Abstract: Re-sampled signals show periodicity in its nth derivative and covariance, and it can be used to estimate the scaling ratio. Calculate the second-order difference along the vertical (horizontal) direction of image. Compute the mean of absolute difference along the horizontal (vertical) direction and obtain a vector, which will be appended with zero to make its length equal to an integer power of 2. Calculate DFT to the vector and produce the frequency-magnitude curve. Detect the peak frequency in the curve and obtain the scaling ratio along the horizontal (vertical) direction. It is applicable to different interpolation schemes, no matter whether the image has been JPEG compressed before or after interpolation. Experimental results show that the proposed method can judge whether or not a photo captured by the digital camera is undergone the digital zoom, and approximately estimate the scaling ratio.

  author       = {Weimin Wei and Shuozhong Wang and Zhenjun Tang},
  url          = {http://www.truemark.cn/publications/Estimation%20of%20Rescaling%20Factor..pdf},
  pages        = {676--679},
  title        = {Estimation of rescaling factor and detection of image splicing},
  year         = {2008},
  keywords     = {Image Forensics; Tamper Detection},
  booktitle    = {11th IEEE International Conference on Communication Technology (ICCT 2008)},