opengl좀 도와주셔요 ㅜㅜ

그림판 만드는 중인데요. 초반이라

지금 소스는 간단해요 Drawmode함수를 통해서 그리고자하는 도형을 선택했고, 그걸 MyDisplay함수에서

if문을 통해 도형을 불러와요.

근데 지금은 한번그리면 계속 그린것이 유지가 안되고 리셋됬다가 다시그리고 , 이런식인데

그린것들을 계속 유지하려면 어떻게 해야하나요?

대충 찾아본바로는 glPushMatrix()랑 glPopMatrix() 이용하는거같은데 잘안되네요 ㅜㅜ

저 소스에 어느곳에 넣어야되는지도 모르겠고.. 도와주셔요

#include <GL\glut.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int DrawMode1, DrawMode2, DrawMode3, DrawMode4, DrawMode5;
GLint TopLeftX, TopLeftY, BottomRightX, BottomRightY;

void DrawMode(unsigned char KeyPressed, int X, int Y) {
switch (KeyPressed){
        case 'w':{   //선 선택
     DrawMode1=1;
  DrawMode2=0;
  DrawMode3=0;
  DrawMode4=0;
  DrawMode5=0;
     }
   break;
        case 'e':{   //사각형 선택
  DrawMode1=0;
  DrawMode2=1;
  DrawMode3=0;
  DrawMode4=0;
  DrawMode5=0;
     }
   break;
  case 'r':{   //이등변삼각형 선택
  DrawMode1=0;
  DrawMode2=0;
  DrawMode3=1;
  DrawMode4=0;
  DrawMode5=0;
     }
   break;
     case 't':{   //직각삼각형 선택
  DrawMode1=0;
  DrawMode2=0;
  DrawMode3=0;
  DrawMode4=1;
  DrawMode5=0;
     }
   break;
  case 'y':{   //원 선택
  DrawMode1=0;
  DrawMode2=0;
  DrawMode3=0;
  DrawMode4=0;
  DrawMode5=1;
     }
   break;
        case 'q':
            glClear(GL_COLOR_BUFFER_BIT);
   break;

}
}

void MyDisplay( ){

glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.5, 0.5, 0.5);
glViewport(0, 0, 500, 500);
   if(DrawMode1==1){
  glBegin(GL_LINES);     //선
  glVertex3f(TopLeftX/500.0,(500.0-TopLeftY)/500.0, 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-BottomRightY)/500.0, 0.0);
  glEnd( );
  glFlush( );

}
else if(DrawMode2==1){
  glBegin(GL_POLYGON);      //사각형
  glVertex3f(TopLeftX/500.0,(500.0-TopLeftY)/500.0, 0.0);
  glVertex3f(TopLeftX/500.0,(500.0-BottomRightY)/500.0 , 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-BottomRightY)/500.0, 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-TopLeftY)/500.0, 0.0);
  glEnd();
  glFlush();
}
else if(DrawMode3==1){
  glBegin(GL_POLYGON);       //이등변삼각형
  glVertex3f(TopLeftX/500.0,(500.0-TopLeftY)/500.0, 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-BottomRightY)/500.0, 0.0);
  glVertex3f(TopLeftX/500.0-((BottomRightX-TopLeftX)/500.0),(500.0-BottomRightY)/500.0, 0.0);
  glEnd();
  glFlush();
}
else if(DrawMode4==1){
  glBegin(GL_TRIANGLES);      //직각삼각형
  glVertex3f(TopLeftX/500.0,(500.0-TopLeftY)/500.0, 0.0);
  glVertex3f(TopLeftX/500.0,(500.0-BottomRightY)/500.0 , 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-BottomRightY)/500.0, 0.0);
  glVertex3f(BottomRightX/500.0, (500.0-TopLeftY)/500.0, 0.0);
  glEnd( );
  glFlush( );
}

/*
else if(DrawMode5==1){

  glutWireSphere(TopLeftX/500*2.0,(500.0-BottomRightY)/500,0.0); //원glutWireSphere(size/2.0, 15, 15);
  glEnd();
  glFlush();
}
*/
}

void MyMouseClick(GLint Button, GLint State, GLint X, GLint Y){
if(Button==GLUT_LEFT_BUTTON && State==GLUT_DOWN){
        TopLeftX = X;
        TopLeftY = Y;

    }
}

void MyMouseMove(GLint X, GLint Y) {
    BottomRightX = X;
    BottomRightY = Y;
    glutPostRedisplay( );

}

int main(int argc, char** argv){
glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_RGB);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(0, 0);
    glutCreateWindow("OpenGL Drawing Example");

    glClearColor (1.0, 1.0, 1.0, 1.0);
    glMatrixMode(GL_PROJECTION);

    glLoadIdentity();
    glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);

glutDisplayFunc(MyDisplay);

glutKeyboardFunc(DrawMode);
glutMouseFunc(MyMouseClick);
glutMotionFunc(MyMouseMove);

glutMainLoop( );

return 0;
}