A computerized method determines a similarity between a first image and a second image. The first image is converted into a first grayscale image, and the second image is converted into a second grayscale image. A first feature vector of the first grayscale image and a second feature vector of the second grayscale image are extracted. A similarity value is calculated indicating the similarity between the first image and the second image according to the first feature vector and the second feature vector. If the similarity value is greater than or equal to the predetermined threshold, the first image is similar to the second image and a determination result is outputted denoting the first image is similar to the second image.