c++ calculate FPS from hooking a function that is called each frame

Question

Ok, so i am making this little 'program' and would like to be able to calculate FPS. I had an idea that if i hook a function that is called each frame i could possibly calculate the FPS?

Here's a complete fail, now that i look at it this code again i see how stupid i was to think this would work:

int FPS = 0;
void myHook()
{
    if(FPS<60) FPS++;
    else FPS = 0;
}

Obviously this is an idiotic attempt, though not sure why i even logically thought it might work in the first place...

But yeah, IS it possible to calculate FPS via hooking a function that is called each frame?

I sat down and was thinking of possible ways to do this but i just couldn't come up with anything. Any info or anything would be helpful, thanks for reading :)

Answer 1

You can call your hook function to do the fps calculation but before being able to do that you should:

1. Keep track of the frames by incrementing a counter each time a redraw is performed

2. Keep track of how much time has passed since last update (get the current time in your hook function)

3. Calculate the following

    frames / time 

Use a high resolution timer. Use a reasonable update rate (1/4 sec or the like).

Answer 2

This should do the trick:

int fps = 0;
int lastKnownFps = 0;

void myHook(){ //CALL THIS FUNCTION EVERY TIME A FRAME IS RENDERED
    fps++;
}
void fpsUpdater(){ //CALL THIS FUNCTION EVERY SECOND
    lastKnownFps = fps;
    fps = 0;
}

int getFps(){ //CALL THIS FUNCTION TO GET FPS
    return lastKnownFps;
}

Answer 3

You can find the time difference between succussive frames. The inverse of this time will give you frame rate. You need to implement a finction getTime_ms() which returns current time in ms.

unsigned int prevTime_ms = 0;
unsigned char firstFrame = 1;
int FPS                  = 0;

void myHook()
{
    unsigned int timeDiff_ms = 0;
    unsigned int currTime_ms = getTime_ms(); //Get the current time.

    /* You need at least two frames to find the time difference. */
    if(0 == firstFrame)
    {
        //Find the time difference with respect to previous time.
        if(currTime_ms >= prevTime_ms)
        {
            timeDiff_ms = currTime_ms-prevTime_ms;
        }
        else
        {
            /* Clock wraparound. */
            timeDiff_ms = ((unsigned int) -1) - prevTime_ms;
            timeDiff_ms += (currTime_ms + 1);
        }

        //1 Frame:timeDiff_ms::FPS:1000ms. Find FPS.
        if(0 < timeDiff_ms) //timeDiff_ms should never be zero. But additional check.
            FPS = 1000/timeDiff_ms;
    }
    else
    {
        firstFrame  = 0;
    }
    //Save current time for next calculation.
    prevTime_ms = currTime_ms;

}