This function lowpass filters pupil data using a Butterworth filter.
This function is called by the exposed wrapper lpfilt()
Arguments
- x
A data frame containing pupil data
- prev_op
The name of the previous operation in the pipeline
- wp
The end of passband frequency in Hz (desired lowpass cutoff)
- ws
The start of stopband frequency in Hz (required lowpass cutoff)
- rp
Required maximal ripple within passband in dB
- rs
Required minimal attenuation within stopband in dB
- fs
The sample rate of the data
- plot_freqz
A flag to indicate whether to display the filter frequency response