Render passes in low and high res images...

[teykey]

Hi I wanted to ask a question:

I wanted to ask if I render the same scene twice - once high res, once lower resolution then after the same amount of passes (which of course will take less amount of time in low res image)
the amount of noise will be the same, right?

I mean basically in both images after the same amount of passes (not time) the noise reduction will be campareable, right?

It's just a question.

[FrostKiwi]

Mathematically yes.
One Pass is each pixel being sampled once in the entire image, after all other rays did their bouncing. So samples per pixel stays the same. GI noise stays the same.

Visually and subjectively you are compressing more information with a lower resolution on a smaller space, which can result in things like moire effect (which can look like noise) and more detail being lost in subpixel level space. These subpixel level detail and moire effects may require more samples per pixel to get the effect of the higher res one being scaled down.

Easier explained: if you make an image 16px by 9 px, it will take a bit longer to get all the different influences "projected" onto the small amount of pixels from the image to get these couple of pixels to look like a scaled down version.

Also Adaptivity was mentioned by devs to be non deterministic (behaves a little bit random each time) though the differences may be even too small to pixel peep.