[ODE] world update - best practices?

Jon Watte (ODE) hplus-ode at mindcontrol.org
Wed May 23 16:00:41 MST 2007

Here's a write-up:



          / h+

Greg Mayer wrote:
> Hello
> I apologize if this issue has alreday been answered in the mailing lists 
> - I did not have any success searching the mailing list for this topic.
> I am evaluating various physics APIs and I'm trying to understand the 
> best way to setup the main update loop in ODE.
> It seems for any game application with more than a small amount of 
> physics, dWorldQuickStep is the way to go.
> The documentation heavily stresses that this function should be called 
> with a constant-size time step.
> Also, there is a function to set dWorldSetQuickStepNumIterations.
> However, there are a few things I'm not clear on.
> Presumably, if you have an app running in real-time, you will have some 
> sort of update loop like:
> UpdateWorld(float dtSeconds)
> {
> }
> In which you do your collision detection and solving and stepping of the 
> dynamic simulation.
> One would hope that this update is being called very frequently and with 
> a value that doesn't vary much. Ideally dtSeconds would be something 
> like 0.016 for a game running at 60fps.
> However, as anyone who's written a game knows, in practice the incoming 
> value will vary slightly, and in bad scenarios (especially during 
> development) it could be several times higher than the desired 0.016.
> Therefore, it seems like good practice to wrap the dWorldQuickStep (and 
> collision detection/solving, and external game force applications) call 
> in a loop which breaks down the incoming dtSeconds into the 
> constant-size time steps that ODE desires.
> Where things get unclear though are the values that are close to, but 
> not exactly, the desired constant-sized step.
> Say for example you have an incoming value of 0.017.
> What is the best practice in this case??
> Does it _really_ matter to ODE if you pass in 0.016 one update, and 
> 0.017 the next? Does it explicitly cache the 0.016
> and have some sort of optimization for the _exact_ same value being used 
> repeatedly.
> This would seem odd to me, but I ask because in the wiki-manual, in 
> large red letters it says "Variable Step Size: Don't!"
> So an alternative in this case would be to have a loop which starts 
> taking 0.016 sized bites out of the incoming step and calls 
> dWorldQuickStep. However, in this case that would mean you make on call 
> with 0.016, and are left with a remainder of 0.001. Now what to do? 
> Making a second call to the update with this value seems to deviate far 
> more from the constant-step-size requirement than to have simply called 
> dWorldQuickStep with 0.017.
> Or one could have some sort of static "remaninder" variable in the 
> update loop and wait until the value accumulates to at least 0.016, but 
> then you are not strictly running in sync with the rendering and the 
> rest of the game (though the difference may not be noticeable)
> A further complication on this is the dWorldSetQuickStepNumIterations - 
> this seems to set an internal counter for substeps so that you do not 
> need to maintain such a loop manually in your WorldUpdate, but shouldn't 
> this number of substeps be dependent on the size of the incoming dtSeconds?
> So are you supposed to call dWorldSetQuickStepNumIterations every frame?
> If someone could please explain these issues in ODE and suggest the best 
> practice I would really appreciate it.
> Thanks in advance
> Greg
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