chipmunk stepper inside cocos2d « cocos2d for iPhone

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chipmunk stepper inside cocos2d

(9 posts) (3 voices)

Started 2 years ago by rolando
Latest reply from coolman

Tags:

chipmunk

rolando
Moderator
Hi all,
Don't remember where, but somewhere in the old discussion group, I found this code to use as a Chipmunk stepper:
```
int steps = 2, i;
cpSpace *space = SPACE(rb_gv_get("space"));
cpFloat dt = delta/(cpFloat)steps;
for (i=0; i < steps; i++)
  cpSpaceStep(space, dt);

cpSpaceHashEach(space->activeShapes, &eachShape, nil);
cpSpaceHashEach(space->staticShapes, &eachShape, nil);
```
The thing is that my simulation was running really jerky. So searching in the chipmunk forums, I found that you should supply a constant dt to cpSpaceStep in order to get a better simulation, so reading a post[1] referenced in the thread[2], I came up with the following code:
```
- (void)chipmunk_step:(ccTime)delta {
	cpSpace *space = SPACE(rb_gv_get("space"));
	cp_accumulator += delta;
	while (cp_accumulator >= cp_dt) {
		cpSpaceStep(space, cp_dt);
		cp_accumulator -= cp_dt;
	}

	cpSpaceHashEach(space->activeShapes, &eachShape, nil);
	cpSpaceHashEach(space->staticShapes, &eachShape, nil);
}
```
It's giving me a lot better results. Of course, you have to schedule chipmunk_step on each tick:
```
[myObj schedule:@selector(chipmunk_step:)];
```
What do you guys use as a stepper?

Oh, I forgot, cp_accumulator and cp_dt are defined as globals:
```
cpFloat cp_dt = 0.01f;
cpFloat cp_accumulator = 0.0f;
```
[1]: http://gafferongames.com/game-physics/fix-your-timestep/
[2]: http://www.slembcke.net/forums/viewtopic.php?f=1&t=356&p=1647&hilit=jerk#p1647

regards,
rolando./
Posted 2 years ago #
edisonslabs
Member

It would give a smoother simulation, but it is also less accurate. For example, shall there be a long time passed before the step function is called, your objects would still only move one "time step". Where as if you apply the time difference dt, the objects would move based on the actual distance that it should have moved during that time period.

I use what you have on the top. I had a discussion with some classmates about this. I think the conclusion was if you want a more realistic simulation (regardless of how smooth it runs), you should use the actual time difference. If you only care about how smooth it runs, then using a constant dt is fine.

Posted 2 years ago #
rolando
Moderator

but that's what the accumulator is for: iterate over a constant dt until the elapsed time has been achieved. In the end, you get the same final dt, but the space was iterated over smaller constants dt...

Posted 2 years ago #
rolando
Moderator

the whole idea is to get consistent and smoother simulation, regardless the FPS.

Posted 2 years ago #
coolman
Member
I would never use this:
```
cpSpaceHashEach(space->activeShapes, &eachShape, nil);
	cpSpaceHashEach(space->staticShapes, &eachShape, nil);
```
In the Sblemke forum tehy say that this is used by convenience for the samples, but not a well practise.

If you don't move our static shapes, this is not needed:

cpSpaceHashEach(space->staticShapes, &eachShape, nil);

Also iterating over the activeshapes is way better having an array of pointers and doing a for with a custom function. I explain this more. Imagine that for active shaped you have bullets and ballons. Sometimes you want to just iterate over the bullets or over the ballons, but not for both of them.

Actually my Chipmunk step is done in this way:
```
-(void) step: (ccTime) dt
{
	int steps = 1;
	cpFloat dt = 1.0f/60.0f/(cpFloat)steps;

	for(int i=0; i<steps; i++){
		cpSpaceStep(space, dt);
	}
}
-(void) draw
{
	for(int i=0; i<cont_Sprites; i++)
	{
		[self eachSprite:i];
	}
```
Posted 2 years ago #
rolando
Moderator

coolman: you're right. Most of the time you would like to iterate and reposition the relevant sprites, but for the game that I'm testing on, there are only 4 active shapes, and I need to reposition them all...
I removed the static shape iteration, and tested my stepper: good but not good enough.

I'm trying your stepper now... Works great!!!! :-D
Thanks!!!

Posted 2 years ago #
coolman
Member

Glad it helped.

Also try to put a 240.0f instead the 60.0f there

So finally how you use this?

Posted 2 years ago #
rolando
Moderator

I assumed the 60.0f correspond to the update frequency of the Director, is that so?...

This is how I'm using it: I scheduled a selector for every tick (in your code, step:) and after the cpSpaceStep look, I called cpSpaceHashEach(space->staticShapes, &eachShape, nil). Anyway, here's the whole code:

http://github.com/funkaster/shinycocos/blob/9ade4b12c23db3f935e13a0b577d80ed4cf23ca0/Integration/SC_CocosNode.m

the stepper is in CocosNode#chipmunk_step (Line 106) and it's scheduled in become_chipmunk_stepper (Line 503).

Again, thanks coolman!
rolando./

Posted 2 years ago #
coolman
Member

>I assumed the 60.0f correspond to the update frequency of the Director, is that so?...

Ah, yes it is.

Yep, this is how I use it, works fine. I didn't checked if having step with a value different than one would make any difference.

Posted 2 years ago #

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