An IDL/ENVI Implementation of the FFT Based Algorithm for Automatic Image Registration

Georeferencing images is a laborious process so schemes for automating this process have been under investigation for some time. Among the most promising automatic registration algorithms aare those based on Fast Fourier Transform (FFT). The displacement between the two given images can be computed by computing the ratio F1*conj(F2)/|F1*F2|, and then applying the inverse Fourier transform. The result is an impulse-like function, which is approximately zero everywhere except at the displacement that is necessary to optimally register the images. Coverting from rectangular coordinates to log-polar coordinates, shifts representing rotation and scaling can be also determined to complete the georectification process. Our FFT-based algorithm has been successfully implemented in IDL (Interactive Data Language) and added as two user functions to an image processing software package - ENVI (ENvironment for Visualizing Images) interface. ENVI handles all pre-processing and post-processing work such as input, output, display, filter, analysis, and file management. To test this implementation, several dozen tests were conducted on both simulated and "real world" images. The results of these tests show advantages and limitations of this algorithm. In particular, our tests show that the accuracy of the resulting registration is quite good compared to current manual methods.