OpenCV - Fastest method to check if two images are 100% same or not

import cv2
import numpy as np
a = cv2.imread("picture1.png")
b = cv2.imread("picture2.png")
difference = cv2.subtract(a, b)    
result = not np.any(difference)
if result is True:
    print("Pictures are the same")
else:
    print("Pictures are different")

You can use a logical operator like xor operator. If you are using python you can use the following one-line function:

Python

def is_similar(image1, image2):
    return image1.shape == image2.shape and not(np.bitwise_xor(image1,image2).any())

where shape is the property that shows the size of matrix and bitwise_xor is as the name suggests. The C++ version can be made in a similar way!

C++

Please see @berak code.

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Notice: The Python code works for any depth images(1-D, 2-D, 3-D , ..), but the C++ version works just for 2-D images. It's easy to convert it to any depth images by yourself. I hope that gives you the insight! :)

Doc: bitwise_xor

EDIT: C++ was removed. Thanks to @Micka and @ berak for their comments.

the sum of the differences should be 0 (for all channels):

bool equal(const Mat & a, const Mat & b)
{
    if ( (a.rows != b.rows) || (a.cols != b.cols) )
        return false;
    Scalar s = sum( a - b );
    return (s[0]==0) && (s[1]==0) && (s[2]==0);
}