Cocos2D projectile speed

Question

I'm new to cocos2d and game developing. Currently I'm following this tutorial and, so far so good, basically:

Monsters spawn off screen and move from right to left and out of screen again.
The player is in a set position and can't move.
The player shoot ninja stars on touch.(still no collision)

The problem: Projectile speed changes (faster the closer i touch the upper or lower border of the screen (game is played horizontally positioned as shown in the image below the code). I belive that's because the target point gets farthest as I touch closer to my character and the upper/lower edges, and the star needs to get to that point in a given time (see the code). I would like the speed to be constant, no matter where I touch. How can I achieve that?. I'm sorry if I'm messy on my explanation.

This is the star/projectile code:

- (void)touchBegan:(UITouch *)touch withEvent:(UIEvent *)event {
    // 1
    CGPoint touchLocation = [touch locationInNode:self];

    // 2
    CGPoint offset    = ccpSub(touchLocation, _player.position);
    float   ratio     = offset.y/offset.x;
    int     targetX   = _player.contentSize.width/2 + self.contentSize.width;
    int     targetY   = (targetX*ratio) + _player.position.y;
    CGPoint targetPosition = ccp(targetX,targetY);

    // 3
    CCSprite *projectile = [CCSprite spriteWithImageNamed:@"projectile.png"];
    projectile.position = _player.position;
    [self addChild:projectile ];

    // 4
    CCActionRotateBy *actionRotate = [CCActionRotateBy actionWithDuration: 0.5f angle:360];
    [projectile runAction:[CCActionRepeatForever actionWithAction:actionRotate]];
    CCActionMoveTo *actionMove   = [CCActionMoveTo actionWithDuration: 1.5f position:targetPosition];
    CCActionRemove *actionRemove = [CCActionRemove action];
    [projectile runAction:[CCActionSequence actionWithArray:@[actionMove,actionRemove]]];
 }

As you can see in the actionMove line, projectile speed depends on the time it takes to reach the targetPosition (this image may help [not enough reputation to post images]): http://cdn5.raywenderlich.com/wp-content/uploads/2014/01/Cocos2D_MonsterMath.png

Thanks in advance, any help or reading material is appreciated.

Joseph Chen Joseph Chen · Accepted Answer · 2014-01-31T01:17:27

If your projectile just moves in a straight line at constant speed then:

distance = speed * time

So if you want to set a constant speed for your ninja stars, the time will vary based on the distance travelled.

Calculate the distance like so:

float distance = sqrtf(powf(targetPosition.x - projectile.position.x, 2.0f) + powf(targetPosition.y - projectile.position.y, 2.0f));

Set a speed and calculate the time:

ccTime flightTime = distance / speed;

Use flightTime in your CCAction:

CCActionMoveTo *actionMove = [CCActionMoveTo actionWithDuration:flightTime 
                                                       position:targetPosition];

Cocos2D projectile speed

1 Answers