Wrong FPS calculations in GLFW game

Question

In my game loop,

m_currentFrameTimestamp = glfwGetTime(); // Get the current timestamp
if ((m_currentFrameTimestamp - m_lastFPSDisplayTimestamp) >= 0.25) { // If 1/4 of a second
                                                         // passed, update the FPS counter
    double renderTime = m_currentFrameTimestamp - m_lastFrameTimestamp; // Frame time
    m_fps = (unsigned int)round(1 / renderTime); // FPS
    std::string title = "MyGame ["s; 
    title += std::to_string(m_fps);
    title += " FPS]"s;
    glfwSetWindowTitle(m_window, title.c_str()); // The title
    m_lastFPSDisplayTime = m_currentFrameTime; // Reset the FPS update timer
}
m_lastFrameTime = m_currentFrameTime; // Reset the frame time timer

calculates the FPS from the frame time and displays it in the window title. However, with just this and glClear() above, the FPS displayed in the title is very unstable (with each FPS update, it changes from 46 FPS to 87 FPS and back), even though back in the initialization step, I enabled VSync, and my display's frequency is 60 Hz. Furthermore, using Fraps, I found out that the program actually runs at a stable 60 FPS. So, what's the problem with my FPS calculation code (more specifically, why is glfwGetTime() producing inaccurate results - my code, being way too simple, just can't be wrong, it's a very simple calculation that cannot give errors of any kind)?

Answer 1

The issue is that you calculate the FPS according to a single measurement, which means that any slight variation will cause a significant shift in results. Since the actual loop time varies due to system considerations, you'll get stabler results by averaging more calculations.

fpsCount = 0;
m_lastFPSDisplayTimestamp = glfwGetTime();
while(shouldRender()) {
    m_currentFrameTimestamp = glfwGetTime(); // Get the current timestamp
    if ((m_currentFrameTimestamp - m_lastFPSDisplayTimestamp) >= 0.25) {
        std::string title = "MyGame ["s; 
        title += std::to_string(fpsCount);
        title += " FPS]"s;
        glfwSetWindowTitle(m_window, title.c_str()); // The title
        fpsCount = 0;
        m_lastFPSDisplayTime = m_currentFrameTime;
    } else {
        fpsCount++;
    }
    m_lastFrameTime = m_currentFrameTime;
}

Answer 2

Could be missing something but am pretty sure a 4th of a second is way bigger than a 60th of a second. Might be weird quirks about that not sure. I'd personally just do something like.

void foo(){
    sleep(1.0f/1000);
    //if last frame is 60th of a second(or some other num)
    //draw window then display fps
}
//or
void foo(){
    //if last frame is 60th of a second(or some other num)
    //draw window then display fps
}

Depending on if your trying to benchmark it or not. Also the sleep function will need tweaking to get the right balance between preserving cpu usage and having stable fps. And there's a whole art to doing this right if your really wanting to.