Convolution Offset and Factor in Frequency Domain
Those lines that keep the range of the R,G,B channels within the valid [0,255] range work on individual pixels. There is a comparison between the pixel value and 0 / 255 levels. Unfortunately, this operation requires the exact pixel value to be known. However the Fourier transform mixes all the individual pixel values into all frequency bins and unless explicitly (and inefficiently) inverted back for a given pixel, this value is not accessible. 

So it's not possible to limit the individual pixel values in the frequency domain.