Generate hash from UIImage

Question

I'm trying to compare two UIImages from the file system to see if they are the same. Obviously, I can't use NSObject's hash method, since this returns a hash of the object, and not the actual image data.

I found code generate an MD5 hash from a string, but I haven't discovered how to implement it for a UIImage.

How should I go about hashing a UIImage? Or is my method for comparing to images totally off?

Answer 1

A less than optimal solution:

[ UIImagePNGRepresentation( uiImage1 ) isEqualToData: 
      UIImagePNGRepresentation( uiImage2 ) ];

This basically compares the PNG encoded data of the 2 images. Since image similarity is a complex subject, better and faster solutions can be devised based on what exactly the end goal is (i.e. are you looking to compare images, pixel by pixel, or just approximate similarity, which could use a downsampled version of the source image, etc).

Answer 2

I wound up using the following code to accomplish the task. Note that this requires that you import <CommonCrypto/CommonDigest.h>:

unsigned char result[16];
NSData *imageData = [NSData dataWithData:UIImagePNGRepresentation(inImage)];
CC_MD5(imageData, [imageData length], result);
NSString *imageHash = [NSString stringWithFormat:
        @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",
        result[0], result[1], result[2], result[3], 
        result[4], result[5], result[6], result[7],
        result[8], result[9], result[10], result[11],
        result[12], result[13], result[14], result[15]
        ];