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 are those based on the fast Fourier transform (FFT). The displacement between two given images can be determined by computing the ratio <f>F1 conj(F2)/|F1F2|</f>, 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. Converting from rectangular coordinates to log-polar coordinates, shifts representing rotation and scaling can also determined to complete the georectification process. A FFT-based algorithm has been successfully implemented in Interactive Data Language (IDL) and added as two user functions to an image processing software package—ENvironment for Visualizing Images (ENVI) interface. ENVI handles all pre- and post-processing works 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. [Copyright &y& Elsevier]