Strange problem with main loop / movement

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Strange problem with main loop / movement

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Started 1 year ago by Martin
Latest reply from Martin

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Martin
Member
Hi! I'm having a really strange problem that I just can't fix. I have a singleton that is supposed to manage every game object. It has the update scheduled by
[self scheduleUpdate];

This is the complete code of the update method of this object:
```
-(void) update:(ccTime)delta
{
	float d = delta; 

	for (MovingObject* obj in objectsArray)
	{
		[obj update:d];
	}
	[self collisionChecks];
	[self combat];
}
```
I think that d = delta makes no sense, right? I added it as a test while trying to solve my problem.
My MovingObject's update method starts like this:
```
-(void) update:(ccTime)delta
	// trying to factor in the time delta for movement:
	float timeDeltaFactor = delta / ( 1.0f/60.0f );
	// capping it at 4 to avoid warping that kills collision detection
	if (timeDeltaFactor > 4) timeDeltaFactor = 4.0f;
	// currentSpeed and speedMultiplier can be ignored for now, they don't change atm
	float speedLeft = currentSpeed * timeDeltaFactor * speedMultiplier;

	// this is what I'm testing with now, the real movement code is more complex.
	self.position = getPointInDirectionFromLocationWithDistance(self.angle, self.position, speedLeft);
	speedLeft = 0;
```
I have MovingObjects that shoot and the projectiles are MovingObjects themselves. I create two next to each other and they should be flying perfectly synchronous, but they don't. Most times but not always the left one is faster, or the right one slower depending on how you see it. But it isn't even an absolute "slower" which would increase the gap between them over time. It's like they start synchronous, after some pixels one of the projectiles starts to lag behind, catches up a little later, lags behind again, but the distance doesn't change noticably over time.
How can this be? How can I debug this?
I thought it might have something to do with how the director works or which type of director I choose. I tried them all out but I can't see a difference.
Any help would be greatly appreciated, I'm stuck on this.
Posted 1 year ago #
Benihana
Member

Try printing the timeDeltaFactor to console (include an identifier so you know which belongs to which projectile). My bet is that it is fluctuating above and below your threshold, so sometimes it is clipping the movement (not moving as far as it should for that step).

Posted 1 year ago #
Martin
Member

Thanks a lot for your reply. I tried this now. My thought was that it should not matter where the timeDeltaFactor is clipped or if it is clipped because it should always be in sync as all objects get the same delta time to respond to. I've also logged the x position of the objects so that you can see the drift.
My new test setup is two scripted spawners that spawn a projectile every 0.5 seconds to the right. They should be in sync, but they are not after a while. This is how my log starts:

2011-04-08 12:57:12.658 cocos2d-project[94819:207] main loop update delta: 0.033317
2011-04-08 12:57:12.692 cocos2d-project[94819:207] timedelta: 2.024640
2011-04-08 12:57:12.693 cocos2d-project[94819:207] x position: 57.086239
2011-04-08 12:57:12.693 cocos2d-project[94819:207] timedelta: 2.024640
2011-04-08 12:57:12.694 cocos2d-project[94819:207] x position: 57.086239
2011-04-08 12:57:12.695 cocos2d-project[94819:207] main loop update delta: 0.033744
2011-04-08 12:57:12.725 cocos2d-project[94819:207] timedelta: 1.975080
2011-04-08 12:57:12.726 cocos2d-project[94819:207] x position: 63.999020
2011-04-08 12:57:12.726 cocos2d-project[94819:207] timedelta: 1.975080
2011-04-08 12:57:12.727 cocos2d-project[94819:207] x position: 63.999020
2011-04-08 12:57:12.728 cocos2d-project[94819:207] main loop update delta: 0.032918
2011-04-08 12:57:12.742 cocos2d-project[94819:207] timedelta: 1.013460
2011-04-08 12:57:12.744 cocos2d-project[94819:207] x position: 67.546127
2011-04-08 12:57:12.748 cocos2d-project[94819:207] timedelta: 1.013460
2011-04-08 12:57:12.750 cocos2d-project[94819:207] x position: 67.546127

The line with main loop update delta is the time delta from my main loop, it marks one cycle of movement. The timedelta lines are the timeDeltaFactor you suggested to log and the x position is the position of the object that is beeing moved. It starts out with two objects being perfectly in sync. Later when there are more shots flying around there should be pairs of projectiles with the same position.
Like this:

2011-04-08 12:57:14.652 cocos2d-project[94819:207] main loop update delta: 0.033332
2011-04-08 12:57:14.675 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.676 cocos2d-project[94819:207] x position: 473.536804
2011-04-08 12:57:14.676 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.677 cocos2d-project[94819:207] x position: 473.536804
2011-04-08 12:57:14.678 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.679 cocos2d-project[94819:207] x position: 368.221680
2011-04-08 12:57:14.679 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.680 cocos2d-project[94819:207] x position: 368.221680
2011-04-08 12:57:14.681 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.681 cocos2d-project[94819:207] x position: 260.032318
2011-04-08 12:57:14.682 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.682 cocos2d-project[94819:207] x position: 260.032318
2011-04-08 12:57:14.683 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.683 cocos2d-project[94819:207] x position: 151.519257
2011-04-08 12:57:14.684 cocos2d-project[94819:207] timedelta: 1.999920
2011-04-08 12:57:14.684 cocos2d-project[94819:207] x position: 151.519257

But then the strange thing happens and the next iteration of the loop shows this:

2011-04-08 12:57:14.685 cocos2d-project[94819:207] main loop update delta: 0.033332
2011-04-08 12:57:14.708 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.709 cocos2d-project[94819:207] x position: 480.509216
2011-04-08 12:57:14.729 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.730 cocos2d-project[94819:207] x position: 375.194092
2011-04-08 12:57:14.730 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.731 cocos2d-project[94819:207] x position: 375.194092
2011-04-08 12:57:14.732 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.734 cocos2d-project[94819:207] x position: 267.004730
2011-04-08 12:57:14.735 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.735 cocos2d-project[94819:207] x position: 267.004730
2011-04-08 12:57:14.736 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.736 cocos2d-project[94819:207] x position: 158.491669
2011-04-08 12:57:14.751 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.751 cocos2d-project[94819:207] x position: 158.491669
2011-04-08 12:57:14.752 cocos2d-project[94819:207] timedelta: 1.992120
2011-04-08 12:57:14.752 cocos2d-project[94819:207] x position: 165.464081

It's still 8 active projectiles, but the first and the last value stand alone, no pairs. Also the first value should maybe not even exist because at more than 480px in x direction projectiles should get destroyed.

This is the next itteration:

2011-04-08 12:57:14.754 cocos2d-project[94819:207] main loop update delta: 0.033202
2011-04-08 12:57:14.775 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.777 cocos2d-project[94819:207] x position: 487.534973
2011-04-08 12:57:14.777 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.778 cocos2d-project[94819:207] x position: 389.192261
2011-04-08 12:57:14.778 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.779 cocos2d-project[94819:207] x position: 281.002899
2011-04-08 12:57:14.779 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.783 cocos2d-project[94819:207] x position: 281.002899
2011-04-08 12:57:14.784 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.784 cocos2d-project[94819:207] x position: 172.489853
2011-04-08 12:57:14.785 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.785 cocos2d-project[94819:207] x position: 179.462265
2011-04-08 12:57:14.786 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.789 cocos2d-project[94819:207] x position: 63.998180
2011-04-08 12:57:14.789 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.790 cocos2d-project[94819:207] x position: 63.998180
2011-04-08 12:57:14.791 cocos2d-project[94819:207] timedelta: 3.999480
2011-04-08 12:57:14.792 cocos2d-project[94819:207] x position: 77.996361

Now there are 9 objects where there should be an even number... I'm beginning to wonder if the problem could be totally unrelated to my movement code and be caused by the way I delete my objects. I'll do some more logging on that one and assume for now that the timedelta is not the problem.

Edit: I tried to solve it by changing the way objects are deleted. This resulted in screwing up the object removal so that they don't get removed at all, but at least their speed stays in sync, which means I'm on the right track I think. :)
Thanks for your help Benihana!

Posted 1 year ago #
Benihana
Member

No problem, I think from the looks of it, it is certainly your deletion code. Are you only allowing for one deletion per cycle? Are you reusing objects (that would have dirty values instead of what you expect). I'd also only use 2 or 3 projectiles for testing.

Posted 1 year ago #
Martin
Member

Yes, it was the deletion code. I made some severe mistakes there. Now instead of deleting objects right away when they crash or leave the screen I only store a reference to them in an array called objectsMarkedForDeletion and the only place where I iterate over that array and remove everything in it from my main objectsArray is in the main loop before I update all my objects, check for collisions etc.. That way hopefully the array does not change when something iterates over it. I also do something similar when checking for line of sights and shooting because shooting is creating new objects is changing the array I'm iterating over. I had that right, only my deletion was faulty.
Also yes, I'm reusing objects heavily, but I always call a reInit method on them that should (and it seems to) reset any value that could be wrong.

Posted 1 year ago #

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