Limiting FPS in C++

Question

I'm currently making a game in which I would like to limit the frames per second but I'm having problems with that. Here's what I'm doing:

I'm getting the deltaTime through this method that is executed each frame:

void Time::calc_deltaTime() {
    double currentFrame = glfwGetTime();
    deltaTime = currentFrame - lastFrame;
    lastFrame = currentFrame;
}

deltaTime is having the value I would expect (around 0.012.... to 0.016...)

And than I'm using deltaTime to delay the frame through the Sleep windows function like this:

void Time::limitToMAXFPS() {

    if(1.0 / MAXFPS > deltaTime)
        Sleep((1.0 / MAXFPS - deltaTime) * 1000.0);
}

MAXFPS is equal to 60 and I'm multiplying by 1000 to convert seconds to milliseconds. Though everything seems correct I'm sill having more than 60 fps (I'm getting around 72 fps)

I also tried this method using while loop:

void Time::limitToMAXFPS() {

    double diff = 1.0 / MAXFPS - deltaTime;

    if(diff > 0) {

        double t = glfwGetTime( );

        while(glfwGetTime( ) - t < diff) { }

    }

}

But still I'm getting more than 60 fps, I'm still getting around 72 fps... Am I doing something wrong or is there a better way for doing this?

Answer 1

How important is it that you return cycles back to the CPU? To me, it seems like a bad idea to use sleep at all. Someone please correct me if I am wrong, but I think sleep functions should be avoided.

Why not simply use an infinite loop that executes if more than a certain time interval has passed. Try:

const double maxFPS = 60.0;
const double maxPeriod = 1.0 / maxFPS;

// approx ~ 16.666 ms

bool running = true;
double lastTime = 0.0;

while( running ) {
    double time = glfwGetTime();
    double deltaTime = time - lastTime;

    if( deltaTime >= maxPeriod ) {
        lastTime = time;
        // code here gets called with max FPS
    }
}

Last time that I used GLFW, it seemed to self-limit to 60 fps anyway. If you are doing anything high performance orientated (game or 3D graphics), avoid anything that sleeps, unless you wanna use multithreading.