c++ - I am writing a program in OpenGL to create an oscillating pendulum -

i have figured out rotation cannot limit rotation particular angle. i'm using idlefunc increment angle theta in background. please suggest alternative or solution limit rotation angle to, say, +90 , -90 degrees. here's code:

#include<stdio.h> #include<gl/freeglut.h> #include<math.h> int n=10; glfloat v[4][3]={{0.0,0.0,1.0},{0.0,0.942809,-0.33333},{-0.816497,-0.471405,-0.33333},{0.816497,-0.471405,-0.333333}}; glfloat a[2][3]={{0.0,0.0,-0.333333},{0.0,3.0,-0.333333}}; void normalize(glfloat *p) {     double d=0.0;     int i;     for(i=0;i<3;i++)         d+=p[i]*p[i];     d=sqrt(d);     if(d>0.0)         for(i=0;i<3;i++)             p[i]/=d; } void triangle(glfloat *a,glfloat *b,glfloat *c) {     glbegin(gl_polygon);         glnormal3fv(a);         glvertex3fv(a);          glvertex3fv(b);         glvertex3fv(c);     glend(); } void divide_triangle(glfloat *a,glfloat *b,glfloat *c,int n) {     glfloat v1[3],v2[3],v3[3];     int j;     if(n>0)     {         for(j=0;j<3;j++)             v1[j]=a[j]+b[j];         normalize(v1);         for(j=0;j<3;j++)             v2[j]=a[j]+c[j];         normalize(v2);         for(j=0;j<3;j++)             v3[j]=c[j]+b[j];         normalize(v3);         divide_triangle(a,v2,v1,n-1);         divide_triangle(c,v3,v2,n-1);         divide_triangle(b,v1,v3,n-1);         divide_triangle(v1,v2,v3,n-1);     }     else         triangle(a,b,c); } void tetrahedron(int n) {     divide_triangle(v[0],v[1],v[2],n);     divide_triangle(v[3],v[2],v[1],n);     divide_triangle(v[0],v[3],v[1],n);     divide_triangle(v[0],v[2],v[3],n); } glfloat theta=0.0; void display(void) {     glclear(gl_color_buffer_bit|gl_depth_buffer_bit);     glclearcolor(1.0,1.0,1.0,1.0);     glbegin(gl_lines);         glvertex3fv(a[0]);         glvertex3fv(a[1]);     glend();     glcolor3f(1.0,0.0,0.0);     tetrahedron(n);     glcolor3f(0.0,0.0,1.0);     glmatrixmode(gl_modelview);     glloadidentity();     gltranslatef(0.0,+3.0,-0.333333);     glrotatef(theta,0.0,0.0,1.0);     gltranslatef(0.0,-3.0,+0.333333);        glflush(); } void myreshape(int w,int h) {     glviewport(0,0,w,h);     glmatrixmode(gl_projection);     glloadidentity();     if(w<=h)             glortho(-4.0,4.0,-4.0*(glfloat)h/(glfloat)w,4.0*(glfloat)h/(glfloat)w,-20.0,20.0);     else             glortho(-4.0*(glfloat)w/(glfloat)h,4.0*(glfloat)w/(glfloat)h,-4.0,4.0,-20.0,20.0);     glmatrixmode(gl_modelview); } void oscillate() { theta+=2.0; glutpostredisplay() } void main(int argc,char** argv) {     glutinit(&argc,argv);     glutinitdisplaymode(glut_rgb|glut_depth);     glutinitwindowsize(1366,768);     glutcreatewindow("pendulum");     glutreshapefunc(myreshape);     glutdisplayfunc(display);     glutidlefunc(oscillate);     glenable(gl_depth_test);     glutmainloop(); }  

quick 'n dirty:

void oscillate() {     static float direction = 1.0f;     if( theta > 90.0f )         direction = -1.0f;     if( theta < -90.0f )         direction = 1.0f;      theta += 2.0 * direction;     glutpostredisplay() }