Hey everyone. I’m having a bit of an issue where whenever I use viewport render or render with a small render region, the scene comes out fine, but whenever I try to render the full scene, or even make the render region slightly bigger, there ends up being a LOT of noise.
Viewport render:
Does anyone have any idea what’s going on?
Thanks in advance!
I think it’s a setup problem
i don’t understand; how could images added as backgrounds be the problem? also, they are both inactive in preview and render.
I was mistaken. 
Attached as an image is visible in the preview but not in the rendering.
However, if you’re having problems with rendering, you start by checking if you have objects with similar settings.
Rendering is related to multiple settings, so it won’t be easy to find without looking at the file unless it’s a simple mistake.
Add…
Denoise is not used in the preview. 
Disable rendering denoise and test it.
it doesn’t seem to be a problem with the objects, it seems to be with the entire render as a whole.
and i tried rendering without the denoiser to no avail:
I haven’t experienced the same phenomenon, so I can’t guess anymore without checking the file. And the setting difference between preview and rendering isn’t that big.
If it’s a problem with the blender itself, you can either use a different version of the blender or try resetting the blender settings.
The size of render has its importance.
The bigger the image is, the more pixels be take into account are numerous, the more complex path tracing is, the more amount of samples required is important.
So, that is perfectly normal, that a region render in viewport ( that has its size dependent of zooming in viewport) may require less samples to denoise than a region render with a bigger fixed amount of pixels.
Possible light paths are also a lot more complicated, if light may bounce several times between different objects than if you only render one surface.
So, you may need stronger lights, less bounces taken into account and/or more samples, a lower threshold to obtain desired result.
that makes sense. the only thing that’s confusing me is why isn’t it happening when looking with viewport render?
This is the kind of noise level I would usually expect from light going through a glass shader or a small opening.
Do you have an object that’s hidden in the viewport but visible at render time?
But even then, I see you have path guiding active, so I have no idea what could possibly be that noisy.
yes, it is going through a small opening (with a portal). but no, there doesn’t seem to be anything hidden in viewport but visible in render.
Did you let the render complete to its full samples in both cases? Is the smaller zone cleaner just because it renders faster?
By the way, portals only work with light coming from the background (HDRIs) and not with light objects.
yes, i did.
i wouldn’t think that’s the issue, since they’re both rendering the exact same thing. (also, while the overall scene may take less time to render, the tiles take the same amount of time, so that wouldn’t change anything.)
i know, i do have a hdri out there.
Looking at the failed image, I see the noise is placed in clustered patterns/patches of light.
This tells me that only a few rays are finding the light source before the noise treshold activates and so there are large parts of the image that don’t get refined, because Cycles didn’t even find light there at all.
Considering the render settings you are using in your images and what you told me, it sounds like path guiding is somehow failing at the larger scales. If you deactivate path guiding, does the render become consistent no matter the size?
i’ve tried that; exact same result.
Some more ideas:
Did you change the sampling settings at any point in the testing? Did you active/deactivate the noise treshold?
Have you tried rendering with different devices? Cpu, Gpu, Optix/Cuda? If it’s a bug, that could have an effect on the end result.
i have tinkered with a few things, but nothing made it any closer to being fixed.
unfortunately, i don’t have a good enough graphics drive to use GPU rendering. but since it worked with CPU processing the first time, that shouldn’t be a problem.
Then, this does seem like a case I have never seen.
Would you be willing to upload your file so I could take a look? If you don’t want the project to spread, you could strip it of any important object or texture.
I have found the problem: it’s because you are rendering with a small tile size. This seems to mess with path guiding, preventing it from working correctly. It works if I use a much bigger tile size, or deactivate tiling completely.
