how to rotate this openGl code - opengl

Question

in this code i'm try to draw simple olympic ring and rotate it... the below work fine but i can't rotate the rings.. help me to solve this problme...
void myReshape (int width, int height)
{
glViewport (0, 0, width, height);
glMatrixMode (GL_PROJECTION);
glLoadIdentity();
gluOrtho2D (-5, 105, -5, 105);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity ();
glTranslatef (0.375, 0.375, 0.0);
}
int main (int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(100,100);
glutInitWindowSize(110*PIXEL_SIZE, 110*PIXEL_SIZE);
glutCreateWindow ("Olymipc Rings || rotation ");
glClearColor(1.0, 1.0, 1.0, 0.0);
glPointSize(PIXEL_SIZE);
glShadeModel (GL_FLAT);
glutDisplayFunc(display);
glutReshapeFunc(myReshape);
glutMainLoop();
return 0;
}

Answer 1

Use glRotatef(axis_x,axis_y,axis,z, angle) function before you draw the rings .
If you want to keep rotating the ring always use glutIdle(myidle) in your main() function and increment the value of angle there and also use glutPostRedisplay().
Use glPushMatrix() before and glPopMatrix() and after your ring drawings if you do not want the rotation to effect other drawings.
for example if you want to rotate your rings about x axis your code will look like
float angle=0;
void display (void) {
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_LINE_LOOP);
glVertex2i(-1,-1);
glVertex2i(100,-1);
glVertex2i(100,100);
glVertex2i(-1,100);
glEnd();
glPushMatrix(); //enters temporarily in a stack
for(int i = 0 ; i <5; i++)
{
glRotatef(1,0,0, angle)
glColor3f(color[i][0],color[i][1],color[i][2]);
draw_circle(center[i][0],center[i][1],ring_radius);
}
glPopMatrix(); // comes out of the stack
glScalef(0.001, 0.001, 0.001);
drawText(MESSAGE);
glFlush();
}
void myidle()
{
angle++; //angle value keeps on increasing
glutPostRedisplay(); // draws your drawing with updated value of angle to the screen
}
int main (int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(100,100);
glutInitWindowSize(110*PIXEL_SIZE, 110*PIXEL_SIZE);
glutCreateWindow ("Olymipc Rings || rotation ");
glClearColor(1.0, 1.0, 1.0, 0.0);
glPointSize(PIXEL_SIZE);
glShadeModel (GL_FLAT);
glutDisplayFunc(display);
glutIdleFunc(myidle); //just like DisplayFunc keeps on getting calls
glutReshapeFunc(myReshape);
glutMainLoop();
return 0;
Read about glPopMatrix(), glPushMatrix() and call back functions like glutIdleFunc().
I hope this will help!!

Answer 2

Try this:
#include <stdlib.h>
#include <stdio.h>
#include <GL/glut.h>
#include <GL/gl.h>
#include <math.h>
#define PIXEL_SIZE 3
#define MESSAGE "hello world !"
void draw_circle(int x, int y, int r);
int ring_radius = 19;
int color[5][3]={{0,0,1}, {0,0,0},{1,0,0}, {1,1,0},{0,1,0}};
int center[5][2]={{15,60},{50,60},{85,60},{33,45},{68,45}};
//=========================================================
void drawText(const char * message)
{
glRasterPos2f((GLfloat)0, (GLfloat)-400);
while (*message)
{
glutBitmapCharacter(GLUT_BITMAP_HELVETICA_12, *message);
glutStrokeCharacter(GLUT_STROKE_ROMAN,*message++);
}
}
void display (void)
{
int ms = glutGet(GLUT_ELAPSED_TIME);
glMatrixMode (GL_PROJECTION);
glLoadIdentity();
gluOrtho2D (-5, 105, -5, 105);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity ();
glTranslatef (0.375, 0.375, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_LINE_LOOP);
glVertex2i(-1,-1);
glVertex2i(100,-1);
glVertex2i(100,100);
glVertex2i(-1,100);
glEnd();
const float deg_per_sec = 60.0f;
float angle = deg_per_sec * ( (float)ms / 1000.0f );
for(int i = 0 ; i <5; i++)
{
glColor3f(color[i][0],color[i][1],color[i][2]);
glPushMatrix();
glTranslatef( center[i][0], center[i][1], 0 );
glRotatef(angle, 0, 0, 1);
glTranslatef( -center[i][0], -center[i][1], 0 );
draw_circle(center[i][0],center[i][1],ring_radius);
glPopMatrix();
}
glScalef(0.001, 0.001, 0.001);
drawText(MESSAGE);
glFlush();
glutSwapBuffers();
}
void draw_circle(int center_x, int center_y , int radius)
{
int r = radius;
int h = 1 - r ; /*initialization */
int x = 0;
int y = r;
int dU=3;
int dD = 5 - 2*r;
int i = center_x;
int j = center_y;
glPointSize(6);
glBegin(GL_POINTS);
while( y > x )
{
if (h<0)
{
dU= dU + 2;
h = h + dU;
x = x + 1;
dD = dD + 2;
}
else
{
dD = 2*(x-y) + 5;
h = h + dD;
x = x + 1;
y = y - 1;
dU = dU + 2;
dD = dD + 4;
}
glVertex2i(x+i, y+j);
glVertex2i(-x+i, y+j);
glVertex2i(x+i, -y+j);
glVertex2i(-x+i,-y+j);
glVertex2i(y+i, x+j);
glVertex2i(y+i, -x+j);
glVertex2i(-y+i, x+j);
glVertex2i(-y+i, -x+j);
}
glEnd();
}
void myReshape (int width, int height)
{
glViewport (0, 0, width, height);
}
void idle()
{
glutPostRedisplay();
}
int main (int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
glutInitWindowPosition(100,100);
glutInitWindowSize(110*PIXEL_SIZE, 110*PIXEL_SIZE);
glutCreateWindow ("Olymipc Rings || rotation ");
glClearColor(1.0, 1.0, 1.0, 0.0);
glPointSize(PIXEL_SIZE);
glShadeModel (GL_FLAT);
glutDisplayFunc(display);
glutReshapeFunc(myReshape);
glutIdleFunc(idle);
glutMainLoop();
return 0;
}

Answer 3

glRotatef