Simple Vertex Problem

[openujs]

Why would the following code draw line but the bottow one does not.
glBegin(GL_LINES);
glVertex3f( 0.0,0.0, 0.0 );
glVertex3f( 10.0, 10.0, 0.0 );

glBegin(GL_LINES);
glVertex3f( 10.0,0.0, 0.0 );
glVertex3f( 10.0, 10.0, 0.0 );

Does every line has to be started from 0,0,0 coordinates?

Even this one does not work-
glBegin( GL_TRIANGLES );
glVertex3f(0.0f, 0.0f, 60.0f);
glVertex3f(-15.0f, 0.0f, 30.0f);
glVertex3f(15.0f,0.0f,30.0f);

Don't forget you need a glEnd(); command after ever line or poly you draw to tell OpenGL you've finished drawing that object.

Try that first.

[openujs]

oh yes. i have glEnd() at the end. but point being why the bottom 2 does not draw, are coordinates off?

[Hlz]

Post the rest of your render code. You've probably done something silly like drawing black lines on a black background, or misconfigured your projection/camera matrix, or tried to begin a primitive in the middle of another one, or some sinister combination of all the above.

Is your app running without errors?

:-)

[openujs]

void drawScene( void )
{

glClear( GL_COLOR_BUFFER_BIT );
//Use black pen
glColor3f( 0.0. 0.0,0.0 );

//Working
glBegin(GL_LINES);
glVertex3f( 0.0,0.0, 0.0 );
glVertex3f( 10.0, 10.0, 0.0 );

//Not working
//glBegin(GL_LINES);
//glVertex3f( 10.0,0.0, 0.0 );
//glVertex3f( 10.0, 10.0, 0.0 );

//Even this one does not work-
//glBegin( GL_TRIANGLES );
//glVertex3f(0.0f, 0.0f, 60.0f);
//glVertex3f(-15.0f, 0.0f, 30.0f);
//glVertex3f(15.0f,0.0f,30.0f);

glFlush();
glutSwapBuffers();
}

void init(void )
{

glClearColor( 1.0,1.0,1.0,1.0);
}

[openujs]

and the last part is

int main( int argc, char** argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE );
glutCreateWindow("test");
glutDisplayFunc( drawScene );
Init();
glutMainLoop();
return 0;
}

[openujs]

hmm, i again forgot glEnd() at my drawScene(), assume it is there.

Each glBegin() has to be closed by an glEnd(). Calling glBegin() twice without an closing glEnd() will result in an GL_INVALID_OPERATION condition and all following commands will be ignored.

So try this instead:

void drawScene( void )
{
  glClear( GL_COLOR_BUFFER_BIT );
  //Use black pen
  glColor3f( 0.0. 0.0,0.0 );
 
  //Working
  glBegin(GL_LINES);
    glVertex3f( 0.0,0.0, 0.0 );
    glVertex3f( 10.0, 10.0, 0.0 );
  glEnd();
 
  glBegin(GL_LINES);
    glVertex3f( 10.0,0.0, 0.0 );
    glVertex3f( 10.0, 10.0, 0.0 );
  glEnd();
 
  glBegin(GL_TRIANGLES);
    glVertex3f(0.0f, 0.0f, 60.0f);
    glVertex3f(-15.0f, 0.0f, 30.0f);
    glVertex3f(15.0f,0.0f,30.0f); 
  glEnd();
 
  glutSwapBuffers();
}

[openujs]

I understand there should be only one glBegin() and glEnd(). My problem is some where, probably with vertex coordinates. Try the following code, if it draws for you or not. Supposed to draw a triangle, but it does not render.

void drawScene( void )

{

glClear( GL_COLOR_BUFFER_BIT );

//Use black pen

glColor3f( 0.0. 0.0,0.0 );

//Working

glBegin(GL_TRIANGLES);

glVertex3f(0.0f, 0.0f, 60.0f);

glVertex3f(-15.0f, 0.0f, 30.0f);

glVertex3f(15.0f,0.0f,30.0f);

glEnd();

glutSwapBuffers();

}

[Relic]

If this is all your code (matrix setup at default) it's obvious why the first line draws (one tenth of its actual length) and no other.
Having the default identity matrix in both projection and modelview means the same as an glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0) setup with no changes to input vertex coordinates.
That is, everything which is not in the range between -1.0 and 1.0 in (x,y,z) is outside the viewing frustum.