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Back to Graphics and GPU Programming

60fps framelock done properly - without duplicated or skipped frames

d h k · 2019-12-12T18:48:35

I am trying to lock my framerate at 60 for a game prototype I am working on* but it seems to be quite difficult to ensure that I always push out a new frame for every monitor vertical retrace, without duplicated or skipped frames. This is my current approach, written in C++ using std::chrono: constexpr auto frameTime = chrono::duration<int64_t, ratio<1, 60>>(1); auto nextFrame = chrono::steady_clock::now() + frameTime; // schedule beginning of next frame while (true) { // process frame here this_thread::sleep_until(nextFrame); // wait until the next frame should start nextFrame += frameTime; } This schedules the time_point when the next frame should start processing, processes the current frame and then sleeps until the next frame is supposed to start. I have measured it and know that my frames do not take too long to process. I have also confirmed that steady_clock and high_resolution_clock are the same on my systems so it's not a clock precision problem. I am moving a sprite 1px to the left/right every frame and it is immediately obvious when frames are duplicated or skipped. It works fine for a couple of seconds and then breaks down with multiple duplications and skips. That goes on for about a second, then it works well for a couple of seconds again. How long it works fine in between 'break downs' appears to depend on the machine. There has to be some way to achieve a better stable framelock, right? What am I missing here? * It's a 2D game with handdrawn animations so I think locking the framerate is reasonable.

Graphics and GPU Programming Programming C++ GameLoop Timing Sync FrameLock FPS

Started by d h k December 06, 2019 12:27 PM

11 comments, last by frob 4 years, 6 months ago

scgrn

168

December 10, 2019 12:23 AM

This may help: https://medium.com/@tglaiel/how-to-make-your-game-run-at-60fps-24c61210fe75

a light breeze

365

December 10, 2019 06:01 AM

I would actually try using vsync if the monitor is 60Hz or a multiple thereof, and a timer-based approach if it isn't. You can ask the OS about the monitor refresh rate, or you can measure it if you don't trust the OS. If the monitor is at 75Hz, you're going to get irregular frame duplication no matter what you do, and if the monitor has a variable refresh rate, then using a timer is how you fix the refresh rate to 60Hz.

d h k

439

Author

December 12, 2019 06:08 PM

Thanks for the suggestions and ideas, everyone. That medium.com article really hit the nail on the head!

I have just converted my game loop to busy wait, like this:

constexpr auto frameTime = chrono::duration<int64_t, ratio<1, 60>>(1);
auto nextFrame = chrono::steady_clock::now() + frameTime;
while (true)
{
  if (std::chrono::steady_clock::now() < nextFrame)
  {
    continue;
  }

  // process frame here

  nextFrame += frameTime;
}

Now I only get a very rare single stutter, pretty much what is described in the article. I will modify my solution and try to get rid of that as well. Maybe I could also sleep for, say, half the frame time, knowing that processing my frames is going to be very fast...

frob

46,236

December 12, 2019 06:48 PM

d h k said:
Maybe I could also sleep for, say, half the frame time

That puts you right back at the mercy of the OS task scheduler, which was part of the problem to begin with. The default scheduler in Windows is about a full frame in duration. Any sleep at all, even those based on higher resolution timers, will still suffer from the task scheduler's resolution.

Blocking for hardware is the best reliable way to do it, and even that is filled with issues and gotchas.