FFT filters are great for dealing with periodic noise. Where do we see this? Latent Print Analysts see this a lot when dealing with lifting prints off of bank checks.
To get us caught up, I'll let the Amped Folks explain periodic noise and Fourier transforms. "To clearly understand what a periodic noise is and how it can be eliminated, we need some background. Them "Fourier transform" is used to decompose an image in its frequency components, while the "Inverse Fourier transform" synthesizes an image from its spectrum of frequency components. An useful analogy is the relationship between a series of pure notes (the frequency components) and a musical chord (the image itself). In an image, the high frequency components are represented by sharp details like, for example, going from black to white suddenly (two neighboring pixels). Low frequency components are instead represented by areas with no details at all."
Ok ... let's move on.
Double clicking on the Fourier filter in the Filters box brings up the Fourier dialog. While ImageJ's FFT functionality is free form, Amped Five's is very precise. "Since the spectrum is symmetrical with respect to the center of the image, the filter will black out the rectangle you selected and its companion. These black rectangles correspond to the frequencies of the spectrum that have been eliminated." You select the area to mask, click Add to add it to the list, and the software does the rest - including documenting the location and size of the mask. Documenting? Yes. But we'll get to that later. Here, we just draw our boxes on one side, and Five fills in the opposite side's boxes as well as the coordinate info and height/width calculations.
Clicking on the Image button lets you check your work. Notice how well we can see the print. Good, but not great. Let's test the image clarification tools.
We can adjust contrast with the easy to use sliders, or input the actual values to test someone else's work.
We can even choose to make a negative of the image.
As we work, Five is building a chain of effects that are completely editable. Just click on any one of them to edit their values - or hit the delete key to remove it.
When you want to save the final file, just use the image writer.
Choose your file type, name, and save location. Then, click on the Play button to save the image.
When you're done, you'll want a report of your activities. Simply select Generate Report from the File menu. Here's a sample portion of the report:
Notice that it lists the filter and what the filter is supposed to do. It also lists the parameters, including the location of the selections. Finally, it gives a text reference for the technique - should you need to cite a published source for your technique. How cool is that? Cooler still, the report is put together by the software automatically.
Enjoy.
