圖片壓縮

http://www.raywenderlich.com/forums/viewtopic.php?f=2&t=8628

- (UIImage *)normalResImageForAsset:(ALAsset*)asset

{
    // Convert ALAsset to UIImage
    UIImage *image = [self highResImageForAsset:asset];
    
    // Determine output size
    CGFloat maxSize = 1024.0f;
    CGFloat width = image.size.width;
    CGFloat height = image.size.height;
    CGFloat newWidth = width;
    CGFloat newHeight = height;
    
    // If any side exceeds the maximun size, reduce the greater side to 1200px and proportionately the other one
    if (width > maxSize || height > maxSize) {
        if (width > height) {
            newWidth = maxSize;
            newHeight = (height*maxSize)/width;
        } else {
            newHeight = maxSize;
            newWidth = (width*maxSize)/height;
        }
    }
    
    // Resize the image
    CGSize newSize = CGSizeMake(newWidth, newHeight);
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    // Set maximun compression in order to decrease file size and enable faster uploads & downloads
    NSData *imageData = UIImageJPEGRepresentation(newImage, 0.0f);
    UIImage *processedImage = [UIImage imageWithData:imageData];
    
    return processedImage;
}