Filter: FFT

 

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.

 

 

Add the "Filter FFT" class in the "Class management"

mode and press "Update DB" to activate the new class.

A new "Filter FFT" mode will be available from the "Mode" menu

 

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 their 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

 

 

 

Normal CT image

W = 0

W = 1%

W = 10%

 

 

From the Graphic Interface

 

 

 

Instance selection

Select which of the instances of the class you want to work on.

 

Demo images

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.

 

High/Low pass

Filtering operations are controlled by the High pass and Low pass parameters.  The sliders use a logarithmic scale.

 

W

 

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.

 

Update DB

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

 

The FFT filter does not play well with some other classes.  For example, it can not be used with the convolution filters.  If used in conjunction with some other classes (such as ROI) the order of the classes in the "Active clases" tree is important.