I have to add a disclaimer to that code I gave above…
proper alpha-aware resizing should weigh a pixel’s source contributions by their alpha values (as well as any bilinear or other interpolation). since OpenCV’s resize doesn’t do that, it will incorporate fully transparent pixels if they’re within the interpolation area. filling those with a plausible color from non-transparent pixels is a hack and mathematically incorrect… but I struggle to come up with a plausible situation where it would be visible, let alone noticeable.