Collision Issues, what to use? « cocos2d for iPhone

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Collision Issues, what to use?

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Started 1 year ago by Domithan
Latest reply from Domithan

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Domithan
Member

Hello all. I'm very sorry to start my first post here with a question, but after taking a good look around I wasn't able to find anything that truly answers my question.

I've been struggling to learn Cocos2d this past week. The whole 'programming on a Mac' experience is very new to me, I've always been a PC guy.

My basic problem is this: I'm trying to make something VERY simple, a ball which starts in the middle of the screen, falls, and then bounces off a paddle on the bottom of the screen. When it bounces off, it should bounce back up to the same height each time. Also, I'd like to use the accelerometer to control the ball's movement left and right, so if and when the ball falls to the left or right of the paddle, you 'lose' this little game.

Now, I'm wondering what the best way to do this is. Should I use the built in physics engines? Or should I just use a plain cocos app and try to sort of make my own physics happen? My only real experience is with the cocos tutorials by Ray Wenderlich, and even that is something I'll have to re-read a few times.

In any case, thanks for any time and consideration, it means a lot to me.

Posted 1 year ago #
Omegas7
Member
Hello and don't worry my first 30 posts here were questions anyway ;).
I don't think you will really need to use a physics engine for this, since you're actually wanting the ball to bounce back to a fixed height, there are no weird calculations to be made (and simple collisions can be checked without physics)

Raywenderlich explains very well how to check for collisions between two CCSprites, but I don't remember him talking about accelerometer usage...

This is what I do to move my object right and left with the accelerometer:

I go to the init method of my scene, somewhere where I can call this once:
```
self.accelerometer = [UIAccelerometer sharedAccelerometer];
self.accelerometer.updateInterval = 0.025;
self.accelerometer.delegate = self;
```
That will let our scene receive accelerometer lectures every 0.025 seconds.
When we do that, these lectures will be sent through a method we have to implement:
```
- (void)accelerometer:(UIAccelerometer*)accelerometer didAccelerate:(UIAcceleration*)acceleration
{
}
```
Inside this method, acceleration.x and acceleration.y will tell me the... well, acceleration value, which is actually a low number from 0 to 1 (I think)
So if I wanted to move my player to the right, I would check if acceleration.x was positive, and left if it was negative. This method works fine with me:

player.position = ccp(player.position.x + (acceleration.x * 12), player.position.y);

I multiply the acceleration by 12 because the value is actually pretty low and you wouldn't notice the movement =/
Also note I didn't change the y position at all. I am leaving Y position manipulation to a different method in charge of the bouncing part.

Hope that helped :D
Posted 1 year ago #
Domithan
Member

Ahhh, okay. Thank you for the quick and friendly reply!

This gives me a good idea as far as what to do with the accelerometer, but I'm still a bit confused as to what to do as far as simulating physics. I'm not really sure how to make the ball keep bouncing to a certain height after colliding with the paddle, and then I need 'gravity' to bring the ball back down as well. Also, in the future I want to add other obstacles for the ball to bounce off. I'm wondering if there's a way to make the ball's collision with just about anything make the ball itself bounce off at the opposite angle.

Posted 1 year ago #
Omegas7
Member
You could have variables like these:
```
int bounced_y
bool bouncing
```
Where bounced_y will be equal to 0 and bouncing will be NO.
Now you just keep checking for collision with the paddle, if it occurs, you turn bouncing YES.

You should have a scheduled method that will be a called every... 0.1 seconds. You schedule a method like this:
[self schedule:@selector(update:) interval:(0.1)]; in your init method. This means a method called update will be called every 0.1 seconds. Now make the method.
```
-(void)update:(ccTime)dt
{
}
```
So inside this method goes our bouncing logic. The collision is telling us (through the bouncing boolean) that the ball has collided, so first we will check if that is true.
```
-(void)update:(ccTime)dt
{
     if (bouncing == YES)
     {
          // Bouncing!
     }
}
```
Now the traveled_y variable tells us how far the ball has gone. Let's put a limit to how far it can go... 150 pixels up? Great, now we check "as long as the ball has not traveled 150 pixels, I want it to keep going up, and when it reaches the 150 pixel limit, I want it to start falling"
```
-(void)update:(ccTime)dt
{
     if (bouncing == YES)
     {
          if (traveled_y < 150)
          {
                ball.position = ccp(ball.position.x,ball.position.y + 1);
                traveled_y += 1;
          } else {
                bouncing = NO;    // Reached limit, not bouncing anymore... now falling!
         }
     } else {
         if (traveled_y > 0)       // We are not bouncing, but we are mid-air!
         {
                ball.position = ccp(ball.position.x,ball.position.y - 1);   // Therefore, fall all you have gone so far.
                traveled_y -= 1;
         }
     }
}
```
Well, you get the idea, I don't know if that actually works since I couldn't test.
Posted 1 year ago #
Domithan
Member

Thank you very much for all the help. I'll be looking at this and the tutorials a bit more to try to understand the concepts and then I'll try building it all together. Once again, thank you, this really helps :D

Posted 1 year ago #

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