Fast fourier transform: A Niche, but critical strategy in optimizing impression evidence

Abstract

The evolution from film to digital as the recording medium for forensic imaging has extended the reach of forensic photographers, both in original capture and postphotography processing. Images of fingermarks and footwear impressions are routinely obstructed by substrates displaying intrusive color and pattern. Such backgrounds are frequently sufficiently intrusive as to prevent the analysis and comparison of the images to known exemplars. Digital techniques such as Fast Fourier Transform (FFT) in postphotography processing can optimize the signal-to-noise ratio to a greater degree than was ever possible when film was the recording standard. Occasionally, unwanted backgrounds can be removed or diminished in Photoshop with techniques such as background subtraction and channel blending. These strategies, however, are dependent on either physical removal of the evidence impression to obtain a second image of the obstructive pattern or on differences in color between the impression and the offending substrate. FFT is unique in its ability to optimize the image signal to noise ratio by suppressing the obstructive background, in that it is not reliant on color, and is not applied in the spatial domain.

Keywords

• Fast Fourier Transform
• image pro premier
• postphotography digital optimization