Creating thumbnail for an image grid

I'm building an app like the photo app by apple in the iPad. I have large full-screen image and I show them using a scrollView for managing zooming and paging. The main problem happen when I try to create a grid with the thumbnail of the images. I create them as UIImageView overlapped on a UIButton. All works great, but when I开发者_高级运维 try the app on the iPad, it requires a lot of memory, I suppose it depend on the rescaling of the Image. There's a way to create a UIImageView with the little image,rescaling the larger image, without using so much memory?

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You can use UIGraphics to create a thumbnail. Here's this code to do it:

UIGraphicsBeginImageContext(CGSizeMake(length, length));
CGContextRef currentContext = UIGraphicsGetCurrentContext();
CGContextClipToRect( currentContext, clippedRect);
CGFloat scaleFactor = length/sideFull;
if (widthGreaterThanHeight) {
    //a landscape image – make context shift the original image to the left when drawn into the context
    CGContextTranslateCTM(currentContext, -((mainImage.size.width - sideFull) / 2) * scaleFactor, 0);
}
else {
    //a portfolio image – make context shift the original image upwards when drawn into the context
    CGContextTranslateCTM(currentContext, 0, -((mainImage.size.height - sideFull) / 2) * scaleFactor);
}
//this will automatically scale any CGImage down/up to the required thumbnail side (length) when the CGImage gets drawn into the context on the next line of code
CGContextScaleCTM(currentContext, scaleFactor, scaleFactor);
[mainImageView.layer renderInContext:currentContext];
UIImage* thumbnail = UIGraphicsGetImageFromCurrentImageContext();