I recently started designing a Polyphase FIR filter in with a lanczos kernel interpolator to resample an image in Python. The goal is to downscale the image using these methods. However, I think I don't understand the intuition behind these processes.
I have a RGB image with dimensions (1000,1910,3) which is sent through the polyphase filter with the lanczos kernel, and I can manage to upsample the image. But downsampling is giving me nightmares. My grand theory is that I just don't understand the basic principle of what interpolation and downscaling actually means when applied with the polyphase filter. It is mostly the tap-parameter in the filter, and its impact, that I just don't seem to get. Is there anyone who could explain the polyphase filters function in regards to image processing simple words?
I am quite new to applied signal processing, so I apologize for my noobines.