FreqDIBHighpass  Method

Description         This method attenuates the low frequency components. It uses Fast Fourier Transform (FFT) algorithm to obtain the Fourier spectrum. In the frequency domain, a filter function attenuates the low frequency components. After that, the inverse FFT algorithm is applied to obtain the result DIB. It works with 4, 8 or 24 bits/pixel. In case of 4 or 8 bits/pixel, the source DIB must contain grayscale images and the palette values in the source DIB must be monotone increasing. It makes no sense to use this function in case of 1 bits/pixel. If the source DIB size is not a power of two, zero padding is used.

Examples

Description: macska                                          Description: macskahighpassed

Syntax                 lRet = [BIFilter.]FreqDIBHighpass Dib Cutoff Type bFFTMethod bShowDialog

Remarks             lRet                         LONGLONG Handle of the newly created DIB on success or 0 on failure.

                            Dib                          LONGLONG Source DIB.

                            Cutoff                    Long                Cutoff frequency.

                            Type                       Long                The filter function in the frequency domain.

1: FILTER_IDEAL
2: FILTER_BUTTERWORTH

                            bFFTMethod         BOOL             Fast Fourier Transform method

                            bShowDialog         BOOL             Show dialog box

 

Cutoff frequency.
This function does not pass any frequency components if the ‘Cutoff’ parameter is greater than the radius of the image. For example if the size of the source image is 128 x 128, then the zero pass frequency is  sqrt(2) * 128 / 2 = 90. Of course this is true only if nType = FILTER_IDEAL.

Ideal highpass filter (‘nType = FILTER_IDEAL)
The name ideal filter indicates that all frequencies outside a circle of radius ‘nCutoff’ are passed with no attenuation, whereas all frequencies inside this circle are completely attenuated.

Butterworth highpass filter  (‘nType’ = FILTER_BUTTERWORTH)
H(u, v) = 1 / ( 1 + ( D0 / D(u, v) ^ 2) )
H(u, v):      Filter function, ‘u’ and ‘v’ is the frequency variable.
D(u, v):      Distance from the origin of the frequency plane:  sqrt( u*u + v*v )
D0:            Cutoff frequency (nCutoff)

bFFTMethod
If bFFTMethod = TRUE, then the Fourier transformed of the image will be processed by Sin and Cos functions, if bFFTMethod = FALSE, then it will be processed by square root functions.

bShowDialog
This parameter specifies that is the dialog box to be showed or not.

See Also