Object without deformations - OpenGL - opengl

Question

Anybody knows how to keep a triangle without deformations and always at the middle of the windows whatever is his size?
I know I have to do one callback with reshape function and then define it, but I'm not sure what is going inside resize function:
void resize(int width, int height) {
viewport(0,0,width,height);
...?
}
I have this main help. glutInitWindowSize(600, 600);

Answer 1

Since the GL calls use normalised vertex coordinates ranging from -1 to +1, it is possible to keep any object in the center of the screen by using the right coordinates independent of the screen pixel sizes.
However, the same independency also brings in the behaviour that, depending on the screen aspect ratio (or window aspect ratio, as the case may be) the object will also change, unless explicitly accounted for. See the discussions in How can i convert an oval to circle openGL ES2.0 Android

Answer 2

Here's a important hint: Don't use the resize callback to do anything with OpenGL.
I know I have to do one callback with reshape function and then define it, but I'm not sure what is going inside resize function:
Then you knew wrong.
It leads to a lot of confusion. OpenGL is a state based drawing API and like all state machines it should be reset into a well known state before you use it. That includes projection and viewport. With that in mind your problem becomes trivial
void display()
{
/* draw some stuff */
glViewport(...);
setup_projection();
setup_modelview();
draw_stuff();
/* draw some other stuff with different projection and modelview */
glViewport(...);
setup_other_projection();
setup_other_modelview();
draw_other_stuff();
/* ... */
swapBuffers();
}
If you're using GLUT you can use glutGet(GLUT_WINDOW_WIDTH) and glutGet(GLUT_WINDOW_HEIGHT) to retrieve the window's size for the viewport calls.
So in your case you'd use a glViewport that covers your whole window and a projection that always maps a certain view space into that viewport. For example
void display()
{
int const win_width = glutGet(GLUT_WINDOW_WIDTH);
int const win_height = glutGet(GLUT_WINDOW_HEIGHT);
float const win_aspect = (float)win_width / (float) win_height;
glViewport(0, 0, win_width, win_height);
/* Using fixed function pipeline for brevity */
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
/* map the vertical range -1…1 to the window height and
a symmetric range -aspect … 0 … +aspect to the viewport */
glOrtho(-win_aspect, win_aspect, -1, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
draw_triangle();
/* ... */
glutSwapBuffers();
}