Fix Your Timestep

Joshua Eirman · 2017-09-04T01:08:06

I am trying to get the time stepping working consistently in an asteroids project. The application runs fairly smoothly but when really looked at there are some pretty slight variances in motion. I have looked at http://www.koonsolo.com/news/dewitters-gameloop/ and https://gafferongames.com/post/fix_your_timestep/. I tried to write something similar to the first link but did not really notice a difference. Here is the first article: const int FRAMES_PER_SECOND = 25; const int SKIP_TICKS = 1000 / FRAMES_PER_SECOND; DWORD next_game_tick = GetTickCount(); // GetTickCount() returns the current number of milliseconds // that have elapsed since the system was started int sleep_time = 0; bool game_is_running = true; while( game_is_running ) { update_game(); display_game(); next_game_tick += SKIP_TICKS; sleep_time = next_game_tick - GetTickCount(); if( sleep_time >= 0 ) { Sleep( sleep_time ); } else { // we are running behind! } } This article said there would be no problems on computer's that are fast enough, and this does not seem to be true. Perhaps the slight variations are as good as it is going to get? Here is the second article: double t = 0.0; double dt = 0.01; double currentTime = hires_time_in_seconds(); double accumulator = 0.0; State previous; State current; while ( !quit ) { double newTime = time(); double frameTime = newTime - currentTime; if ( frameTime > 0.25 ) frameTime = 0.25; currentTime = newTime; accumulator += frameTime; while ( accumulator >= dt ) { previousState = currentState; integrate( currentState, t, dt ); t += dt; accumulator -= dt; } const double alpha = accumulator / dt; State state = currentState * alpha + previousState * ( 1.0 - alpha ); render( state ); } As I understand it this program takes whats left over in the accumulator and blends it with an interpolation. I have read that this is a pretty famous article. I am wondering about the variables previousState and currentState. I assume the passed in variable to integrate is a reference to hold the value for next time? And in the final line before the render, the 2 states that are being multiplied by the 2 alphas are what, maybe my x and y? The variable Previous is previousState and current is currentState? Will this be even any better though as my understanding is I would need a previous and current x and y for every object and there is a function call for each draw. I would also have to run the while over and over again and and than the two lines below it over and over again. These two procedures in themselves would seem to slow the game down more than what it is now. I could really use some help, please. Thank you, Josheir

For Beginners C++

Started by Josheir August 28, 2017 09:10 PM

11 comments, last by Josheir 7 years, 2 months ago

Rutin

1,701

September 03, 2017 07:14 PM

1 hour ago, Josheir said:

Well it's interesting I think. Both programs (mine and your sample) work smoothly. I'm interested in why (more feedback from you.) By running on the draw side you mean that their code for SFML drawing already uses interpolation? Thanks also for those great tips and the excellent code sample.

Have a super Labor Day weekend,

Josheir

Hello Josheir,

What I meant was that you're seeing smooth motion on your program because you're using interpolation in your code, otherwise you're running the render frames very close to your refresh rate. Video cards can lock applications to run in VSYNC so I don't have enough information to go on, but in most cases when you run any game loop with a variable frame rate not locked down, you will never get smooth motion unless you use interpolation, or extrapolation without locked rates.

If you toggle off all your interpolation code (only draw based on actual positions), run something like FRAPS so you can report back the FPS you're getting. It's extremely odd to get smooth motion unless something is capping your FPS close to your refresh rate.

Other methods people use to get smooth fps in their loops would be rendering at same rate as your refresh rate, example 60 FPS for 60 hertz, or just use VSYNC so it matches your monitor refresh.

SFML does not handle interpolation by default.

Have a great long weekend as well, let me know what FRAPS shows.

Programmer and 3D Artist

Josheir

Author

489

September 04, 2017 01:08 AM

Just understood what you meant when I changed the ticks per second to 10.f on your sample!

Thank you so much,

Josheir

Fix Your Timestep

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Fix Your Timestep

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