to floats and the same thing happened.

I've also tried using glVertex2f instead of 3f but the problem remained the same. The windows displayed nothing.

Below is how I called the DrawCircleApproximationFunction.

int DrawGLScene(GLvoid) // Here's Where We Do All The Drawing
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity(); DrawCircleApproximation(5,5,false); //calling the function
return TRUE;
}

The first argument of the function is the radius. The second argument is the number of sides while the third argument determines whether to use GL_POLYGONS or GL_LINE_STRIP.

Could the problem lie with the calling function?

[Keith_VB6]

You said that you have the answer from the book. How does the book suggest that you do it ? What is the difference between your code and their code ?

[fartman_900]

I found the problem. I forgot to change the Z value of the glVertex3f function. So it was drawing it more towards the viewer instead of further away. I just tried it on my computer and it works good with z=-20.0f. So it should be

glVertex3f(cosf(angle)*radius, sinf(angle)*radius, -20.0f);
...
if(edgeOnly)
     glVertex3f(radius,0.0,-20.0f);

You need to have that last vertex with the same z value as the others, or it messes up. I tried it and it works fine.

[LiLo]

Hi,i've tried out the suggestion and it works. Thanks!