OpenGL not displaying 2D lines - c++

Question

I am using Ubuntu 18.04 LTS and trying to implement 2d lines using OpenGl but it isn't displaying anything. i am using ubuntu compiler. I have installed the library using terminal.
here is my code
#include <GL/glut.h>
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_LINES);
glVertex2i(10,10);
glVertex2i(100,100);
glEnd();
glFlush();
}
int main(int argc, char** argv)
{
glutInit(&argc,argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow ("points and lines");
glutDisplayFunc(display);
glutMainLoop();
return 0;
}
this is how i am compiling and running my program
g++ jamshaid.cpp -o jamoo -lglut -lGLU -lGL
./jamoo
Update 1
added init2D and tried other answer as well but not working. it even doesn't changing the title of the window.
update 2
after updating the code from this site. i now have this code and it's working. can you please explain the reason why is that so. thanks
#include <GL/glut.h>
void init2D(float r, float g, float b)
{
glClearColor(r,g,b,0.0);
glMatrixMode (GL_PROJECTION);
gluOrtho2D (0.0, 200.0, 0.0, 150.0);
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINES);
glVertex2i(10,10);
glVertex2i(100,100);
glEnd();
glFlush();
}
int main(int argc,char *argv[])
{
glutInit(&argc,argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow ("points and lines");
init2D(0.0,0.0,0.0);
glutDisplayFunc(display);
glutMainLoop();
return 0;}

Answer 1

As shown in the exemple here, you need a correct init2D function to initialize the 3D projection matrix before drawing.
void init2D(float r, float g, float b)
{
glClearColor(r,g,b,0.0);
glMatrixMode (GL_PROJECTION);
gluOrtho2D (0.0, 200.0, 0.0, 150.0);
}

Answer 2

As Tung Le Thanh told you, you need a correct projection matrix initialization, and this is what it does:
glMatrixMode (GL_PROJECTION)
glMatrixMode — specify which matrix is the current matrix
GL_PROJECTION
Applies subsequent matrix operations to the projection matrix stack.
GL_TEXTURE
Applies subsequent matrix operations to the texture matrix stack.
GL_COLOR
Applies subsequent matrix operations to the color matrix stack.
To find out which matrix stack is currently the target of all matrix
operations, call glGet with argument GL_MATRIX_MODE. The initial
value is GL_MODELVIEW.
gluOrtho2D
gluOrtho2D — define a 2D orthographic projection matrix
Specification
void gluOrtho2D(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top);
Parameters
left, right
Specify the coordinates for the left and right vertical clipping planes.
bottom, top
Specify the coordinates for the bottom and top horizontal clipping planes.
Description
gluOrtho2D sets up a two-dimensional orthographic viewing region.