Moiré patterns in scanned halftone pictures

We present a Fourier-transform analysis of the moire-pattern formation commonly observed in images reconstructed from scanned halftone pictures. The analysis takes into account the halftone screen frequency, the scanner sampling frequency, the nonzero size of the scanner aperture, the angle between the scan direction and the halftone screen and the shape (square or parallelogram) of the scanner sampling lattice. A fortran program based on the mathematical analysis is shown to permit not only simulation of a wide variety of moire phenomena actually observed in practice but also explanation of their origin. Several important observations are discussed, including the following: (1) Moire fringes can result from the beating together of clusters of Fourier spatial frequencies. (2) Moire patterns other than square-grid fringe arrays are associated with parallelogram (nonsquare) sampling lattices. (3) Fixed-value thresholding intensifies moire patterns.