Winapi: GetTickCount() can't get ELAPSED TIME

Question

Hi I got a simple FPS counter in my animation. FPS counts fine but I don't know why I can't measure the animation time. Normaly I get that time by glutGet(GLUT ELAPSED TIME) but since I have to go Winapi I would like to (or must) use GetTickCount() Here is my code:

int Frames=0;
int CurrentTime=0;
int PreviousTime=0;
int IntervalTime=0;
int CounterTime=0;
float FPS;

void fpscount(){
    Frames++;
    CurrentTime = GetTickCount();
    IntervalTime = CurrentTime - PreviousTime;

    if(IntervalTime >= 1000)
    {
        FPS = Frames / (IntervalTime / 1000.0f);
        PreviousTime = CurrentTime;
        Frames = 0;
        CounterTime += IntervalTime;

    }
}

I measure by line: CounterTime += IntervalTime; and get strange values like: 44834406 44835420

If I define that line as CounterTime = IntervalTime+CounterTime; the CounterTime gets values as above.

If I define that line as CounterTime = IntervalTime; the CounterTime gets values: 1014 1014 but they are are not summed.

The correct values should be: 1014 2028 . . .

What I'am doing wrong?

Answer 1

I can't follow your thought pattern, but the way I do it is like so:

int gFpsStartTime;
int gFpsEndTime;
int gFpsFrames;
float gFPS;

void fpscount(){
    if (gFpsEndTime == 0) gFpsEndTime = GetTickCount();
    gFpsStartTime = GetTickCount();
    ++gFpsFrames;
    if (gFpsStartTime - gFpsEndTime > 0) {
        gFPS = gFpsFrames / ((gFpsStartTime - gFpsEndTime) / 1000.0f);
    }
}

This has the benefit of giving you fractions of FPS too, which gives a better average.

Answer 2

You're initialising IntervalTime like this:

CurrentTime = GetTickCount();
IntervalTime = CurrentTime - PreviousTime;

But on the very first iteration, PreviousTime is 0. This means that on the first iteration, IntervalTime is assigned the value of GetTickCount(), which is the number of milliseconds since the system was booted.

You need to initialise PreviousTime to GetTickCount() when your animation starts playing.

Answer 3

Try this instead:

int Frames = 0;
DWORD PreviousTime = 0;
float FPS = 0.0f;

void fpscount()
{
    Frames++;

    DWORD CurrentTime = GetTickCount();
    DWORD IntervalTime = (CurrentTime >= PreviousTime)
        ? (CurrentTime - PreviousTime)
        : ((MAXDWORD - PreviousTime) + CurrentTime);

    if (IntervalTime >= 1000)
    {
        FPS = Frames / (IntervalTime / 1000.0f);
        PreviousTime = CurrentTime;
        Frames = 0;
    }
}

And don't forget to initialize PreviousTime with the current time when you first start processing frames.

Answer 4

Try using timeGetTime for better timer precision.