(RTX4090) Rendering sci-fi scene- What's wrong here?

Hey guys,

My question is: Is there any way to check with a tool, or analysis, or some plugin what’s being super heavy in my scene that’s forcing such extreme render times?

I made a video of one of the issues too, I believe this is related:

So this is a general question about render times and not really understanding why something takes an insane amount of time to render, with pretty much exact same settings.

I am rendering out some frames from a sci-fi animation/scene that I have. The frames take around 1-2 minutes each usually, great high quality and fast with the RTX4090.

All of the sudden, around after I tweak lighting, tweak a bit of volume shaders, with no apparent reason, the frames start taking up towards 1 hour to render out. I do have adaptive subdivision, so I’m not sure if that has anything to do with it.

This is a test render here, and it’s taking a whopping 1 hour to render out, when it usually takes 1-2 minutes.

With my 1-2 minute renders of this scene, I’m using 0.01 noise threshold, 2048 samples, 1920x1080x200%. So this scene should take a couple minutes, but for some reason it’s going up to 1 hour now.

It does seem that the scene is rendering out much more finely than the others, so maybe the GPU is confused as to what the noise threshold is, and it’s not showing up properly in the settings tab?

Cheers! W.

I also rendered out a full 900 frame animation. Frames 0-800 rendered super fast, great, beautiful, etc. But then starting from 820 ish and forward, it just gets 10-20-30x the render times.

Is this perhaps the Adaptive Subdivision at play? There’s no obvious difference between the frames (816, and 820), yet render speed is greatly increased with the latter:

Perhaps when the pyramid in the scene is out of the frame, adaptive subdivision tries to work really hard? I’m just not sure why pure desert simple models would render out so slow.

hard to tell without seeing the scene, but i’m thinking maybe overlapping volumes? if you disable adaptive subdivision, and re-render a “fast” and “slow” frame, are they hugely different in rendertime?

Will try that, and a few more things. Reporting in a min.

Okay here I made a video of one of the issues here with some frames just taking insane amounts of time for no obvious reason:

Thanks for your help guys!

Such a strange issue – Does anyone have any idea here? Made the video up there. Anytime the pyramid object goes out of frame, render times spike super high.

Without the pyramid, the scene if more uniform, it might have something to do with the noise threshold.
Did you try different values? What happens if you turn it off, and use say 1024 or 2048 samples?

I did, I tried turning it off, turning it up to 0.1, etc. But it still renders very slow comparatively. Perhaps I could try exporting key elements of the scene and recreate it to see if there’s something else going on here.

I don’t think that problem is the pyramid.
I think that this angle of camera.
The more, it is looking to the horizon, the more there are geometry to render.
Adaptive subdivision should help.
But because of camera move, at each frame, adaptation is changed.

So, motion blur is probably requiring a lot of adaptive subdivision computations.

With a LOD manually created or a motion blur at compositing, maybe that would be less problematic.

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It’s just so strange that even having a tiny part of the pyramid visible will make the render time 1-2 min ish, but as soon as the tiny edge of it goes outside the camera, render times skyrocket as in the video. But you’re definitely on to something. I’ll see if I can do something with LOD. I don’t have any adaptive subdiv anywhere else.

Usually those with super fast cards forget all the little speed tweaks we mortals with cheap cards have to set to get render time down…

Try your render with the Volumes section in the rendering tab set to max steps - 100. Then click Fast Gi Approximation and set that to render bounces 3.

Under Light Paths advanced set pattern to progressive multi-jitter and click scrambling distance automatic and calculate light threshold from exposure and try the multiplier at .50

Make sure persistent data is checked, as well. And you might also tick Use Spatial Splits.