Drawing multiple objects in OpenGL using C - c

Question

I have an OpenGL program to draw a circle by mouse click. The program works fine except when I try to draw multiple circles the previous circle disappears. Here is the code:
#include <GL/glut.h>
#include<math.h>
#include<stdio.h>
struct position
{
float x;
float y;
};
typedef struct position Position;
Position start;
Position finish;
void setPixel(int x, int y)
{
glBegin(GL_POINTS);
glVertex2f(x, y);
glEnd();
}
void circle(int a0, int b0, int a1, int b1)
{
int i, x, y, x1, y1, r, p;
x1 = (a0+a1)/2;
y1 = (b0+b1)/2;
r = sqrt((((a1-x1)*(a1-x1))+((b1-y1)*(b1-y1))));
p = (5/4-r);
x = 0;
y = r;
while(x <= y)
{
setPixel( x+x1, y+y1);
setPixel( x+x1, -y+y1);
setPixel(-x+x1, -y+y1);
setPixel(-x+x1, y+y1);
setPixel( y+x1, x+y1);
setPixel( y+x1, -x+y1);
setPixel(-y+x1, -x+y1);
setPixel(-y+x1, x+y1);
x = x+1;
if (p<0)
p = p+2*x+1;
else {
y = y-1;
p = p+2*x-2*y+1;
}
}
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glPushMatrix();
circle(start.x, start.y, finish.x, finish.y);
glPopMatrix();
glutSwapBuffers();
}
void reshape( int w, int h)
{
glViewport( 0, 0, w, h);
glMatrixMode( GL_PROJECTION);
glLoadIdentity();
glOrtho( 0, w, h, 0, -1, 1);
}
void mouse(int button, int state, int x, int y)
{
switch(button)
{
case GLUT_LEFT_BUTTON:
if(state == GLUT_DOWN)
{
start.x = x; //x1
start.y = y; //y1
}
if(state == GLUT_UP)
{
finish.x = x; //x2
finish.y = y; //y2
}
break;
glutPostRedisplay();
}
}
void motion( int x, int y)
{
finish.x = x;
finish.y = y;
glutPostRedisplay();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(100, 100);
glutCreateWindow("");
glutMouseFunc(mouse);
glutMotionFunc(motion);
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
How can I display multiple images together?

Answer 1

If you want to draw multiple things, then draw multiple things. You're only drawing one circle. If you change the position of that circle, it will just be drawn in a different place.
However I really wouldn't recommend drawing like this in GL. Using the fixed-function pipeline to draw individual pixels as GL_POINTS is exceedingly inefficient. GL is not designed as a raster drawing API.