如何在没有物理引擎的cocos2d中模拟一个sprite的重力？

问题描述

我想为一个sprite模拟重力。

I would like to simulate "gravity" for one sprite.

我定义了一个保存重力值的CGPoint。

I defined a CGPoint which holds the gravity values. And I have tick method.

//in my init I defined this: gravity = ccp(0.0f, -1.0f);
-(void)tick:(ccTime) dt {
if (enableGravity_) {
    //simulate gravity
    velocity_.x += gravity_.x;
    velocity_.y += gravity_.y;
    self.position = CGPointMake(position_.x + velocity_.x, position_.y + velocity_.y);
}
if (isAcceleratingEnabled_) { //I move my sprite here
    float angle = self.rotation;
    float vx = cos(angle * M_PI / 180) * acceleratingSpeed_;
    float vy = sin(angle * M_PI / 180) * -acceleratingSpeed_;
    CGPoint direction = ccp(vx, vy);

    [self setPosition:ccpAdd(self.position, direction)];
}

}

EDIT ：
现在的问题是...如果isAccelerationEnabled为YES，我将移动我的sprite。当我移动精灵像这样不顺利。
如果我向上移动我的sprite，而禁用isAcceleration，它不会再向上移动，但重力会永远拉我的sprite下来。我不知道如何使用速度与此代码：

EDIT:the problem now is..I'm moving my sprite if "isAccelerationEnabled" is YES. And it's not smooth when I move the sprite like that.If I move my sprite upwards and than disable the "isAcceleration" it won't move upwards anymore but the gravity will INSTANTLY pull my sprite down. I don't know how to use the velocity with this code:

[self setPosition:ccpAdd(self.position, direction)];

---

现在这不是一个真正平滑的解决方案模拟重力。

---

Now this isn't a really smooth solution to simulate gravity.

这会将重力瞬间施加到精灵。

This applies the gravity force instantly to the sprite.

但我想要的效果是精灵顺利下降。像真实的物体落地。

But I want the effect that the sprite falls smoothly. Like real objects fell to earth.

Btw：我也对sprite使用一些向上的力。

Btw: I also apply some upward forces to the sprite.

推荐答案

问题是你应用重力作为速度而不是加速度。你需要给你的sprite一个速度，然后做像...。

The problem is that you're applying gravity as a velocity rather than an acceleration. You need to give your sprite a velocity, then do something like...

sprite.velocity = ccp(self.velocity.x+gravity.x, self.velocity.y+gravity.y);
sprite.position = ccp(sprite.position.x+sprite.velocity.x, sprite.position.y+sprite.velocity.y);

此外，如果你只是想做重力向下拉，它不需要2D，它可以只是一个标量，总是应用在负Y方向。

Also if you're only ever going to do gravity pulling downward, it doesn't need to be 2D, it can just be a scalar that always applies in the negative Y direction.

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