UIImage category to compare two images pixel by pixel.

UIImage+HPIsEqualToImage.h

//
//  UIImage+HPIsEqualToImage.h
//
//  Created by Hermes Pique on 20/02/14.
//  Copyright (c) 2014 Hermes Pique. All rights reserved.
//
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
//

#import <UIKit/UIKit.h>

@interface UIImage (HPIsEqualToImage)

- (BOOL)hp_isEqualToImage:(UIImage*)image;

- (NSData*)hp_normalizedData;

@end

UIImage+HPIsEqualToImage.m

//
//  UIImage+HPIsEqualToImage.m
//
//  Created by Hermes Pique on 20/02/14.
//  Copyright (c) 2014 Hermes Pique. All rights reserved.
//
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
//

#import "UIImage+HPIsEqualToImage.h"

@implementation UIImage (HPIsEqualToImage)

- (BOOL)hp_isEqualToImage:(UIImage*)image
{
    NSData *data = [image hp_normalizedData];
    NSData *originalData = [self hp_normalizedData];
    return [originalData isEqualToData:data];
}

- (NSData*)hp_normalizedData
{
    const CGSize pixelSize = CGSizeMake(self.size.width * self.scale, self.size.height * self.scale);
    UIGraphicsBeginImageContext(pixelSize);
    [self drawInRect:CGRectMake(0, 0, pixelSize.width, pixelSize.height)];
    UIImage *drawnImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return UIImagePNGRepresentation(drawnImage);
}

@end

This is what I use in my unit tests to compare images. It's not very efficient, so only use it in production code if performance is not a concern.

Is there a difference between [image hp_normalizedData] and UIImagePNGRepresentation(image)?

@rsaunders Yes. There is no guarantee that two images with the same pixel values will have the same PNG data representation.