Using this submenu, it is possible to filter a buffer (or data already in the memory array) in wave number space. After selection of a valid filter type, the user is prompted for the source buffer (-1 gives the data already in memory), destination buffer (which may equal the source buffer; -1 suppresses the copying from memory to a buffer of the real part of the result). The filter may be applied in either the horizontal, vertical or both directions. The transformation to and from wave number space may be suppressed if appropriate. As noted earlier, these filter operations are of somewhat limited value due to the reduced 256x128 pixel resolution at which they operate. Moreover, as all FFT operations are performed by the CUP and maths coprocessor, they are much slower than if an array processor were used.
In the following discussion, Pij represents the amplitude of the Fourier coefficient with a wave number i in the vertical and j in the horizontal.
Isotropic low pass filter. Each Fourier component is divided by the magnitude of the associated wave number.
Isotropic low pass filter. Each Fourier component is divided by the square of the magnitude of the associated wave number.
Low pass filter which does not affect the horizontal information. Each Fourier component is divided by the magnitude of the vertical wave number.
Low pass filter which does not affect the vertical information. Each Fourier component is divided by the magnitude of the horizontal wave number.
Low pass filter which does not affect the horizontal information. Each Fourier component is divided by the square of the magnitude of the vertical wave number.
Low pass filter which does not affect the vertical information. Each Fourier component is divided by the square of the magnitude of the horizontal wave number.
Low pass filter which eliminates all Fourier components with a wave number greater than a specified limit.
High pass filter which eliminates all Fourier components with a wave number less than a specified limit (except for the zero wave number component which is maintained).
This option allows the user to specify explicitly a filter function.