Calculating a trajectory with chipmiunk

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Calculating a trajectory with chipmiunk

(8 posts) (2 voices)

Started 2 years ago by gargantuan
Latest reply from gargantuan

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gargantuan
Member

I need to implement a little bit of Ai that can apply an impulse to a chipmunk in order to hit a target. Like a cannon ball being fired from a cannon.

My maths is terrible, as is my physics, but i've been reading up and came up with the following (from this wikipedia entry)

float x = target.x = launchPos.x; float y = target.y = launchPos.y; float g = 9.8; float v = 100; float angle1, angle2;
float tmp = pow(v, 4) - g * (g * pow(x, 2) + 2 * y * pow(v, 2)); if(tmp < 0){ NSLog(@"No Firing Solution"); }else{ angle1 = atan2(pow(v, 2) + sqrt(tmp), g * x); angle2 = atan2(pow(v, 2) - sqrt(tmp), g * x); } // Split the velocities float vVel = v * sin(angle2); //NSLog(@"Vertical Velocity: %f", vVel); float hVel = v / tan(angle2); //NSLog(@"Horizontal Velocity: %f", hVel);
CGPoint force = cpv(hVel, vVel);

Which should give me the angle, from which I can calculate the horizontal and vertical velocities needed to launch the projectile.

However, it's not working, which doesn't surprise me at all for a number of reasons. Firstly because I'm terrible at Maths and Physics, but also because I'm confused by a couple of other things.

1. This method doesn't seem to take mass into account. Should it? I would have thought that was quite important?

2. Box2d has PTM_RATIO, but I can't find anything like that in chipmunk, so how do my values correspond to the space values in chipmunk?

3. I know of radians and how they differ to degrees, and how to convert between the two. But which should I be using here? should I be converting angle1 and angle2 to degrees? Even if I do, It still doesn't work.

In summary, there's a lot about physics and chipmunk that I don't understand. So I'm here, asking for help. I did find this thread which seems to touch on the same issue, but there's no mention of angles, and it doesn't seem to work in this case either.

Is there something in chipmunk that I can use to figure this out, or if anyone has had to figure this out themselves, I'd really appreciate some help.

cheers
-t

Posted 2 years ago #
mobilebros
Moderator
I'm pretty sure you don't need anything like PTM_RATIO, just use the coordinate system cocos uses. I think your math is good up to the point of calculating the angles (in radians btw) so.....

to get the angle as vertical and horizontal components, just do:
```
CGPoint direction = cpvforangle(angle2);
```
Now you have a direction and a velocity, the direction is normalized, so all you have to do is set the velocity on your body and you should be golden:
```
body->v = cpvmult(direction, v);
```
Now as long as your gravity in chipmunk is (0,-9.8) and the damping is set to 1.0, you should be ok I think....
Posted 2 years ago #

Thanks for the pointers, but it appears Im still getting totally the wrong results here, so i'm doing something totally wrong somewhere.

I'm using SpaceManager to handle the chipmunk stuff which is set up like this

spmgr = [[SpaceManager alloc] init];
spmgr.constantDt = 1/55.0;
spmgr.gravity = ccp(0,-980);
spmgr.damping = 1.0;
[spmgr start];

The class that needs to figure out the required angle is called CPUPlayer and the method is called like this

CPUPlayer *cpu = [[CPUPlayer alloc] init];
[cpu calculateShotForTarget:cpv(1340,40) from:cpv(200,150)];

inside CPUPlayer, the method now looks like this

- (void) calculateShotForTarget:(CGPoint)target from:(CGPoint)launchPos
{
	float x = target.x - launchPos.x;
	float y = target.y - launchPos.y;
	float g = 9.8;
	float v = 1099;
	float angle1, angle2;

	float tmp = pow(v, 4) - g * (g * pow(x, 2) + 2 * y * pow(v, 2));

	if(tmp < 0){
		NSLog(@"No Firing Solution");
	}else{
		angle1 = atan2(pow(v, 2) + sqrt(tmp), g * x);
		angle2 = atan2(pow(v, 2) - sqrt(tmp), g * x);
	}

	cpVect direction = cpvforangle(angle2);

        // The projectile has a Mass of 1, so there's no need to multiply v
	CGPoint force = cpvmult(direction, v);

	LaunchData *shot = [[[LaunchData alloc] initWithForce:force andPosition:launchPos] autorelease];
	[[NSNotificationCenter defaultCenter] postNotificationName:PlayerDidFinishAiming object:shot];

}

LaunchData is just a handy way to pass around several CGPoints but it looks like this.

@implementation LaunchData

@synthesize forceX, forceY, positionX, positionY;

- (id) initWithForce:(CGPoint)pForce andPosition:(CGPoint)pPosition
{
	if( (self = [super init]) )
	{
		forceX = pForce.x;
		forceY = pForce.y;
		positionX = pPosition.x;
		positionY = pPosition.y;
	}
	return self;
}

@end

and finally, the force is applied to the rigid body like this

- (void) launchBall:(NSNotification *)notification
{

	LaunchData *ldata = [notification object];
	ballShape = [spmgr addCircleAt:cpv(ldata.positionX,ldata.positionY) mass:1.0 radius:32];
	ballShape->e = 0.9;

	ball = [BallSprite spriteWithShape:ballShape manager:physicsSpritesManager rect:CGRectMake(226, 67, 64, 64)];
	[physicsSpritesManager addChild:ball];

	[ball applyImpulse:cpv(ldata.forceX, ldata.forceY)];

}

I think that's about everything. Neither angle1 or angle2 reach the target. Angle1 fires almost straight up. I'm completely lost with this now.

Posted 2 years ago #

mobilebros
Moderator
Ok so gravity is -980 is this example, I believe your g variable should also be -980 then (not sure about negative), and also 1099 for v seems like way to much for a mass of 1, perhaps 50 or so would be better.

Did you try removing the line:
```
[ball applyImpulse:cpv(ldata.forceX, ldata.forceY)];
```
and putting:
```
ball.shape->body->v = cpv(ldata.forceX, ldata.forceY);
```
because it actually is the velocity you calculated.

p.s. Yes I know, ugly C code, someday the spacemanager stuff will making setting velocity, etc look obj-c-ish.
Posted 2 years ago #
gargantuan
Member

awesome, it seems to be working now. I'm a bit embarrassed I didn't think to change 'g' to -980 myself, but ho hum. This has been bugging me for weeks and i've just woken up to this answer, which means I'm in for a good days coding. Cheers

Also, keep up the awesome work on space manager, it's a game changer as far as I'm concerned.

Posted 2 years ago #

gargantuan
Member

Just incase anyone stumbles across this and has the same issue, the final method looks like this

- (void) calculateShotForTarget:(CGPoint)target from:(CGPoint)launchPos
{

	float x = target.x - launchPos.x;
	float y = target.y - launchPos.y;
	float g = 980;
	float v = 1000;
	float angle1, angle2;

	float tmp = pow(v, 4) - g * (g * pow(x, 2) + 2 * y * pow(v, 2));

	if(tmp < 0){
		NSLog(@"No Firing Solution");
	}else{
		angle1 = atan2(pow(v, 2) + sqrt(tmp), g * x);
		angle2 = atan2(pow(v, 2) - sqrt(tmp), g * x);
	}

	cpVect direction = cpvforangle(angle1);
	CGPoint force = cpvmult(direction, v);

	NSLog(@"force = %@", NSStringFromCGPoint(force));
	NSLog(@"direction = %@", NSStringFromCGPoint(direction));

	LaunchData *shot = [[[LaunchData alloc] initWithForce:force andPosition:launchPos] autorelease];
	[[NSNotificationCenter defaultCenter] postNotificationName:PlayerDidFinishAiming object:shot];

}

Posted 2 years ago #

mobilebros
Moderator

Hey no problem, just wanted to add that you're calculating two angles but only using one, so you'd be avoiding a somewhat expensive cpu operation by getting rid of the calculation for angle2.

Posted 2 years ago #
gargantuan
Member

I'll be using both angles in the final version to add a bit of flair to the AI. I just needed to test that both were working as expected in this method.

Posted 2 years ago #

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