Whats the best way to draw a shaded circle?

I want to draw a circle that starts out dark at the top and slower gets lighter towards the bottom. I am using gl_lines to draw out the circle and change each lines color.

    glVertex2f(0.0, 0.0);
    glVertex2f(0.05, 0.0);
    glVertex2f(-0.02, -0.01);
    glVertex2f(0.07, -0.01);
    glVertex2f(-0.04, -0.02);
    glVertex2f(0.09, -0.02);
    glVertex2f(-0.05, -0.03);
    glVertex2f(0.1, -0.03);
    glVertex2f(-0.06, -0.04);
    glVertex2f(0.11, -0.04);
    glVertex2f(-0.06, -0.05);
    glVertex2f(0.11, -0.05);

Thats some of what I got. But that’s just not practical. Is there a better way to get this done? Should I use a for loop or while loop?

Are we discussing a problem with the shading of the circle, or programming language features and coding style?

Assuming it’s the later: Both the points on the circle and your shading effect can be calculated. It’s definitely better generating the points in a loop.
On the one hand you can controll a lot of parameters (e.g. how many line segments should the circle have, etc…) without code changes and on the
other hand, a loop is definitely going to be smaller than a wall of glVertex* calls.

Should I use a for loop or while loop?

They are equivalent. It doesn’t matter.

I am trying to code the circle in a more effective way. i am new to opengl so i dont know if GL_LINES is the best method to get this done.

I also want to try to make the circle as smoothed as possible. Right now i get a circle but the edges are not smooth.

Perhaps, GL_LINE_LOOP might be an easier choice for programmatical generation of a circle. When using GL_LINE_LOOP, every point you specify by calling glVertex*
is connected to the last point specified and when you call glEnd( ), the last point is automatically connected to the first point you specified.

You could for instance write a simple for loop that loops over degress from 0 to <360 and compute a position on the circle. Here’s some pseudo-C for that simplistic approach:

int i, step=10;

glBegin( GL_LINE_LOOP );
for( i=0; i<360; i+=step )
    glVertex2f( cos( ((float)i)*DEGTORAD ), sin( ((float)i)*DEGTORAD ) );
glEnd( );

The smaller “step”, the more line segments and thus the smoother the circle.

Thanks a lot.

How do I now make it so that the circle is dark at the top and slower gets lighter downward, eventually becoming white at the bottom?

I was making each line individually and choosing the color for it.

If the color is bright at the bottom and dark at the top, it must obviously be a function of y.

Why not try something linear:

int i, step=10;
float x, y, c;

glBegin( GL_LINE_LOOP );
for( i=0; i<360; i+=step )
    x = cos( ((float)i)*DEGTORAD );
    y = sin( ((float)i)*DEGTORAD );
    c = y * (-0.5f) + 0.5f;
    glColor3f( c, c, c );
    glVertex2f( x, y );
glEnd( );

I understand pretty much everything in the pseudo code except for DEGTORED. Is that pi? Or does that convert degrees to radian?

I worked on it and this is what i have now.

float x, y, z;
    for (float i = 0.0; i < 360; i+=1) {
        x = cos(((float) i) * (3.1415/180));
        y = sin(((float) i) * (3.1415/180));
        z = y * (-0.5f) + (0.5f);
        glColor3f(z, z, z);
        glVertex2f(x, y);

It does exactly what i want it to do but location and size of the circle is not where i want them to be.

My window is currently 500 by 500 and the circle takes up the entire window.

What i want to know is how would i change the radius and how do i change the location of the circle? I would like to place more circles in the same window.

How familiar are you with the following things:
[li]OpenGL®[/li][li]The C programming language[/li][li]Trigonometry[/li][li]Linear algebra[/li][/ul]

There are many ways to resize/move the circle in the image. If it fills the entire window, you obviously have your transformation and projection matrices set to identity. You could multiply your x and y values with the desired radius and add an offset, or you could use glTranslatef/glRotatef to do the same thing (basics of legacy OpenGL®). When you resize your window to a non rectangular shape, you will probably notice that the circle is no longer a circle, since your projection matrix is identity.

Basic knowledge of linear algebra and the legacy OpenGL® API you are using can be very helpfull here. In this case, a little knowledge of trignometry got you the solution of how to programmatically generate circle points.

You initially asked if you should use a for-loop or a while-loop and replaced the loop counter with a float, that you compare against an integer and add an integer to and then cast in to a float. How familiar are you with the C programming language you are using? (Or programming languages in general, since pretty much every imparative programming language has for and while loops). When you want to do OpenGL® based 3D graphics, fundamental understanding of the programming language you use is essential.

I am not familiar with OpenGL. I am new to it and I think that is what I was confused with. I have programed in C++ but not in a while. I have a good understanding of for loops and while loops. I just didn’t think of using cos and sin to create a circle. That made it much more simple. I was trying to pick coordinates and drawing individual lines, which was making it confusing and more difficult. I was trying to figure out a simpler way and using sin and cos was it.