Question

I have tried many different techniques of applying a realistic looking gravity feature in my game but have had no luck so far. I plan on offering a 100 point bounty on this for someone who can show me or (share) some code that applies gravity to a CCSprite in Cocos2D.

What I have done so far has just been ugly or unrealistic and I have asked in many different places on what the best approach is but I just have not found any good looking gravity techniques yet.

Anyway, can anyone offer some tips/ideas or their approach only applying gravity to a CCSprite in Cocos2D without using a physics engine?

Thanks!

Answer 1

A effective approach without having to explicitly use any physics engine is to step the velocity and position of your sprite manually in your update loop. This is essentially Euler Integration.

// define your gravity value
#define GRAVITY -0.1

// define a velocity variable in the header of your Game class/CCSprite Subclass (neater)
float velocity_y;

-(void) update: (ccTime) dt
{
    // Step the y-velocity by your acceleration value (gravity value in this case)
    velocity_y += GRAVITY *dt; // drop the dt if you don't want to use it 

    // Step the position values and update the sprite position accordingly
    sprite.position.y += velocity_y* dt; // same here
}

In the code snippet above, I defined a velocity_y variable to keep track of my sprite’s current velocity along the y-direction. Remember to initialize this value to 0 in your init method.

Next comes the crux of Euler. At every time step:

1. Advance your velocity by your acceleration (which is your gravity) multiplied by dt to find your new velocity.
2. Advance your position by your newly computed velocity value multiplied by dt to find your new position.

You can experiment whether using delta-time or not (see LearnCocos2D’s excellent comment on the cons of using it) works for your game. While multiplying by delta-time allows your object motion to more accurately take into account varying framerates, as LearnCocos2d pointed out, it might be wise to avoid using it in a real-time game where system interrupts (new mail, low battery, ads pop-out) can occur and subsequently cause the game simulation to jump forward in an attempt to make up.

So if you are dropping the dt, remember to tweak (scale down) your GRAVITY value accordingly to retain the right feel, since this value is no longer multiplied by the value of delta-time (which is ~ 1/60).

Aditional Tip: To apply an impulse to move the sprite (say via a swipe), you can affect the velocities directly by changing the values of velocity_x (if you have this) and velocity_y.

I have used this approach in my games, and it works very well. hope it helps.