The FFT filter use the FFT transform to filter the images. An FFT transform of the selected frames is computed. Then we use the High pass and Low pass parameter to force part of the FFT transform to zero. An inverse transform is then computed from the FFT and the resulting image is the filtered frame.
If we do a straight transition to zero when the FFT is outside the high pass / low pass range, we will introduce "ringing" in the filtered image. We use the parameter "W" to smooth the transition using a sinus function.
For example, for the low pass filter, if we have a frequency cutoff of "L" then the FFT values will be attenuated by the factor "fact" depending on thier distance "Dist" to the center of the image (0=center, 100%=farthest from the center):
if ( Dist > L + W ) then, fact = 0
if ( Dist < L - W ) then, fact = 1
else fact = (1 - Sin( π/2 * (Dist - L) / W ) ) / 2
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Normal CT image |
W = 0 |
W = 1% |
W = 10% |
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The "Filter FFT" class is enabled through the 2D Mode: "DB Class management" interface. Once installed, a new button will be visible in the "All clases" list. |
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From the Graphic Interface
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Select which of the instances of the class you want to work on.
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Three demo images are provided to help understand the filter. The image on the left is unfiltered, the image in the center show the FFT of the image with the region of the FFT that is forced to zero by the high pass and low pass sliders. the image on the right is filtered by doing the FFT inverse on the FFT in the center image.
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Filtering operations are controlled by the High pass and Low pass parameters. The sliders use a logarithmic scale.
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W
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Size of the low pass / high pass transition. The low/high pass filters force the FFT to zero. A rough transition will induce rigning in the filtered image. We use the parameter "W" to prevent this.
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Clicking on this button will apply this filter to the selected images. |
From the Display Area
There is no display area interaction specific to this class.
From the Keyboard
There is no keyboard interface specific to this class.
From the Command Line
There is no command line for this class.
Technical Note
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