glutTimerFunc and FPS

[Zack]

I'm just learning opengl but one thing that is bugging me is I keep
running into glutTimerFuc being used to generate a specified FPS. Even
if I set it to update at 1 FPS my cpu is being 100% consumed. Is
glutTimerFunc implemented with busy spinning? What is the preferred way
to control FPS? I guess I could always use my specific OS's timer
utilities but is there any better way to do this with the opengl api?

[phibre]

Post by Zack
I'm just learning opengl but one thing that is bugging me is I keep
running into glutTimerFuc being used to generate a specified FPS. Even
if I set it to update at 1 FPS my cpu is being 100% consumed. Is
glutTimerFunc implemented with busy spinning? What is the preferred way
to control FPS? I guess I could always use my specific OS's timer
utilities but is there any better way to do this with the opengl api?

A timer function is not a good way to do this, because your code is locked down on a specific
framrate. Normally fps measurement is done that way:
GLUT provides a function, called glGet( GLUT_ELAPSED_TIME ). This function returns the number of
milliseconds since the program started or glGet was called the first time. Let's say, you have a
idleFunc(). This is a good place to put all stuff for fps measurement. The idle func would look like
this:

void idleFunc(){

static int lastUpdate = 0;
static int frames = 0;
char buf[20];

glutPostRedisplay(); // calls your display callback function
glutSwapBuffers();

int currentTime = glutGet( GLUT_ELAPSED_TIME );
frames++;

// is the time difference between lastUpdate and current time > one second ( 1000 ms )?
if ( ( currentTime - lastUpdate ) >= 1000 ){

sprintf( buf, "FPS: %d", frames );
glutSetWindowTitle( buf );
frames = 0;
lastUpdate = currentTime;

}

// you could also put a sleep here, to avoid glut consuming full processor time
// relaxing things
Sleep(1);

}


greets,
benjamin

[Zack]

Post by phibre
// you could also put a sleep here, to avoid glut consuming full processor time
// relaxing things
Sleep(1);

Is there a way to throttle opengl's cpu usage w/o giving up my apps time
slice like that.
Say I want to make a lightweight opengl application that shows some
basic animation say a stop watch that displays fractions of seconds. It
also has some user interface (clicking the start and stop button). Also
say this is a small app that shouldn't be taking up needless CPU time.

[fungus]

Post by Zack
I'm just learning opengl but one thing that is bugging me is I keep
running into glutTimerFuc being used to generate a specified FPS.

Specified frames/sec is usually a bad way to do things
(mainly because no matter how low you set it there's
always somebody slower than that and the program will
go crazy on their machine).

Post by Zack
if I set it to update at 1 FPS my cpu is being 100%
consumed. Is glutTimerFunc implemented with busy spinning?

You can put in a call to Sleep() in your idle function
but you're only delaying the day when you figure out
that timers are bad.

Post by Zack
What is the preferred way to control FPS?

Best way is not to do it. Much better to calculate
elapsed time per frame and move things by that
amount.

Post by Zack
is there any better way to do this with the opengl api?

OpenGL only does rendering. There's no timers or
even "SwapBuffers" in OpenGL - they're OS functions.