C++ Frames per Second

Foreword

When making a game we need to make sure that it's running smooth all the time. We usually measure the smoothness with Frames per Second.

What are Frames per Second

Frames per second (FPS) means how many pictures does the game throw at us per second. If we play a very graphic intense game on a very old computer then we can see what happens if we don't have enough frames per second in a game. Everything seems to be stuck and it's not really a game but a slideshow of pictures. The difference between a slideshow and a video are simply the frames per second. A slideshow just has one or two per second, a video has about 25, that's why it looks smooth.

The usual goal is to have about 60 frames per second in our games, so they don't appear like a slideshow at all. Hard core gamers will even see a difference between 60 and 100 frames per second, so we need to make sure to get them as high as possible. If we can get it to 60 FPS it's usually okay, but if more are possible then we should try to get more.

We can achieve high FPS rates by having very fast algorithms in our game, so calculating a single frame (picture) happens as fast as possible. The purpose of this tutorial will be to calculate the frames per second and not to make them as high as possible, since this is far more complicated and depends on a lot of things.

The Math behind it

Calculating the FPS in a game is very simple. All we need to do is increase a counter every time a new picture is being drawn, and after one second has elapsed we get the FPS by taking the value of that counter.

The Implementation

For simplicity we will implement the FPS class in one file instead of a .h and a .cpp file (which does not mean that you shouldn't split it in a .h and .cpp file).

To find out if one second has passed, we will use the Interval class that we implemented in this tutorial: C++ Time Measurement.

Here is the code:

class Fps

{

protected:

    unsigned int m_fps;

    unsigned int m_fpscount;

    Interval m_fpsinterval;



public:

    // Constructor

    Fps() : m_fps(0), m_fpscount(0)

    {

    }



    // Update

    void update()

    {

        // increase the counter by one

        m_fpscount++;



        // one second elapsed? (= 1000 milliseconds)

        if (m_fpsinterval.value() > 1000)

        {

            // save the current counter value to m_fps

            m_fps         = m_fpscount;

            

            // reset the counter and the interval

            m_fpscount    = 0;

            m_fpsinterval = Interval();

        }

    }



    // Get fps

    unsigned int get() const

    {

        return m_fps;

    }

};

In the constructor we need to initialize the counter and the m_fps value to zero. The m_fps value always holds the FPS that we had last second.

The update() function has to be called every time the game updates (draws a new picture), more about that later. What happens in the update() function is that we increase the counter, and if one second has elapsed, we simply store the current counter value and start counting again. To start counting again, we just have to reset the counter and the interval that measure how much time has elapsed. The interval can be reset by assigning a new object to it like in line 28.

Example

Let's take a look at a practical example. Every game has an update loop that is called every time a new picture is being generated. All we have to do is create a Fps object and then call its update() method every time the game updates. If we want to get the current FPS then all we need to do is call its get() method.

Some pseudo-code:

// we need one Fps object stored globally

Fps fps;



void onGameUpdate()

{

    // update player movement

    // ...

    

    // update monster ai

    // ...

    

    // add special effects

    // ...

    

    // update frames per second

    fps.update();

}



void processInput()

{

    // print out the fps if f12 was pressed

    if (keyPressed("F12") {

        std::cout<<"Fps: "<<fps.get()<<'\n';

    }

}

Our FPS get updated each time the game updates, and they will be printed if the F12 key is pressed.

Note: we need to make sure to store the Fps object globally and not inside the onGameUpdate() function. Otherwise it would be created, count one fps and then it would be destroyed again.

About V-Sync and Displays

If we are trying to get our game above 60 FPS but it doesn't seem to be possible, we need to make sure to check our graphics card driver or game window settings. Sometimes V-Sync is enabled, which synchronizes the games frames per second with our displays refresh rate. If our display has a refresh rate of 60 Hz then this means that the game would never get above 60 FPS as long as V-Sync is turned on. This is not bad at all since the display wouldn't show more than 60 pictures per second anyway, but for performance tests it's a good idea to turn V-Sync off.

Summary

Now we have a lightweight class that helps us to find out how fast our game really is. The implementation was rather easy because of our already existing Interval class that measures time for us.