How to make background repeat in vertical direction using parallax - iphone

Question

I have been looking to make background repeat in vertical direction but there is always some flickering..
I searched a lot on goggle and some book for reference but not able to find a proper reply
There are lot of proper reply for background repeat on horizontal direction
-(void) update:(ccTime)delta
{
CCSprite* sprite;
CCARRAY_FOREACH([spriteBatch children], sprite)
{
NSNumber* factor = [speedFactors objectAtIndex:sprite.zOrder];
CGPoint pos = sprite.position;
pos.y -= scrollSpeed * [factor floatValue] * (delta * 50);
if (pos.y < -screenSize.height)
{
pos.y += (screenSize.height * 2) - 2;
}
sprite.position = pos;
}
}

Answer 1

Loading new background each time? I strongly suspect flickering is caused by loading time in texture. You can use threaded loading to avoid flickering.
Here is once other question with something similar problem.
Continuous-horizontal-scrolling-of-background-in-cocos2d: Refer my answer
Continuous vertical scroll:
#define IS_IPAD (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define MM_BG_SPEED_DUR ( IS_IPAD ? (6.0f) : (2.0f) )
-(void)onEnter
{
[super onEnter];
[self initBackground];
[self schedule: #selector(tick:)];
}
-(void)initBackground
{
NSString *tex = #"BG/Background.png";//[self getThemeBG];
mBG1 = [CCSprite spriteWithFile:tex];
mBG1.position = ccp(s.width*0.5f,s.height*0.5f);
[self addChild:mBG1 z:LAYER_BACKGROUND];
mBG2 = [CCSprite spriteWithFile:tex];
mBG2.position = ccp(s.width*0.5f,s.height*0.5f+mBG1.contentSize.Height);
mBG2.flipY = true;
[self addChild:mBG2 z:LAYER_BACKGROUND];
}
-(void)scrollBackground:(ccTime)dt
{
CGSize s = [[CCDirector sharedDirector] winSize];
CGPoint pos1 = mBG1.position;
CGPoint pos2 = mBG2.position;
pos1.y -= MM_BG_SPEED_DUR;
pos2.y -= MM_BG_SPEED_DUR;
if(pos1.y <=-(s.height*0.5f) )
{
pos1.y = pos2.y + mBG2.contentSize.height;
}
if(pos2.y <=-(s.height*0.5f) )
{
pos2.y = pos1.y + mBG1.contentSize.height;
}
mBG1.position = pos1;
mBG2.position = pos2;
}
-(void)tick:(ccTime)dt
{
[self scrollBackground:dt];
}