BigViewPageView.m

-(void)initOffscreenContext
{
    CGSize layerSize = [self bounds].size;
    layerSize.height = floorf(layerSize.height);
    layerSize.width = floorf(layerSize.width);
    
    if (layerSize.height <= 0 || layerSize.width <=0) 
    {
        self.offscreenContext = nil;
        return;
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef ctx = (CGContextRef) [(id) CGBitmapContextCreate(NULL, layerSize.width, layerSize.height, 8, layerSize.width*4, colorSpace, kCGImageAlphaPremultipliedLast) autorelease];
    CGColorSpaceRelease(colorSpace);
    CGContextTranslateCTM(ctx, 0, layerSize.height);
    CGContextScaleCTM(ctx, 1.0, -1.0);
    
    CGFloat tx = layerSize.width * (1.0 - scale) * 0.5;
    CGFloat ty = layerSize.height * (1.0 - scale) * 0.5;
    
    CGRect tempbounds = CGRectZero;
    tempbounds.size = layerSize;
    tempbounds = CGRectIntegral(CGRectInset(tempbounds, tx, ty));
    CGContextSetShadow(ctx, CGSizeMake(5,5), 5);
    
    CGContextSetFillColorWithColor(ctx, [[UIColor whiteColor] colorWithAlphaComponent:0.5].CGColor);
    CGContextFillRect(ctx, tempbounds);
    
    self.offscreenContext = (id) ctx;
}