Translating a single object opengl - c

Question

I am writing code to draw the figure
but my code gives
as you can see the middle circle is missing.
My code:
#include <stdio.h>
#include <stdlib.h>
#include <GL/glut.h>
#include <math.h>
float width, height, r = 0.3, change = 0;
void draw(float tx, float ty)
{
glBegin(GL_LINE_LOOP);
for(int i = 1; i <= 1200; i++)
{
float x1, y1, theta;
theta = (2 * 3.14159 * i) / 1200;
x1 = r * cosf(theta) * height / width;
y1 = r * sinf(theta);
glVertex3f(x1 , y1 ,0);
}
glEnd();
glTranslatef(tx, ty, 0);
}
void display()
{
float p[6][2];
int j = 0;
if (change == 0)
change = 1;
else if (change == 1)
change = 0;
width = glutGet(GLUT_WINDOW_WIDTH);
height = glutGet(GLUT_WINDOW_HEIGHT) ;
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(1.0f, 0.0f, 0.0f);
glMatrixMode(GL_MODELVIEW);
glBegin(GL_LINE_LOOP);
for(int i = 1; i <= 1200; i++)
{
float theta, x1, y1;
theta = (2 * 3.14159 * i) / 1200;
x1 = r * cosf(theta) * height / width;
y1 = r * sinf(theta);
glVertex3f(x1, y1, 0);
if (i == 100 | i == 300 | i == 500 | i == 700 | i == 900 | i == 1100)
{
if(change == 0){
p[j][0] = x1;
p[j][1] = y1;
j++;
}
}
}
glEnd();
for(int i=0;i<6 && change == 0;i++){
draw(p[i][0],p[i][1]);
}
glutSwapBuffers();
}
void main(int argc,char *argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(700,500);
glutCreateWindow("circles");
glutDisplayFunc(display);
glutMainLoop();
}
Issue is when i translate first circle in draw function the center circle drawn is also translated to that point which is merged with other circle.My doubt is how to translate only one circle not the center one i tried translating by using push and pop matrix but it doesn't work.
Thank you.

Answer 1

glTranslatef() changes the current matrix by appending a translation. So your translations will just accumulate. And since you do not have a transform between the first two circles, they will appear at the same positions. Your program basically does the following:
Draw Circle
draw()
Draw Circle
Move up, right (p[0])
draw()
Draw Circle
Move up (p[1])
draw()
Draw Circle
Move up left (p[2])
...
If you want absolute positioning, you have to reset the transform in between. This can either be done with glLoadIdentity() or with the matrix stack. And be sure to draw after setting the transform.
I guess you know, but in any case a little reminder: The entire matrix stack functionality is deprecated in modern OpenGL and you will need to manage the matrices yourself. I assume, when you do this, everything gets a bit clearer. So I'm not sure if there is a good reason to try to understand the interface of the matrix stack functionalities.
If you want to place each circle at a specific location, you can do something like the following:
void drawCircle()
{
glBegin(GL_LINE_LOOP);
for(int i = 1; i <= 1200; i++)
{
float x1, y1, theta;
theta = (2 * 3.14159 * i) / 1200;
x1 = r * cosf(theta) * height / width;
y1 = r * sinf(theta);
glVertex3f(x1 , y1 ,0);
}
glEnd();
}
void display()
{
// ...
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(1.0f, 0.0f, 0.0f);
glMatrixMode(GL_MODELVIEW);
drawCircle();
for(int i = 0; i < 6; ++i)
{
float angle = M_PI / 3 * i;
float tx = r * sin(angle);
float ty = r * cos(angle);
glPushMatrix(); //save the current matrix
glTranslatef(tx, ty, 0); //move to the desired location
drawCircle();
glPopMatrix(); //restore the old matrix
}
glutSwapBuffers();
}