gistfile1.m

#define GravityDirection -1 // -1 for down, 1 for up
#define ParticleWidth 2
#define ParticleHeight 2


// Import the interfaces
#import "HelloWorldLayer.h"

// Needed to obtain the Navigation Controller
#import "AppDelegate.h"

struct SandParticle
{
    CGPoint Position;

    BOOL CMTop;
    BOOL CMBottom;
    BOOL CMLeft;
    BOOL CMRight;
};


// HelloWorldLayer implementation
@implementation HelloWorldLayer
{
    struct SandParticle SandMatrix[5000];
    
    NSMutableArray *DrawLines;
    int CurrentLineIndex;
    
    int SandCount;
    
    bool StartDrop;
    
    CGSize WindowSize;
    
    CCLabelTTF *label;
    
    Byte ScreenBuffer[960*640*4];
    
    UIColor *ClearColor;
}

// Helper class method that creates a Scene with the HelloWorldLayer as the only child.
+(CCScene *) scene
{
	CCScene *scene = [CCScene node];
	
	HelloWorldLayer *layer = [HelloWorldLayer node];
	
	[scene addChild: layer];
	
	return scene;
}

-(id) init
{
    self = [super init];
    
    if (self)
    {
        WindowSize = [[CCDirector sharedDirector] winSize];
        
        ClearColor = [[UIColor alloc] initWithRed:1 green:0 blue:0 alpha:1];
        
        [self setIsTouchEnabled:YES];
        
        DrawLines = [[NSMutableArray alloc] init];
        
        CCLayerColor *Demo = [CCLayerColor layerWithColor:ccc4(255, 0, 0, 255)];
        [self setContentSize:CGSizeMake(WindowSize.width, WindowSize.height)];
        [self addChild:Demo z:-10];
    
        [self schedule:@selector(AddNewParticle:) interval:0.03];
        [self schedule:@selector(UpdateParticles:) interval:0.03];
        
        
        label = [CCLabelTTF labelWithString:@"Pixeldrop" fontName:@"Game Over" fontSize:150];
        [label setAnchorPoint:CGPointZero];
        [label setPosition:CGPointMake(WindowSize.width/2 - label.boundingBox.size.width/2, WindowSize.height - label.boundingBox.size.height)];
        [label setColor:ccc3(0, 255, 0)];
        
        [self addChild:label];
    
    }
    return self;
}

#define MaxParticles 2000

- (void) AddNewParticle: (ccTime) dt
{
    if (SandCount < MaxParticles && StartDrop) 
    {
        struct SandParticle NewParticle;
                
        NewParticle.Position = CGPointMake(WindowSize.width/2, WindowSize.height- label.boundingBox.size.height + 10);
        NewParticle.CMBottom = YES;
    
        SandMatrix[SandCount] = NewParticle;
        
        SandCount++;
    }
    
    glReadPixels(0,0,WindowSize.width,WindowSize.height,GL_RGBA,GL_UNSIGNED_BYTE,&ScreenBuffer);
}



- (BOOL) GetColorAtPoint: (int) GetX : (int)GetY
{
    int YSize = (int)WindowSize.width*4;
    
    Byte R = ScreenBuffer[(YSize*GetY) + 0 + (4 * GetX)];
    Byte G = ScreenBuffer[(YSize*GetY) + 1 + (4 * GetX)];
    Byte B = ScreenBuffer[(YSize*GetY) + 2 + (4 * GetX)];
    Byte A = ScreenBuffer[(YSize*GetY) + 3 + (4 * GetX)];
    
    return (R==255 && G == 0 && B == 0 && A == 255);
}

- (void) UpdateParticles: (ccTime) dt
{    
    for (int CPLoop = 0; CPLoop <= SandCount; CPLoop++)
    {
        struct SandParticle CurrentParticle = SandMatrix[CPLoop];
        
        
        if (CurrentParticle.Position.y < ParticleHeight + 1) continue;
        
        CurrentParticle.CMBottom = [self GetColorAtPoint:CurrentParticle.Position.x:CurrentParticle.Position.y-1];
        CurrentParticle.CMLeft = [self GetColorAtPoint:CurrentParticle.Position.x-ParticleWidth*1.5:CurrentParticle.Position.y-1];
        CurrentParticle.CMRight = [self GetColorAtPoint:CurrentParticle.Position.x+ParticleWidth*1.5:CurrentParticle.Position.y-1];
        
        int Randomized = arc4random_uniform(2);
 
        if (CurrentParticle.CMBottom)
        {
            CurrentParticle.Position.y -= ParticleHeight;
            
            if (Randomized == 1)
                CurrentParticle.Position.x += 1;
            else 
                CurrentParticle.Position.x -= 1;
            
        }
        else if (CurrentParticle.CMLeft && CurrentParticle.CMRight)
        {
            CurrentParticle.Position.y -=ParticleHeight;
            
            if (Randomized == 1)
                CurrentParticle.Position.x += 1;
            else 
                CurrentParticle.Position.x -= 1;
        }
        else if (CurrentParticle.CMLeft)
        {
            CurrentParticle.Position.x -=ParticleWidth;
            CurrentParticle.Position.y -=ParticleHeight;
        }
        
        else if (CurrentParticle.CMRight)
        {
            CurrentParticle.Position.x +=ParticleWidth;
            CurrentParticle.Position.y -=ParticleHeight;
        }
         
 
        SandMatrix[CPLoop] = CurrentParticle;
    }
     
}

-(void)ccTouchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{   
    NSMutableArray *TouchArray = [[NSMutableArray alloc] init];
    [DrawLines addObject:TouchArray];
    
    StartDrop = YES;
    
}

-(void)ccTouchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch  *Touch = [touches anyObject];
    CGPoint CurrentTouchLocation = [Touch locationInView:[Touch view]];
    CGPoint PreviousTouchLocation = [Touch previousLocationInView:[Touch view]];
    
    
    NSMutableArray *NewTouch = [[NSMutableArray alloc] init];
    
    CurrentTouchLocation = [[CCDirector sharedDirector] convertToGL:CurrentTouchLocation];
    PreviousTouchLocation= [[CCDirector sharedDirector] convertToGL:PreviousTouchLocation];
    
    [NewTouch addObject:NSStringFromCGPoint(CurrentTouchLocation)];
    [NewTouch addObject:NSStringFromCGPoint(PreviousTouchLocation)];
    
    NSMutableArray *CurrentLine = [DrawLines objectAtIndex:CurrentLineIndex];
    [CurrentLine addObject:NewTouch];
    [DrawLines replaceObjectAtIndex:CurrentLineIndex withObject:CurrentLine];
    
}

-(void)ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    CurrentLineIndex++;
}

- (void) draw
{    
    [super draw];
    
      
    for (int CPLoop = 0; CPLoop <= SandCount; CPLoop++)
    {
        struct SandParticle CurrentParticle = SandMatrix[CPLoop];
                
        if (CurrentParticle.Position.x > 0 && CurrentParticle.Position.y > 0)
        ccDrawSolidRect(CGPointMake(CurrentParticle.Position.x, CurrentParticle.Position.y), CGPointMake(CurrentParticle.Position.x+ParticleWidth, CurrentParticle.Position.y+ParticleHeight), ccc4f(255, 255, 255, 255));
        else break;
        
        
    }
    
   
    for (NSArray *Line in DrawLines)
    {
        for (NSArray *Point in Line)
        {
            
            CGPoint CurrentTouchLocation = CGPointFromString([Point objectAtIndex:0]);
            CGPoint PreviousTouchLocation = CGPointFromString([Point objectAtIndex:1]);
            
            
            glLineWidth(5.0f);
            ccDrawLine(PreviousTouchLocation, CurrentTouchLocation);

        }
    }
}


@end

• (BOOL) GetColorAtPoint: (int) GetX : (int)GetY
  {
  //int YSize = (int)WindowSize.width_4;
  //int i = (YSize_GetY) + (4 * GetX);
  int i = (WindowSize.width_GetY + GetX)_4;
  if( i < 0 || i > bufferSize ) return NO;

  Byte R = ScreenBuffer[i];
  Byte G = ScreenBuffer[++i];
  Byte B = ScreenBuffer[++i];
  Byte A = ScreenBuffer[++i];

  return (R==255 && G == 0 && B == 0 && A == 255);
  }

Если затык действительно в GetColorAtPoint, то можно попробовать или перевести GetColorAtPoint на C-функцию, или хотя бы объединить 3 вызова в один: адрес пикселя CMLeft вычисляется умножением, а две CMBottom и CMRight сложением.