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Bilinear down/upsampling, aligning pixel grids, and that infamous GPU half pixel (2021)
See this ugly pixel shift when upsampling a downsampled image? My post describes where it can come from and how to avoid those! It’s been more than two decades of me using bilinear texture filterin…
Some parts of it like phase shifting are so tricky that a famous blog post of “magic kernel” comes up every few years and again, experts re(read) it a few times to figure out what’s going on there, while the author simply rediscovered the bilinear! I’ll have a look at the second interpretation of this filter – it’s [0.125 0.375 0.375 0.125] in disguise 🕵️♀️, but first with this intro, I think it’s time to make the main claim / statement: we need to be careful to use same reference coordinate frames when discussing images of different resolutions. Which is a double edge sword – both great for an easy entry point for beginners, but also a source of confusion once you start getting deeper into it and analyzing what’s going on or do things like fractional or nearest neighbor downsampling (or e.g. create a non-interpolable depth map pyramid…).
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