Question about running a program at same speed in any computer

Question

I made a program (in C++, using gl/glut) for study purposes where you can basically run around a screen (in first person), and it has several solids around the scene. I tried to run it on a different computer and the speed was completely different, so I searched on the subject and I'm currently doing something like this:

Idle function:

    start = glutGet (GLUT_ELAPSED_TIME);
    double dt = (start-end)*30/1000;

    <all the movement*dt>

    glutPostRedisplay ();

    end = glutGet (GLUT_ELAPSED_TIME);

Display function:

    <rendering for all objects>

    glutSwapBuffers ();

My question is: is this the proper way to do it? The scene is being displayed after the idle function right?

I tried placing end = glutGet (GLUT_ELAPSED_TIME) before glutSwapBuffers () and didn't notice any change, but when I put it after glutSwapBuffers () it slows down alot and even stops sometimes.

EDIT: I just noticed that in the way I'm thinking, end-start should end up being the time that passed since all the drawing was done and before the movement update, as idle () would be called as soon as display () ends, so is it true that the only time that's not being accounted for here is the time the computer takes to do all of the movement? (Which should be barely nothing?)

Sorry if this is too confusing..

Thanks in advance.

Answer 1

I don't know what "Glut" is, but as a general rule of game development, I would never base movement speed off of how fast the computer can process the directives. That's what they did in the late 80's and that's why when you play an old game, things move at light speed.

I would set up a timer, and base all of my movements off of clear and specific timed events.

Answer 2

Set up a high-resolution timer (eg. QueryPerformanceCounter on Windows) and measure the time between every frame. This time, called delta-time (dt), should be used in all movement calculations, eg. every frame, set an object's position to:

obj.x += 100.0f * dt; // to move 100 units every second

Since the sum of dt should always be 1 over 1 second, the above code increments x by 100 every second, no matter what the framerate is. You should do this for all values which change over time. This way your game proceeds at the same rate on all machines (framerate independent), rather than depending on the rate the computer processes the logic (framerate dependent). This is also useful if the framerate starts to drop - the game doesn't suddenly start running in slow-motion, it keeps going at the same speed, just rendering less frequently.