This is a web app that computes the 2D Fourier transforms (FTs) of images. After a FT is computed, an image is generated representing the magnitudes of each of the constituent sinusoids.
Check out a live demo at: http://turbomaze.github.io/JS-Fourier-Image-Analysis/
Low frequencies are in the center of this image, per usual. Entering a value in the "Low pass radius" box removes all sinusoids that are more than that many pixels away from the center: those with high frequency. Setting the "High pass radius" removes the sinusoids that are within the specified number of pixels. If you enter both a low and high pass radius, a band filter will be applied. That is, only sinusoids between the low and high radii are kept.
Once you click the reconstruct button to restore the image, you can see how the result differs from the original. Green areas are the same as the original, blue areas are darker, and red areas are brighter. Currently (due to CORS issues), only images that are hosted on the same web server as the web app can be transformed.
For more info about Fourier transforms/an example app that uses this code, check out this blog post about evolutionary art.
To use this module, include the js/fourier.js file in your webpage.
To compute the FFT of an input array, call the Fourier.transform(data, out) function where data is the array of vaalues you'd like to FFT and out is an empty, pre-declared array that will be filled with the transform.
For the inverse FFT, use Fourier.invert(transform, sig) similarly.
To compute the FFT of an image, first, draw it to a canvas and get the image data with [CanvasRenderingContext2D].getImageData. Then you can run the above functions on a copy of the resulting array. See the demo code in js/main.js for examples.
You can compute ta low pass/high pass filter with the Fourier.filter(data, dims, lowPass, highPass) function. data is the FFT output, dims is a two-element array representing the dimensions of the original image, lowPass is the optional low pass radius and highPass is the optional high pass radius.
MIT License: http://igliu.mit-license.org/