rtf_init_filter man page
rtf_init_filter — Reinitialize the filter with given data
void rtf_init_filter(hfilter filt, const void* data);
rtf_init_filter() initializes the internal states of the filter with one sample of data as if the filter has been applied indefinitely to constant data consisting by this only sample.
data is allowed to be NULL. In such case, the filter is initialized with null signals (i.e. initialized with an input full of 0.0 values).
If data is not NULL, the array pointed by data should be organized the same way as it would be if used in a call to rtf_filter(3) with one sample.