Computerized analysis of optical fringe patterns has become an accepted method of reducing the time and tedium associated with extracting quantitative data from phase encoded intensity distributions. The computing power of today's personal computers makes digital image processing of moire, holographic, photoelastic, and speckle interferometric fringe patterns both economical and efficient. We review several automatic and semiautomatic image processing algorithms that enable the reduction of both speckle and full-field fringe patterns to quantitative data. The speckle patterns are analyzed using morphological properties, while the full-field fringe patterns are analyzed using standard thinning and fringe order seeding, sinusoidal fitting, Fourier transform, and phasestepping techniques. The mapping that each fringe pattern experiences when being digitized is also discussed so that proper system calibration can be developed. Analysis of geometric moiré, moiré interferometric, photoelastic, and speckle interferometric fringe patterns are presented as examples of digital image processing based data reduction.