How to correctly resize image with a different linear interpolation scheme

C++
2

I will just ignore all that code you posted and recommend that you use warpAffine() with a suitable transformation matrix, and the WARP_INVERSE_MAP flag, so the matrix directly describes the calculation from destination coordinate to source coordinate.

afaik, INTER_AREA only works for resize(), not for any of the warps

if you need a lowpass, do the lowpass explicitly.

or use pyrDown()