Manually resetting body positions [Box2d]

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Manually resetting body positions [Box2d]

(8 posts) (2 voices)

Started 2 years ago by eshirt
Latest reply from eshirt

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eshirt
Member

I'm a little confused here. It seems like this should work, but I'm not getting the desired results. My game has two "teams" and after a round, the players' positions need to get reset to a pre-determined position (named here, startingLine). I have everything working fine with collisions, filtering user data, etc., but when I call my "movePlayerPositions" method, the bodies on the screen don't update.

Here's the relevant code:

- (void) movePlayerPositions {
	// get new position array
	playerPos = [[self createPlayerPositions] mutableCopy];

	int i = 0;

	// for each body in the world
	for (b2Body* b = world->GetBodyList(); b; b = b->GetNext()) {
	// get the fixture list
		for (b2Fixture* f = b->GetFixtureList(); f; f = f->GetNext()) {
		// if the user data matches the right team
			if ([(AtlasSprite *)f->GetUserData() isEqual:@"mySprite"]) {
				CGPoint p = [[playerPos objectAtIndex:i] CGPointValue];
				b2Vec2 point = b->GetPosition();
				NSLog(@"Location of Body before transform is %f, %f", point.x, point.y);
				b->SetTransform(b2Vec2(p.x, p.y), 0);
				point = b->GetPosition();
				NSLog(@"Location of Body AFTER transform is %f, %f", point.x, point.y);

				i++;
			}
		}

	}

}

- (NSArray *) createPlayerPositions {
	NSArray *tmp = [NSArray arrayWithObjects:	[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 1), 9.5)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 1), 7.125)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 1), 4.75)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 1), 2.375)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 1), 0)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine - 3), 8.3125)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine -  3), 5.9375)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine -  3), 3.5625)],
					[NSValue valueWithCGPoint:CGPointMake((myGame.startingLine -  3), 1.1875)],
					nil];
	return tmp;
}

// to call this in my game, I'm using
[self movePlayerPositions];

Basically, I have an array of points which make up the "lineup." I need to move each body on the team to one of those points. In the console, I am showing that the positions of the body are actually getting changed by the b->SetTransform() method correctly. The problem is the Sprites and bodies on the screen are not getting updated in the World.

Am I missing an "update" or "draw" type command that will reset my view?

Thanks in advance!

Posted 2 years ago #

eshirt
Member

Ok, after doing some more research, I'm even more confused. Everywhere I find mentions of SetTransform() or its previous version SetXForm() it's used exactly as I use it here. The only other suggestion I've found was to call b->WakeUp() after the transform. Tried that and it didn't change anything. I've also tried calling [self draw] with no change.

So, my methods are being called (all my NSLog statements are displaying in the console) but the bodies and Sprites aren't getting updated. I can post more of my code if it helps you figure it out, but I really don't know where the problem could be.

Posted 2 years ago #
blackmouth
Member
eshirt,

Do you have debug draw data turned on for Box2D? When I look at your code it seems like it should work. The only thing I would verify is if the box2d bodies do actually move using debug draw. I'm not sure if this matters but I would also store the new positions in a variable and pass it to SetTransform, as well as the previous angle.
```
float32 angle = b->GetAngle();
const b2Vec2 pos(p.x,p.y); // maybe (p.x/PTM_RATIO,p.y/PTM_RATIO), verify with NSLog
b->SetTransform(pos, angle);
NSLog(@"Body AFTER transform, Body.x:%f,Body.y:%f",b->GetPosition().x,->GetPosition().y);
```
Posted 2 years ago #
eshirt
Member

Yeah, debug draw is on - and the bodies aren't moving at all after I call setTransform() even though the NSLog statement shows that the position has moved. I'm pretty much stumped here.

Posted 2 years ago #
eshirt
Member
Ok, I am even more confused now, if that's even possible. I have tried just about everything. I know for sure that there are bodies in my game world that match some specific UserData. I'm able to use ApplyForce to them by iterating through the bodyList then the fixtureList. I'm trying to do the same thing here:
```
for (b2Body* b = world->GetBodyList(); b; b = b->GetNext()) {
		for (b2Fixture* f = b->GetFixtureList(); f; f = f->GetNext()) {
			if ([(AtlasSprite *)f->GetUserData() isEqual:@"mySpriteSel"] || [(AtlasSprite *)f->GetUserData() isEqual:@"mySprite"]) {
				NSLog(@"THE POSITION OF THE BODY IS %f, %f", b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
			}
		}
	}
```
And what I get in the console is this:
```
THE POSITION OF THE BODY IS -126.621368, 38.000000
THE POSITION OF THE BODY IS -126.621368, 114.000000
THE POSITION OF THE BODY IS -126.621368, 190.000000
THE POSITION OF THE BODY IS -126.621368, 266.000000
THE POSITION OF THE BODY IS -62.621368, 0.000000
THE POSITION OF THE BODY IS -62.621368, 76.000000
THE POSITION OF THE BODY IS -62.621368, 152.000000
THE POSITION OF THE BODY IS -62.621368, 228.000000
THE POSITION OF THE BODY IS -62.621368, 304.000000
```
Now, I KNOW these are wrong because on the screen, I have bodies scattered all over the x-axis, not in two distinct lines. How could I be getting different position values than what's showing up on my screen? I'm in debug mode and I can see the bodies under my Sprites. They're all synced up by the tick: method.

Is there a "synchronize" command or something that I don't know about? This is driving me bonkers! :)

EDIT: I've also tried using b->GetWorldCenter() and those are also "wrong".
Posted 2 years ago #
eshirt
Member

So, I set a NSLog() in my tick: method to track my "selected" player. That works fine.

For some reason, when I iterate through the bodies and fixtures outside of the tick: method, I am getting different points. Not only are they different points, but they don't seem to correspond to any shapes on the screen at all.

If anyone has any insight into this, I'm dying over here :)

Posted 2 years ago #
eshirt
Member
Can anyone tell me why the same code:
```
for (b2Fixture* f = b->GetFixtureList(); f; f = f->GetNext()) {
if ([(AtlasSprite *)f->GetUserData() isEqual:@"mySpriteSel"]) {
	NSLog(@"DEBUG: POSITION OF  SPRITE IS %f, %f", f->GetBody()->GetPosition().x, f->GetBody()->GetPosition().y);
}
}
```
when put in my tick: method shows me the correct position, but when placed in another method and called from my contact listener, does not show anything?

For some reason, my coordinates are only accurate within the tick method.
Posted 2 years ago #
eshirt
Member

Ok, I think the problem was actually in my ContactListener. I was calling [GameLayer node] doSomethingWithPoint] to trigger the "movePlayerPositions" method above. Then I realized that +node actually calls alloc & init on the Layer, thus creating a second init of my GameLayer.

Obviously, this created havoc on the position of the bodies since they were created in the init method.

I started a different thread here:

http://www.cocos2d-iphone.org/forum/topic/2800

to discuss possible solutions. Just posting it here to wrap up the thread for others who may be having the same problem.

Posted 2 years ago #

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