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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.

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    $\begingroup$ good news: the math is exactly the same for 1D as for 2D signals, so simply look at how decimators with polyphase kernel/core filters are implemented. $\endgroup$
    – Marcus Müller
    Jan 24, 2019 at 13:55
  • $\begingroup$ If you have trouble understanding down/upsampling I would start with a normal structure of the filter and once you understand how this works advance to polyphase filters. $\endgroup$
    – Irreducible
    Jan 25, 2019 at 9:59
  • $\begingroup$ Turns out, as Marcus Müller pointed out, that the math was in fact exactly the same! Thank you. $\endgroup$
    – ewol
    Feb 4, 2019 at 9:06

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