opengl and c++ drawing shape using circle algorthim - c++

Question

how can drawing small circle in side big circle using algorthim of circle
each circle consist of 9 or 8 point and link each 4point to gather and link this with 4 under and so on
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Answer 1

You need to generate the inner and outer points in one loop.
Give this a shot:
#include <GL/glut.h>
#include <cmath>
void Torus2d( float inner, float outer, unsigned int pts )
{
glBegin( GL_QUAD_STRIP );
for( unsigned int i = 0; i <= pts; ++i )
{
float angle = ( i / (float)pts ) * 3.14159f * 2.0f;
glVertex2f( inner * cos( angle ), inner * sin( angle ) );
glVertex2f( outer * cos( angle ), outer * sin( angle ) );
}
glEnd();
}
void display()
{
glClear( GL_COLOR_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
double w = glutGet( GLUT_WINDOW_WIDTH );
double h = glutGet( GLUT_WINDOW_HEIGHT );
double ar = w / h;
glOrtho( -4 * ar, 4 * ar, -4, 4, -1, 1);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glColor3ub( 255, 0, 0 );
Torus2d( 2, 3, 20 );
glutSwapBuffers();
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutInitWindowSize( 640, 480 );
glutCreateWindow( "GLUT" );
glutDisplayFunc( display );
glutMainLoop();
return 0;
}

Answer 2

Find below a faster (less trigono) solution in Go (can be easily translated to c++) which was inspired by Sieglord's Abode article
func Torus(cx, cy, inner, outer float64, num_segments int) {
theta := 2 * math.Pi / float64(num_segments)
c := math.Cos(theta) //precalculate the sine and cosine
s := math.Sin(theta)
t_in := 0.0
t_out := 0.0
x_in := inner //we start at angle = 0
x_out := outer //we start at angle = 0
y_in := 0.0
y_out := 0.0
gl.Begin(gl.QUAD_STRIP)
for ii := 0; ii <= num_segments; ii++ {
gl.Vertex2f(float32(x_in+cx), float32(y_in+cy)) //output vertex inner
gl.Vertex2f(float32(x_out+cx), float32(y_out+cy)) //output vertex outer
//apply the rotation matrix
t_in = x_in
t_out = x_out
x_in = c*x_in - s*y_in
x_out = c*x_out - s*y_out
y_in = s*t_in + c*y_in
y_out = s*t_out + c*y_out
}
gl.End()
}