How does one compare one image to another to see if they are similar by a certain percentage, on the iPhone?

Question

I basically want to take two images taken from the camera on the iPhone or iPad 2 and compare them to each other to see if they are pretty much the same. Obviously due to li

Answer 1

As a quick, simple algorithm, I'd suggest iterating through about 1% of the pixels in each image and either comparing them directly against each other or keeping a running average and then comparing the two average color values at the end.

You can look at this answer for an idea of how to determine the color of a pixel at a given position in an image. You may want to optimize it somewhat to better suit your use-case (repeatedly querying the same image), but it should provide a good starting point.

Then you can use an algorithm roughly like:

float numDifferences = 0.0f;
float totalCompares = width * height / 100.0f;
for (int yCoord = 0; yCoord < height; yCoord += 10) {
    for (int xCoord = 0; xCoord < width; xCoord += 10) {
        int img1RGB[] = [image1 getRGBForX:xCoord andY: yCoord];
        int img2RGB[] = [image2 getRGBForX:xCoord andY: yCoord];
        if (abs(img1RGB[0] - img2RGB[0]) > 25 || abs(img1RGB[1] - img2RGB[1]) > 25 || abs(img1RGB[2] - img2RGB[2]) > 25) {
            //one or more pixel components differs by 10% or more
            numDifferences++;
        }
    }
}

if (numDifferences / totalCompares <= 0.1f) {
    //images are at least 90% identical 90% of the time
}
else {
    //images are less than 90% identical 90% of the time
}

Answer 2

Based on aroth's idea, this is my full implementation. It checks if some random pixels are the same. For what I needed it works flawlessly.

- (bool)isTheImage:(UIImage *)image1 apparentlyEqualToImage:(UIImage *)image2 accordingToRandomPixelsPer1:(float)pixelsPer1
{
    if (!CGSizeEqualToSize(image1.size, image2.size))
    {
        return false;
    }

    int pixelsWidth = CGImageGetWidth(image1.CGImage);
    int pixelsHeight = CGImageGetHeight(image1.CGImage);

    int pixelsToCompare = pixelsWidth * pixelsHeight * pixelsPer1;

    uint32_t pixel1;
    CGContextRef context1 = CGBitmapContextCreate(&pixel1, 1, 1, 8, 4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipFirst);
    uint32_t pixel2;
    CGContextRef context2 = CGBitmapContextCreate(&pixel2, 1, 1, 8, 4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipFirst);

    bool isEqual = true;

    for (int i = 0; i < pixelsToCompare; i++)
    {
        int pixelX = arc4random() % pixelsWidth;
        int pixelY = arc4random() % pixelsHeight;

        CGContextDrawImage(context1, CGRectMake(-pixelX, -pixelY, pixelsWidth, pixelsHeight), image1.CGImage);
        CGContextDrawImage(context2, CGRectMake(-pixelX, -pixelY, pixelsWidth, pixelsHeight), image2.CGImage);

        if (pixel1 != pixel2)
        {
            isEqual = false;
            break;
        }
    }
    CGContextRelease(context1);
    CGContextRelease(context2);

    return isEqual;
}

Usage:

[self isTheImage:image1 apparentlyEqualToImage:image2
accordingToRandomPixelsPer1:0.001]; // Use a value between 0.0001 and 0.005

According to my performance tests, 0.005 (0.5% of the pixels) is the maximum value you should use. If you need more precision, just compare the whole images using this. 0.001 seems to be a safe and well-performing value. For large images (like between 0.5 and 2 megapixels or million pixels), I'm using 0.0001 (0.01%) and it works great and incredibly fast, it never makes a mistake.

But of course the mistake-ratio will depend on the type of images you are using. I'm using UIWebView screenshots and 0.0001 performs well, but you can probably use much less if you are comparing real photographs (even just compare one random pixel in fact). If you are dealing with very similar computer designed images you definitely need more precision.

Note: I'm always comparing ARGB images without taking into account the alpha channel. Maybe you'll need to adapt it if that's not exactly your case.