Display window and line segment in OpenGL - c++

Question

The line isn't showing up. What is wrong with the code?
#include<windows.h>
//#ifdef __APPLE__
//#include <GLUT/glut.h>
//#else
#include <GL/glut.h>
//#endif
//#include <stdlib.h>
void init(void){
glClearColor(1.0, 1.0,1.0,0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0, 200.0, 0.0,150.0);
}
void lineSegment(void){
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0f, 0.0f, 0.0f); // Red
//glColor3f(0.2, 0.4, 0.2);
glBegin(GL_LINE);
glVertex2i(180,15);
glVertex2i(10,145);
glEnd();
glFlush();
}
int main(int argc, char* argv[]){
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE); // Enable double buffered mode
//glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowPosition(50,100);
glutInitWindowSize(400,300);
glutCreateWindow("An example OpenGL Program");
init();
glutDisplayFunc(lineSegment);
glutMainLoop();
return 0;
}

Answer 1

You're requesting a GLUT_DOUBLE-buffered window but are failing to call glutSwapBuffers() at the end of your glutDisplayFunc() callback. glFlush() is not sufficient.
GL_LINE is not a valid argument for glBegin(). You're thinking of GL_LINES.
All together:
#include <GL/glut.h>
void init()
{
glClearColor( 1.0, 1.0, 1.0, 0.0 );
glMatrixMode( GL_PROJECTION );
gluOrtho2D( 0.0, 200.0, 0.0, 150.0 );
}
void lineSegment()
{
glClear( GL_COLOR_BUFFER_BIT );
glColor3f( 1.0f, 0.0f, 0.0f ); // Red
glBegin( GL_LINES );
glVertex2i( 180, 15 );
glVertex2i( 10, 145 );
glEnd();
glutSwapBuffers();
}
int main( int argc, char* argv[] )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_DOUBLE ); // Enable double buffered mode
glutInitWindowSize( 400, 300 );
glutCreateWindow( "An example OpenGL Program" );
init();
glutDisplayFunc( lineSegment );
glutMainLoop();
return 0;
}