fixed time step movement

Excelcius

Hi all Im currently struggling with a physics based sprite class which includes gravity and vector based collision detection. I started using time based movement but have hit a brick wall where by even if I use an elasticity value of 1.0 the object keeps losing or gaining height randomly. Ive calculated things like the exact x y of the collision, the overshoot after a collision, to bounce the object back the correct distance and when the velocity changes from a negative to a positive value to make sure the objects position is relative to where it would have been had the update moved it to the correct height before falling back again but still it behaves erratically more often than not loosing height.

I was about to ditch my work and just use plain frame based movement but As I understand it the SPMovieclip class uses a fixed time step system of animation (at least as far as I can tell) that being the case my animation and movement will inevitable fall out of sync, so my question is a) how does everyone else deal with this issue and b)is there any inbuilt time step feature in Sparrow ie instead of the onenterframe passing back elapsed time, passing back the number of frames passed.

Thanks in advance

Daniel

Yeah, this stuff is challenging to get right! 😐

So you mean you want an SPMovieClip that's not advanced the actual passed time of the current frame, but a fixed value, right?

There are two ways to do this:

a) Make a subclass of SPMovieClip that overrides the "advanceTime:" method and calls "[super advanceTime:TIME_STEP]" with a constant TIME_STEP.

b) Alternatively, don't add the clip to the juggler, but call it's "advanceTime:" method manually, also with a fixed time value.

Daniel

Oh, and if you want one solution that works with all your game elements: just don't use the default juggler, but create your own Juggler. In the ENTER_FRAME event, advance that juggler with a fixed time step. If you add all your animations and objects to that juggler, you'll have a fixed time step animation system.

Excelcius

Hi Daniel thanks, your not wrong about it being a challenge mate Ive spent 3 weeks tearing my hair out trying to get my head around it 😉 I have written my own movieclip class which has a few added features I have kept your time step solution as I think its the best solution when animating. What I want to do is have my physics sprite use the same time step system, hopefully that will resolve the inconsistencies assuming its the result of floating point precision, whilst maintaining synchronicity.

Im a bit reluctant to use the juggler as Ive set my project up to use one main ENTER_FRAME event which propagates down keeping it abstract I say that from the perspective of my understanding of the juggler is that its used in conjunction with tweens which I tend to avoid preferring to write the movement code in the object itself, maximising reuse of objects as opposed to creating a tween and throwing it away after use.

My brain is a bit frazzled and was looking for a bit of code to implement the time step in my main ENTER_FRAME event, and bring a bit of sanity to the proceedings, so I don't spend weeks going down another blind alley, this coding lark can be a complete nightmare sometimes 😉