Looping in OpenGL with glutTimerFunc(...) - glut

Question

I want to create a program that draws some objects to scene using OpenGL, where I am continually changing the position of those object manually. To achieve this, I need to run some kind of loop where on each loop, it changes the position of the objects, and then draws to the screen, before repeating.
Given that glutMainLoop() is a non-returning function, and is also compulsory to run an OpenGL program, I need to run my loop with some sort of timer.
Now my solution which works is similar to the following:
void Render()
{
// Draw some objects using OpenGL
// ......
// ......
}
void Loop
{
// Update the positions of the objects
// ......
// ......
glutPostRedisplay();
glutTimerFunc(1, Loop, 0);
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
GlutCreateWindow("Test");
glutDisplayFunc(Render);
glutMainLoop();
glutTimerFunc(1, Loop, 0);
}
However, I am not sure I understand why there is the need for the glutTimerFunc() call in main(). Why can I not just replace it with a direct call to Loop()?
For example:
int main(int argc, char** argv)
{
glutInit(&argc, argv);
GlutCreateWindow("Test");
glutDisplayFunc(Render);
glutMainLoop();
Loop();
}
This does not work. The objects do not move on the screen as expected, and furthermore, then OpenGL window is totally unresponsive; I cannot even close it by clocking the cross on the title bar.
Surely glutTimerFunc(1, Loop, 0); just calls Loop() in the same way I have done in the second example, so why does this not work...?

Answer 1

Basically, the glutTimerFunc waits for a number of milliseconds to pass before calling the callback (in your case, Loop()). In this way it acts like a refresh operation.
Check out →GLUT API for more detail
void glutTimerFunc(unsigned int numMilliseconds, functionCallback, value);
So, that means you can pass in something like this:
glutTimerFunc(1000 / SCREEN_FPS, Loop, 0);
For extra info on main loops
deWitter's Gameloop
However, as your project gets more complex, you might find that you want the objects in your scenes rendering at constant speed. In addition to the glutTmerFunc, you can make you animations work with elapsed time - found by calculating the difference between the current time and previously current time.
Hope this helps!

Answer 2

Here's a really simple glutTimerFunc() example from a real program.
I wanted to hi-jack the title bar for command input. When you typed stuff, it appeared in the title bar and when you pressed return, it executed the command. I wanted a blinking cursor.
char title[80]; // This is where our keystrokes go.
char * cursor = NULL; // This points to the cursor character.
void cursorBlink(int id) {
if (cursor) {
*cursor = (*cursor == ' ') ? '_' : ' ';
glutSetWindowTitle(title);
glutTimerFunc(1000.0, cursorBlink, 0);
}
}
This isn't quite the whole thing. The ESC key gets us into command line entry mode. This sets up the title string and the cursor pointer, and it also has to make the first call to cursorBlink() to get the loop started. When command entry is done, the cursor point gets set back to NULL, and the loop shuts itself down.
The cursorBlink argument is not used. This is common with simple animations. Don't worry about it.