UIImage will not reduce file size when archived with NSKeyedArchiver

Question

Im using images from my iphone library in an app.The following method work well but my problem is when I then archive the image using NSData. When the image is unarchived later on the UIImage size reverts back to the original size?

NSData *data = [[NSData alloc]init];
UIImage *reducedImage = [self imageWithImage:selectedImage      scaledToSize:CGSizeMake(50,50)];
data = [NSKeyedArchiver archivedDataWithRootObject:reducedImage];

and here the method Im using;

 - (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
UIGraphicsBeginImageContext(newSize);
UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0);
[image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
 }

why are you calling UIGraphicsBeginImageContext twice ? (once only with size, once with options?) — Vinzzz, Feb 11 '13 at 14:31

score 0 · Answer 1 · answered Feb 11 '13 at 14:24

0

Are you asking if it should revert back to the original size? If so, yes it should. NSKeyedArchiver is used to convert objects to just data so it is easier to save them in core data or other methods of storage. It is not a form of compression.

EDIT: The scale to size that you are using is also not a form of compression, it just shrinks the visible image, all of the image data is still there.

answered Feb 11 '13 at 14:24

Kris Gellci

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yes it reverts back to the full original size. I want it to remain a smaller size. The data (NSData) is then being used with NSUserDefaults. When use the selected images are required later on I only require very small images (small file size). If this is not possible - whats the best way to reduce a UIImage file size ?? – pete Feb 11 '13 at 14:33
Thanks for your help - The NSKeyedArchiver is not be used to try to compress the data. Im using it because it is be used with an NSArray and NSUserDefaults. So are you saying that the method I am using does not actually change the file size ?? – pete Feb 11 '13 at 14:43
Right, to actually change the size of the image, you can create a new image. Heres a link to help you with that: http://stackoverflow.com/a/4469273/1684508 – Kris Gellci Feb 11 '13 at 20:33

score 0 · Answer 2 · answered Feb 11 '13 at 14:25

it doesn’t mean making a compressed like zip file, it means putting all the relevant data together and then saving it to a single file.

NSKeyedArchiver, a concrete subclass of NSCoder, provides a way to encode objects (and scalar values) into an architecture-independent format that can be stored in a file. When you archive a set of objects, the class information and instance variables for each object are written to the archive. NSKeyedArchiver’s companion class, NSKeyedUnarchiver, decodes the data in an archive and creates a set of objects equivalent to the original set.

A keyed archive differs from a non-keyed archive in that all the objects and values encoded into the archive are given names, or keys. When decoding a non-keyed archive, values have to be decoded in the same order in which they were encoded. When decoding a keyed archive, because values are requested by name, values can be decoded out of sequence or not at all. Keyed archives, therefore, provide better support for forward and backward compatibility.

The keys given to encoded values must be unique only within the scope of the current object being encoded. A keyed archive is hierarchical, so the keys used by object A to encode its instance variables do not conflict with the keys used by object B, even if A and B are instances of the same class. Within a single object, however, the keys used by a subclass can conflict with keys used in its superclasses.

An NSArchiver object can write the archive data to a file or to a mutable-data object (an instance of NSMutableData) that you provide.

score 0 · Answer 3 · answered Feb 11 '13 at 14:39

0

UIImage drawInRect just fits the bitmap image attached to UIImage into the area you give it. The data itself hasn't changed...

What you're looking for is a compression. See UIKit UIImagePNGRepresentation or UIImageJPEGRepresentation functions.

answered Feb 11 '13 at 14:39

Vinzzz

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UIImage will not reduce file size when archived with NSKeyedArchiver

3 Answers3