I'm trying to get rid of some periodic flicker noise through post-processing of the recorded images. The reason for these artifacts is that the electronic rolling shutter of the camera reads each line sequentially, and this mechanism results in brightness variations in the image, when the picture is taken under fluorescent lighting (due to 50 or 60 Hz AC power frequency).
I tried calculating the Fourier transform of the image and suppressing the components that might cause flickering. I inspected the magnitude spectrum (as you can see below for an example image) and set the vertical components around the DC component to a very small value (I preserved the DC component). In the end, I managed to eliminate the flicker because I suppressed the flicker frequency, but also the image was sort of deformed since I, probably, also suppressed some other frequencies that contributed to the signal's energy.
I would like to know what would be a better way to detect the flicker frequency and eliminate/suppress only that frequency? In my current implementation, I could be doing something fundamentally wrong, so I would appreciate any guidance and hints. Below, I show the resulting image that I got for the kind of filtering that I explained above. You can also find my Python code below.
import cv2 import numpy as np from matplotlib import pyplot as plt img = cv2.imread('images/flicker2.jpg',0) f = np.fft.fft2(img) fshift = np.fft.fftshift(f) # calculate amplitude spectrum mag_spec = 20*np.log(np.abs(fshift)) r = f.shape/2 # number of rows/2 c = f.shape/2 # number of columns/2 p = 3 n = 1 # to suppress all except for the DC component fshift2 = np.copy(fshift) # suppress upper part fshift2[0:r-n , c-p:c+p] = 0.001 # suppress lower part fshift2[r+n:r+r, c-p:c+p] = 0.001 # calculate new amplitude spectrum mag_spec2 = 20*np.log(np.abs(fshift2)) inv_fshift = np.fft.ifftshift(fshift2) # reconstruct image img_recon = np.real(np.fft.ifft2(inv_fshift)) plt.subplot(131),plt.imshow(img, cmap = 'gray') plt.title('Input Image'), plt.xticks(), plt.yticks() plt.subplot(132),plt.imshow(mag_spec, cmap = 'gray') plt.title('Magnitude Spectrum'), plt.xticks(), plt.yticks() plt.subplot(133),plt.imshow(mag_spec2, cmap = 'gray') plt.title('Magnitude Spectrum after suppression'), plt.xticks(), plt.yticks() plt.show() plt.figure() plt.subplot(121),plt.imshow(img, cmap = 'gray') plt.title('Input Image'), plt.xticks(), plt.yticks() plt.subplot(122),plt.imshow(img_recon, cmap = 'gray') plt.title('Output Image'), plt.xticks(), plt.yticks() plt.show()