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
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