A lot of defects can be effectively visualized and corrected in the frequency domain.
This gallery explores a couple of them.
In all examples the image on the left is spatial domain image and the image on
the right is the magnitude of its Fourier Transform. They were obtained using the
Save Image & Transform command. More noisy images can be found
and their corresponding corrected versions in the
Coherent Noise causes periodically repeating defects in the image. This can be
caused by electrical equipment interfering with the camera sensors. These photographs
were captured by Lansat 7
ETM+ . More information can be found on Landsat's page on
Coherent Noise . Similar images can be found using
Earth Explorer and selecting the L7 ETM+ SLC-off data set.
The image exhibits a grid of black dots littered over majority of its area.
The noise is manifested into the frequency domain by extra(except the centre) sharp peaks in the
The stray peaks have been eliminated by zeroing them out. The recurring grid
of black dots in the image is visibly reduced.
The image exhibits a similar defect as the last image. Notice the similarity
in the placement of peaks in the frequency domain. This points towards
the same source contributing to noise in both the images.
Image corrected by zeroing out stray peaks.
are observed in a variety of places, one of them is on printed media. It is usually
the result of halftone screens of insufficient density used during printing.
The following 2 examples, attempt to correct Moiré patterns seen in scanned photographs.
The Moiré pattern on the image is clearly visible. The corresponding artifacts
on the frequency domain image are the peaks observed in a circle around
Zeroing out the peaks visibly reduces the Moiré pattern. This could also
be achieved with a band reject filter.
The Moiré pattern is evident on the face of the cartoon. Since the image
was scanned from the same magazine as the last one, the position of peaks
in the frequncy domain is similar
Correction is done in a similar manner as the last image.