A Review of Hashing based Image Copy Detection Techniques <s> Benchmark images <s> In this paper, we propose a robust image hash algorithm by using the invariance of the image histogram shape to geometric deformations. Robustness and uniqueness of the proposed hash function are investigated in detail by representing the histogram shape as the relative relations in the number of pixels among groups of two different bins. It is found from extensive testing that the histogram-based hash function has a satisfactory performance to various geometric deformations, and is also robust to most common signal processing operations thanks to the use of Gaussian kernel low-pass filter in the preprocessing phase. <s> BIB001 </s> A Review of Hashing based Image Copy Detection Techniques <s> Benchmark images <s> A robust image hashing using locally linear embedding (LLE) is presented. The image is first rescaled to a fixed size. Then the image is divided into key-dependent overlapping blocks. Every blocks are reshaped to vectors and combined to the matrix. The LLE algorithm is applied to the matrix. The reconstruction weights coefficients reflect the intrinsic geometric properties of image and are invariant basicly to ordinary image processing and several geometric transformation. The quantization scheme is devised to compress the coefficients matrix and form the binary image hash. Test results indicate that the proposed method is robust against content-preserving modifications such as JPEG compression, filtering and affine transformation. Hashes of different images have very low collision probability. <s> BIB002
This section discusses the benchmark images used for evaluating the performance of different hashing algorithms. Table 5 below gives a list of the benchmark images used by various image hashing algorithms. It can be seen that Lena is the most popular image used for evaluating the robustness of different algorithms, followed by Baboon, Peppers, Airplane, House  BIB002 BIB001 . All the reviewed techniques employ more than one benchmark image which is given in Table 5 . 