How to create square thumbnails using iPhone SDK

- (UIImage *)thumbWithSideOfLength:(float)length {

NSString *subdir = @”my/images/directory”;
NSString *filename = @”myOriginalImage.png”;
NSString *fullPathToThumbImage = [subdir stringByAppendingPathComponent:[NSString stringWithFormat:@"%dx%d%@",(int) length, (int) length,filename];
NSString *fullPathToMainImage = [subdir stringByAppendingPathComponent:filename];

UIImage *thumbnail;

NSFileManager *fileManager = [NSFileManager defaultManager];

if ([fileManager fileExistsAtPath:fullPathToThumbImage] == YES) {
thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];

}
else {
//couldn’t find a previously created thumb image so create one first…
UIImage *mainImage = [UIImage imageWithContentsOfFile:fullPathToMainImage];

UIImageView *mainImageView = [[UIImageView alloc] initWithImage:mainImage];

BOOL widthGreaterThanHeight = (mainImage.size.width > mainImage.size.height);
float sideFull = (widthGreaterThanHeight) ? mainImage.size.height : mainImage.size.width;

CGRect clippedRect = CGRectMake(0, 0, sideFull, sideFull);

//creating a square context the size of the final image which we will then
// manipulate and transform before drawing in the original image
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];

thumbnail = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();
NSData *imageData = UIImagePNGRepresentation(thumbnail);

[imageData writeToFile:fullPathToThumbImage atomically:YES];

thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];
}
return thumbnail;
}