Darker images render faster

[Magnum_design]

I am optimizing a scene for animation. This is quite interesting what to find out trying different settings/ hdri and materials. First of all when I render the scene bright and beautiful the way it should be, it takes 13 min / frame inkl noisereduction. But when I lower lower ISO and render ot a darker image it takes only 7 min. After noise reduction and postprocessing it looks almost exactly the same.

Anyone else that is using this trick without any drawbacks / flickering ? Another interesting thing is how much changing Hdri:s can change the rendertime. Even Hdri from the same serie looking not too much different in renders... I got it down 30% by swithing... This´s hell of a rendertime saving in big animations altogether

[Magnum_design]

[Morne]

Materials with RGB values too high, or very bright lights, can cause excessive render times.
Darker images at same settings, will render faster, but with INCREASED noise issues, which can clearly be seen here by the post brightened image, which will be more evident with animation

[glorybound]

Yep, I agree. No white brighter than 200,200,200 and no blacks darker than 10,10,10.

[Muhammed_Hamed]

But when I lower lower ISO and render ot a darker image it takes only 7 min

underexposing your renders is pointless. Like what Morne said above, you will get a lot of noise just like using a higher noise threshold.
Overexposing your renders then adjusting the exposure in post works the same. Renders will take more time to finish, but they will be cleaner, just like using a lower noise threshold.
So why not just rely on your noise threshold for this? do a side by side comparison , you will get the exact same render times.

[Magnum_design]

Yes you´re correct guys. I focused on wrong parts of the image. When I study it again I see the noise clearly. Well atleast I learned how important it is to not have overbright materials in the scene. Thanks

[Magnum_design]

underexposing your renders is pointless. Like what Morne said above, you will get a lot of noise just like using a higher noise threshold.
Overexposing your renders then adjusting the exposure in post works the same. Renders will take more time to finish, but they will be cleaner, just like using a lower noise threshold.
So why not just rely on your noise threshold for this? do a side by side comparison , you will get the exact same render times.

It´s easy to say that if you have free fast renderpower and clients that don´t care about the price. Unfortunately it´s not the case here I´, trying to find a middleway...

[Magnum_design]

I don´t think we should look away from this totally though. If noisereduction could handle the dark wall parts more smooth instead of producing blotches I think this might work quite well. Need some animationtesting though

[^Lele^]

https://forums.chaosgroup.com/forum/...t-bonus-script

[Neilg]

If you rendered the first one with lower settings so it had the same level of noise as the re-exposed dark one pre-noise reduction they would take the exact same amount of time to render.
your noise reduced one looks worse, of course it was quicker.

[dgruwier]

It´s easy to say that if you have free fast renderpower and clients that don´t care about the price. Unfortunately it´s not the case here I´, trying to find a middleway...

It has nothing to do with render power or clients, they're not suggesting that you just live with the longer render times, but that you use cleaner method to get the same reduction in render time:
When you reduce the brightness of the image, render times are lower and noise levels are higher, because VRay allows more noise in darker areas of an image (and if the entire image is darker, it will allow more noise in the entire image). Instead of reducing the brightness, you can just directly tell VRay to simply allow more noise in your existing image, by increasing the noise threshold. Your render will still be faster, you're just skipping the whole step of rendering it dark and brightening it again.

[Magnum_design]

It has nothing to do with render power or clients, they're not suggesting that you just live with the longer render times, but that you use cleaner method to get the same reduction in render time:
When you reduce the brightness of the image, render times are lower and noise levels are higher, because VRay allows more noise in darker areas of an image (and if the entire image is darker, it will allow more noise in the entire image). Instead of reducing the brightness, you can just directly tell VRay to simply allow more noise in your existing image, by increasing the noise threshold. Your render will still be faster, you're just skipping the whole step of rendering it dark and brightening it again.

Ok thanks. I get it now I thought I had found a better way than to lower the noise threshold. But I was wrong

[^Lele^]

When you reduce the brightness of the image, render times are lower and noise levels are higher, because VRay allows more noise in darker areas of an image (and if the entire image is darker, it will allow more noise in the entire image).

This isn't exactly right.
The details as to how and why are mired in tough maths, but V-Ray strives to ensure noise level is uniform across the screen.
Add a noise level RE, let a render run to a given Noise threshold, with enough max AA subdivs, and you will see the noise is very uniform.
It's also not exactly accurate that an overall very dark image will converge quicker, as there are biasing methods to reduce overall tracing coming into play which may skew the results.

What *is* quicker is an image with darker shaders, as the GI won't bounce for as long, leading to less divergence and less overall energy content, and converging quicker.
This won't darken the image per se', as much as contrast it (two different mathematical operations).

[vlado]

In the darker image, the contrast between the pixels is lower when there is noise so V-Ray doesn't sample as much as it does for brighter image. You might get the same result simply by raising the noise threshold a little bit instead of making the image darker.

Best regards,
Vlado