C++ Jittery game loop - how to make it as smooth as possible? - c++

Question

This is my gameloop code:
while (shouldUpdate)
{
timeSinceLastUpdate = 0;
startTime = clock();
while (timeAccumulator >= timeDelta)
{
listener.handle();
em.update();
timeAccumulator -= timeDelta;
timeSinceLastUpdate += timeDelta;
}
rm.beginRender();
_x->draw();
rm.endRender();
timeAccumulator += clock() - startTime;
}
It runs almost perfectly, but it has some jitter to it, a few times a second instead of _x (a test entity that all it does in update is x++) moving 1 pixel to the right, it actually moves 2 pixels to the right and it's a noticeable lag/jitter effect. I'm guessing clock() isn't accurate enough. So what could I do to improve this game loop?
If it matters I use SDL and SDL_image.
EDIT: Nothing changed after doing something more accurate than clock. BUT, what I have figured out, is that it's all thanks to timeDelta. This is how timeDelta was defined when I made this post:
double timeDelta = 1000/60;
but when I defined it as something else while messing around...
double timeDelta = 16.666666;
I noticed for the first few seconds the game started, it was smooth as butter. But just a few seconds later, the game stuttered hugely and then went back to being smooth, and repeated itself like that. The more 6's (or anything after the . really) i added, the longer the game was initially smooth and the harder the lag hit when it did. It seems floating errors are attacking. So what can I do then?
EDIT2: I've tried so much stuff now it's not even funny... can someone help me with the math part of the loop? Since that's what's causing this...
EDIT3: I sent a test program to some people, some said it was perfectly smooth and others said it was jittery like how I described it. For anyone that would be willing to test it here it is(src): https://www.mediafire.com/?vfpy4phkdj97q9j
EDIT4: I changed the link to source code.

Answer 1

It will almost certainly be because of the accuracy of clock()
Use either std::chrono or SDL_GetTicks() to measure time since epoch.
I would recommend using std::chrono just because I prefer C++ api's, here's an example:
int main(){
using clock = std::chrono::high_resolution_clock;
using milliseconds = std::chrono::milliseconds;
using std::chrono::duration_cast;
auto start = clock::now(), end = clock::now();
uint64_t diff;
while(running){
diff = duration_cast<milliseconds>(end - start).count();
start = clock::now();
// do time difference related things
end = clock::now();
}
}
To only update after a specified delta, you'd do your loop like this:
int main(){
auto start = clock::now(), end = clock::now();
uint64_t diff = duration_cast<milliseconds>(end - start).count();
auto accum_start = clock::now();
while(running){
start = clock::now();
diff = duration_cast<milliseconds>(end - start).count();
if(duration_cast<nanoseconds>(clock::now() - accum_start).count() >= 16666666){
// do render updates every 60th of a second
accum_start = clock::now();
}
end = clock::now();
}
}
start and end will both be of the type std::chrono::time_point<clock> where clock was previously defined by us as std::chrono::high_resolution_clock.
The difference between 2 time_point's will be a std::chrono::duration which can be nanoseconds, milliseconds or any other unit you like. We cast it to milliseconds then get the count() to assign to our uint64_t. You could use some other integer type if you liked.
My diff is how you should be calculating your timeDelta. You should not be setting it as a constant. Even if you are certain it will be correct, you are wrong. Every frame will have a different delta, even if it is the smallest fraction of a second.
If you want to set a constant frame difference, use SDL_GL_SetSwapInterval to set vertical sync.
EDIT
Just for you, I created this example git repo. Notice in main.cpp where I multiply by diff to get the adjusted difference per frame. This adjustment (or, lack of), is where you are getting your jitteriness.