My Sim: Timing

Hi there. Although this project of mine is now finished, I am still tinkering around with it I hear a lot of mention about changing the physics frequency to 5 hz or 1 hz or whatever What exactly does this mean? In my sim, the time step is calculated as the time difference between program cycles, not given as a constant time. It performs the physics using the calculated timestep, draws the screen, calculates a new time step, does the physics, draws the screen.... etc. So how is a fixed timestep implementable without making the game run in slow mo as it would if I put the timestep to 1 hz (effectively making it run at a 1000th of a second each frame Alex Smith

"Alex Smith" <[email hidden]> wrote in message news:abrgfi$7ks$[email hidden]... > Hi there. > > Although this project of mine is now finished, I am still tinkering around > with it > > I hear a lot of mention about changing the physics frequency to 5 hz or 1hz > or whatever Now THAT would be slow! :-) > What exactly does this mean? Quite simply, it means how many times per second (1 Hz = once per second) you update the physics. For arcade physics, a few dozen Hz might do, but for a real physics simulation it needs to be in the hundreds to maintain stable (= cars not launching to Jupiter & beyond) and accurate simulation. With special integration techniques like implicit integration (which I haven't tried) it's possible to use less. In my sim, I use 250 Hz. > In my sim, the time step is calculated as the > time difference between program cycles, not given as a constant time. > It performs the physics using the calculated timestep, draws the screen, > calculates a new time step, does the physics, draws the screen.... etc. The way you're doing it, your physics are dependent on your graphics refresh rate - if you get 100 fps, your physics are also updated at 100 Hz which might be enough - but if your graphics slow down due to a lot of alpha blending or other stuff, your physics calculations will slow down also, which could lead to stability problems. In my view, it's best to keep graphics and physics updates separate, especially if you intend to implement some sort of replay mode. > So how is a fixed timestep implementable without making the game run inslow > mo as it would if I put the timestep to 1 hz (effectively making it run ata > 1000th of a second each frame You shouldn't have much trouble modifying your existing code to do the following: Loop physics calculations with a fixed timestep until it catches up with realtime, draw the screen, loop physics again, draw the screen... Petri Blomqvist

Basically, you have a variable that keeps track of the time elapsed. Each frame, you see if the time elapsed since the last update is bigger than a certain value. If it isn't, do nothing and loop around. If it is, reset the elapsed time and do the physics calculations. Then repeat. This basically decouples the sim frequency from the speed of the computer running the program. It also should allow you to do a replay just by saving the control inputs on each frame (as the game will reproduce exactly the same behaviour). There is a great article on gamasutra.com about fixing the frequency of a sim for replays (http://www.gamasutra.com/features/20010713/dickinson_01.htm). You might have to register (for free) to read it. This is ripped out of the Quake 2 source code as an example, this is done in WinMain. oldtime = Sys_Milliseconds (); while (1) { do { newtime = Sys_Milliseconds (); time = newtime - oldtime; } while (time < 1); Qcommon_Frame (time); oldtime = newtime; } Hope that helps, Nick.

I'd be interested in the answer to this as well. I'm basically doing what your doing but I thought if I needed smaller time steps I could possibly run through the physics sim loop say 2 or more times per graphics loop. In other words, do the physics simulation twice then update the screen, then repeat... is this how everyone else is doing it? or am I missing something?

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> Hi there. > > Although this project of mine is now finished, I am still tinkering around > with it > > I hear a lot of mention about changing the physics frequency to 5 hz or 1 hz > or whatever > > What exactly does this mean? In my sim, the time step is calculated as the > time difference between program cycles, not given as a constant time. >

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>I'd be interested in the answer to this as well. I'm basically doing >what your doing but I thought if I needed smaller time steps I could >possibly run through the physics sim loop say 2 or more times per >graphics loop. > >In other words, do the physics simulation twice then update the >screen, then repeat... is this how everyone else is doing it? or am I >missing something? >

Yep, but I do it a lot more than twice per graphics frame usually. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm

You're absolutely right, my sim is very unstable at times, especially low speed, when I get about 80fps. so is this a solution??? // Physics frequency in ms fixedTimeStep = 0.002; // Counter for how much time has passed in the physics world physicsTimeElapsed = 0.0; // Actual system time elapsed between frames realTimeElapsed = thisFrameTime - lastFrameTime (from system clock) // While the physics is not in sync with system time while (physicsTimeElapsed < realTimeElapsed { do_physics physicsTimeElapsed+=fixedTimeStep } This is just off the top of my head, so I might have the wrong end of the physics stick :)

On 15 May 2002 01:15:47 GMT, [email hidden] (J. Todd Wasson) wrote:

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>>I'd be interested in the answer to this as well. I'm basically doing >>what your doing but I thought if I needed smaller time steps I could >>possibly run through the physics sim loop say 2 or more times per >>graphics loop. >> >>In other words, do the physics simulation twice then update the >>screen, then repeat... is this how everyone else is doing it? or am I >>missing something? >> > > Yep, but I do it a lot more than twice per graphics frame usually.

Though sometimes I run at 250Hz and the framerate at certain simple tracks is catching up with the physics! I think it draws the same frame twice in that case, hm. Wow. :) I'll move to 500Hz/1000Hz anyway. Gotta keep ahead of those gfx boards! ;-) Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

I have implemented the said pseudocode and it works lovely! Thanks for the input guys!