Fixed Update Speed

Question

I've been looking through the internet a long time now for an easy solution to a rather easy problem.

I have this function running as a thread, and what I want is quite simple. There is plenty of pseudo code around that explains how it's done, but I'm having trouble implementing this in C/C++.

What I want: I have a loop, which I want to run 20 times a second. I have done this before, but I cannot remember for the life of me.

Here is the code I have.

void* Tick::startBeating() {
    mNext = clock();
    mRunning = 1;

    // Loop dat
    while (mRunning) {
        mT = clock();
        while ((mT) >= mNext) {
            // do updates here
            std::cout << "Tick." << std::endl;

            mNext = mT + SKIP_TICKS;
        }


        sleep(1);
    }

    return NULL;
}

This doesn't work, nothing seems to work. A simple solution in C is no where to be found on the internet, and that surprises me.

Yes, I know about CLOCKS_PER_SEC but I just don't know anymore.

Any help?

Answer 1

I do it like this

float mAccumulatedTime;
int mLastTime;

void Init()
{
  mAccumulatedTime = 0.f;
  mLastTime=clock();
}

void Tick(float DeltaTime)
{
  mAccumulatedTime += DeltaTime;
  while (mAccumulatedTime > FixedTimeStep)
  {
    DoFixedTimeStepStuff();
    mAccumulatedTime -= FixedTimeStep;
  }
}

void Update()
{
  int CurrentTime=clock();
  float DeltaTime=(CurrentTime-mLastTime)/(float)CLOCKS_PER_SEC;
  Tick(DeltaTime);
  mLastTime=CurrentTime;
}

You need to take care to handle NaNs and "spirals of death" appropriately, but this will be enough to get you started.