Zoom Layer centered on a Sprite - objective-c

Question

I am in process of developing a small game where a space-ship travels through a layer (doh!), in some situations the spaceship comes close to an enemy, and the whole layer is zoomed in on the space-ship with the zoom level being dependent on the distance between the ship and the enemy. All of this works fine.
The main question, however, is how do I keep the zoom being centered on the space-ship?
Currently I control the zooming in the GameLayer object through the update method, here is the code:
-(void) prepareLayerZoomBetweenSpaceship{
CGPoint mainSpaceShipPosition = [mainSpaceShip position];
CGPoint enemySpaceShipPosition = [enemySpaceShip position];
float distance = powf(mainSpaceShipPosition.x - enemySpaceShipPosition.x, 2) + powf(mainSpaceShipPosition.y - enemySpaceShipPosition.y,2);
distance = sqrtf(distance);
/*
Distance > 250 --> no zoom
Distance < 100 --> maximum zoom
*/
float myZoomLevel = 0.5f;
if(distance < 100){ //maximum zoom in
myZoomLevel = 1.0f;
}else if(distance > 250){
myZoomLevel = 0.5f;
}else{
myZoomLevel = 1.0f - (distance-100)*0.0033f;
}
[self zoomTo:myZoomLevel];
}
-(void) zoomTo:(float)zoom {
if(zoom > 1){
zoom = 1;
}
// Set the scale.
if(self.scale != zoom){
self.scale = zoom;
}
}
Basically my question is: How do I zoom the layer and center it exactly between the two ships? I guess this is like a pinch zoom with two fingers!

Answer 1

Below is some code that should get it working for you. Basically you want to:
Update your ship positions within the parentNode's coordinate system
Figure out which axis these new positions will cause the screen will be bound by.
Scale and re-position the parentNode
I added some sparse comments, but let me know if you have any more questions/issues. It might be easiest to dump this in a test project first...
ivars to put in your CCLayer:
CCNode *parentNode;
CCSprite *shipA;
CCSprite *shipB;
CGPoint destA, deltaA;
CGPoint destB, deltaB;
CGPoint halfScreenSize;
CGPoint fullScreenSize;
init stuff to put in your CCLayer:
CGSize size = [[CCDirector sharedDirector] winSize];
fullScreenSize = CGPointMake(size.width, size.height);
halfScreenSize = ccpMult(fullScreenSize, .5f);
parentNode = [CCNode node];
[self addChild:parentNode];
shipA = [CCSprite spriteWithFile:#"Icon-Small.png"]; //or whatever sprite
[parentNode addChild:shipA];
shipB = [CCSprite spriteWithFile:#"Icon-Small.png"];
[parentNode addChild:shipB];
//schedules update for every frame... might not run great.
//[self schedule:#selector(updateShips:)];
//schedules update for 25 times a second
[self schedule:#selector(updateShips:) interval:0.04f];
Zoom / Center / Ship update method:
-(void)updateShips:(ccTime)timeDelta {
//SHIP POSITION UPDATE STUFF GOES HERE
...
//1st: calc aspect ratio formed by ship positions to determine bounding axis
float shipDeltaX = fabs(shipA.position.x - shipB.position.x);
float shipDeltaY = fabs(shipA.position.y - shipB.position.y);
float newAspect = shipDeltaX / shipDeltaY;
//Then: scale based off of bounding axis
//if bound by x-axis OR deltaY is negligible
if (newAspect > (fullScreenSize.x / fullScreenSize.y) || shipDeltaY < 1.0) {
parentNode.scale = fullScreenSize.x / (shipDeltaX + shipA.contentSize.width);
}
else { //else: bound by y-axis or deltaX is negligible
parentNode.scale = fullScreenSize.y / (shipDeltaY + shipA.contentSize.height);
}
//calculate new midpoint between ships AND apply new scale to it
CGPoint scaledMidpoint = ccpMult(ccpMidpoint(shipA.position, shipB.position), parentNode.scale);
//update parent node position (move it into view of screen) to scaledMidpoint
parentNode.position = ccpSub(halfScreenSize, scaledMidpoint);
}
Also, I'm not sure how well it'll perform with a bunch of stuff going on -- but thats a separate problem!

Answer 2

Why don't you move the entire view, & position it so the ship is in the centre of the screen? I haven't tried it with your example, but it should be straight forward. Maybe something like this -
CGFloat x = (enemySpaceShipPosition.x - mainSpaceShipPosition.x) / 2.0 - screenCentreX;
CGFloat y = (enemySpaceShipPosition.y - mainSpaceShipPosition.y) / 2.0 - screenCentreY;
CGPoint midPointForContentOffset = CGPointMake(-x, -y);
[self setContentOffset:midPointForContentOffset];
...where you've already set up screenCentreX & Y. I haven't used UISCrollView for quite a while (been working on something in Unity so I'm forgetting all by Obj-C), & I can't remember how the contentOffset is affected by zoom level. Try it & see! (I'm assuming you're using a UIScrollView, maybe you could try that too if you're not)