Imagine you were using a 4K by 3K (12-megapixel) photo from my camera as a background, but you're rendering 800 by 600. So for every rendered pixel, there are 25 background image pixels. Those have to be dealt with somehow. If you don't do texture filtering, then you're only pulling out one of every 5 background pixels from each row or column to put in the background of the render.

So super-sampling definitely applies to background images.

What doesn't make sense, and I mentioned this, is that if the background image dimension exactly matches the render dimension, then there is no reason to be using a reduced-resolution version of it. I don't know why that happens.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)