Create a mask from difference between two images (iPhone)

Question

How can I detect the difference between 2 images, creating a mask of the area that's different in order to process the area that's common to both images (gaussian blur for example)?

EDIT: I'm currently using this code to get the RGBA value of pixels:

+ (NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy count:(int)count
{
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:count];

    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = malloc(height * width * 4);
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                    bitsPerComponent, bytesPerRow, colorSpace,
                    kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);

    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
    for (int ii = 0 ; ii < count ; ++ii)
    {
        CGFloat red   = (rawData[byteIndex]     * 1.0) / 255.0;
        CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
        CGFloat blue  = (rawData[byteIndex + 2] * 1.0) / 255.0;
        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
        byteIndex += 4;

        UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
        [result addObject:acolor];
    }

  free(rawData);

  return result;
}

The problem is, the images are being captured from the iPhone's camera so they are not exactly the same position. I need to create areas of a couple of pixels and extracting the general color of the area (maybe by adding up the RGBA values and dividing by the number of pixels?). How could I do this and then translate it to a CGMask?

I know this is a complex question, so any help is appreciated.

Thanks.

Answer 1

I think the simplest way to do this would be to use a difference blend mode. The following code is based on code I use in CKImageAdditions.

+ (UIImage *) differenceOfImage:(UIImage *)top withImage:(UIImage *)bottom {
    CGImageRef topRef = [top CGImage];
    CGImageRef bottomRef = [bottom CGImage];

    // Dimensions
    CGRect bottomFrame = CGRectMake(0, 0, CGImageGetWidth(bottomRef), CGImageGetHeight(bottomRef));
    CGRect topFrame = CGRectMake(0, 0, CGImageGetWidth(topRef), CGImageGetHeight(topRef));
    CGRect renderFrame = CGRectIntegral(CGRectUnion(bottomFrame, topFrame));

    // Create context
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    if(colorSpace == NULL) {
        printf("Error allocating color space.\n");
        return NULL;
    }

    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 renderFrame.size.width,
                                                 renderFrame.size.height,
                                                 8,
                                                 renderFrame.size.width * 4,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorSpace);

    if(context == NULL) {
        printf("Context not created!\n");
        return NULL;
    }

    // Draw images
    CGContextSetBlendMode(context, kCGBlendModeNormal);
    CGContextDrawImage(context, CGRectOffset(bottomFrame, -renderFrame.origin.x, -renderFrame.origin.y), bottomRef);
    CGContextSetBlendMode(context, kCGBlendModeDifference);
    CGContextDrawImage(context, CGRectOffset(topFrame, -renderFrame.origin.x, -renderFrame.origin.y), topRef);

    // Create image from context
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    UIImage * image = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);

    CGContextRelease(context);

    return image;
}