When creating a digital forgery

it is often necessary to combine several images which were originally of different JPEG compression qualities.This paper proposes a passive approach to detect digital forgeries by checking inconsistencies of blocking artifact.We calculate the second-order difference edge map by convolving the blocky image with a Laplacian kernel.Then compute the mean of the results along the horizontal (vertical) direction and obtain a vector.Discrete Fourier transform of this vector produces a normalized spectrum curve and builds a blocking artifact measure.Divide the composite image into overlapped blocks and calculate the measure of each block.The forged area can be effectively exposed by checking inconsistencies of blocking artifact.Simulation results show the efficiency of proposed algorithm.