blinking on rotation in opengl - c++

Question

here is my code
#include "stdafx.h"
#include <GL/glut.h>
#include <gl/GL.h>
#include <gl/GLU.h>
#include <math.h>
#define pi 3.14;
float angle=3;
void reshape(int w, int h)
{
if(h==0) h=1;
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0,w,0,h,-1,1);
}
void display(void)
{
glDepthMask(GL_TRUE);
glClear(GL_COLOR_BUFFER_BIT);
glRotatef(angle++,0,1.0,1.0);
glBegin(GL_POLYGON);
glVertex2f(10,10);
glVertex2f(30,20);
glVertex2f(30,30);
glVertex2f(10,30);
glEnd();
glRotatef(angle++,0,1.0,1.0);
glBegin(GL_TRIANGLE_FAN);
glVertex2f(300,300);
int segments=20;
GLfloat angle=0;
for(int i=0;i<=segments; i++)
{
angle=i*2*pi; angle=angle / (segments);
glVertex2f(300+cos(angle)*30,300+sin(angle)*30);
}
glEnd();
glFlush();
glutSwapBuffers();
}
int main(int argc, char** argv)
{
glutInit(&argc , argv);
glutInitDisplayMode(GLUT_DEPTH|GLUT_RGB|GLUT_DOUBLE);
glutInitWindowSize(640,480);
glutCreateWindow("first");
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutIdleFunc(display);
glutMainLoop();
return(0);
}
here the problems are
cannot see the rotation correctly just blinking the rectangle. Don't know is it rotating or not.
2.the circle is not rotating even there is code for rotation. why?
3.what change I made if I want only rectangle or circle is rotated?

Answer 1

1. cannot see the rotation correctly just blinking the rectangle.
You're probably running waaaaay too fast. Use a timer callback to issue glutPostRedisplay()s every 16ms or so.
2. the circle is not rotating even there is code for rotation. why?
glRotatef(angle++,0,1.0,1.0);
^^^
You're rotating outside the X/Y plane and exceeding the depth limits of your glOrtho() call resulting in clipped fragment. Bump your nearVal/farVal to accommodate:
glOrtho(0,w,0,h,-1000,1000);
3. what change I made if I want only rectangle or circle is rotated?
Remove one of the glRotatef()s.
All together:
#include <GL/glut.h>
#include <cmath>
using namespace std;
const double pi = 3.14159;
float angle=3;
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
double w = glutGet( GLUT_WINDOW_WIDTH );
double h = glutGet( GLUT_WINDOW_HEIGHT );
glOrtho(-w,w,-h,h,-1000,1000);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glDepthMask(GL_TRUE);
//glRotatef(angle++,0,1.0,1.0);
glBegin(GL_POLYGON);
glVertex2f(10,10);
glVertex2f(30,20);
glVertex2f(30,30);
glVertex2f(10,30);
glEnd();
glRotatef(angle++,0,1.0,1.0);
glBegin(GL_TRIANGLE_FAN);
glVertex2f(300,300);
const int segments=20;
for(int i=0;i<=segments; i++)
{
const GLfloat angle = ( i*2*pi ) / (float)segments;
glVertex2f(300+cos(angle)*30,300+sin(angle)*30);
}
glEnd();
glutSwapBuffers();
}
void timer( int value )
{
glutPostRedisplay();
glutTimerFunc( 16, timer, 0 );
}
int main(int argc, char** argv)
{
glutInit(&argc , argv);
glutInitDisplayMode(GLUT_DEPTH|GLUT_RGB|GLUT_DOUBLE);
glutInitWindowSize(640,480);
glutCreateWindow("first");
glutDisplayFunc(display);
glutTimerFunc( 0, timer, 0 );
glutMainLoop();
return(0);
}