What's the best way to render dark scenes with minimal light?

[dylan86.exe]

I'm at work so I'll upload my image when I get home, but I'm doing a series of dark home theatre rooms at the moment and I was wondering what the best approach is to do dark night interior scenes with mood lighting only and no light coming through the windows would be? I'm trying to capture the mood of the room when it's dimly lit.

I did a test image last night for the 1st time using CUDA and the result with .005 noise threshold was a little noisy for my liking. I reduced it to .001 today before I left for work so I'll see how that goes but there was also a bit of blotchiness going on too.
I've been told before that the best way for darker scenes is to pump as much light into the scene as possible and reduce the exposure in post? But how can you do that with minimal lighting? Should I be adjusting the lighting to the point where they're clipping and then reduce the exposure? Are there any issues with doing this?

Here's one I did a while ago on CPU render. It's turned out ok. But this is an idea of what I'm going for.

[Neilg]

But how can you do that with minimal lighting? Should I be adjusting the lighting to the point where they're clipping and then reduce the exposure?

use reinhard with a low burn if you're cheating, it'll make it easier. I would also be tempted to go over-bright and darken in post. if you lit it correctly and had to lift any part of it a tiny amount, you'll be pulling up noise.

[glorybound]

How about light select elements with really low light settings and tweak in post.

[piotrus3333]

there is a point where VRay starts having trouble with low intensity lights and realistic camera exposure. In my case same ibl lit dusk exteriors. I just multiply all the lights by 10 and correct that with iso and noise is gone.

[dylan86.exe]

use reinhard with a low burn if you're cheating, it'll make it easier. I would also be tempted to go over-bright and darken in post. if you lit it correctly and had to lift any part of it a tiny amount, you'll be pulling up noise.

Is there any render element I can use to control strickly the lighting? I'm still rather new..
So you would crank the intensity up on those lamps and turn the exposure down in post?

This is what I came back to when I got home from work. It's pretty noisy and notice the blotchiness in the top right corner on the curtain? This is with .01 on the noise thresh and colour mapping with a default burn value on Reinhard

[dylan86.exe]

And would someone mind explaining to me what is actually happening behind the scenes when I turn the burn value down on the Reinhard setting? Does it work the same way the highlight burn adjustment slider works?

[piotrus3333]

And would someone mind explaining to me what is actually happening behind the scenes when I turn the burn value down on the Reinhard setting? Does it work the same way the highlight burn adjustment slider works?

same thing.

[Jiri.Matys]

I had the same problems on some scenes. Sampling in dark areas was totaly wrong, noise was terrible, like some threshold light problem. I solve it only by one thing - higher EV on camera - like 6 or higher. Then render was dark, but without any problem. Then I use exposure settings in VFB to correct the exposure and it was ok. But I think its a bug.

[dgruwier]

Have you checked your max subdivs? If they're too low, noise threshold doesn't matter. 0.001 sounds extreme for such a noisy image, it should be almost perfectly clean regardless of brightness. That being said, those splotches look like GI flicker, not sampling noise. Try upping your light cache subdivs (assuming that's what you're using).


Anyway, even if that doesn't work, you shouldn't have to mess with the Color mapping in the VRay settings, and you probably wouldn't want to. From the documentation:

Changing the Color Mapping settings might be desirable for artistic purposes, but doing so will deviate from the linear correspondence between user actions and the rendered result, and will also veer away from physical accuracy in the scene.

The Color Mapping bakes the corrections into the render and affects how the render is calculated. The sliders in the frame buffer can do similar corrections, but as a post process, which is almost always better. In fact, ideally you'd be saving .exr files, skipping the frame buffer, and doing this in compositing/grading somewhere else (that way you can apply your corrections to animation and to multiple frames), but if that's overkill, just use the sliders in the frame buffer.

I believe there IS a way to use Color Mapping to have VRay judge noise based on how the image is going to look when graded, rather than how it looks when linear, which is a way to solve problems like this. That's more complicated and something I haven't tried, and I think it's overkill to setup for something like this. I'd suggest sticking to the simpler method of just brightnening everything using a physical camera or by changing the intensity of your lights, then bring it back down in post.

[^Lele^]

there is a point where VRay starts having trouble with low intensity lights and realistic camera exposure. In my case same ibl lit dusk exteriors. I just multiply all the lights by 10 and correct that with iso and noise is gone.

No, there is not anymore.
Set all your lights and shaders' cutoffs to 0.0, and V-Ray will (slowly, likely) be equally efficient across the whole lighting range.
Remember than post 3.3 the Noise Threshold is a percentage, not an absolute value.
The CutOff value, however, expresses the minimum absolute value below which to stop sampling for that light or shader.
CutOffs will catch the sampler before it will have a chance to do its job, and when doing very low-lighting shots, many areas will get within that cutoff range.

However, it's really not efficient an approach (not just for V-Ray, either.), as it'd also force low NTs and high sampling across the rest of the image.
It's likely better to expose up a bit the whole image (with the "allowed" part of pre-rendering colorMapping: the Camera Exposure.), to then undo the operation after the render has completed (the old "Wax on, Wax off" approach.).

below: the original render, a close-up, overexposed crop of cutoffs in action, and the same bit with cutoffs to 0.0 and noise threshold and max AA correspondingly lowered and raised.

[PIXELBOX_SRO]

this usually happens to me with HDRI mapped dome lights, less with sunlight.
Do you think Lele things will get better with the new dome light borrowed from Corona guys?

Working with cut-offs across the whole scene to me looks like a lot of unnecessary hustle - should not vray be smarter than that in such areas when rendering darker areas?

[^Lele^]

Well, others do exactly the same thing, or something quite similar along the lines.
I personally know of no solution which is "smarter", currently (or ever before) on the market, and the one doing it slightly differently is rMan, which adds a small brightening to said dark pixels.

This said, cutoffs are a one-line maxscript to initiate change across your scene (two, one for the lights too.), so if someone needs a tool to that effect, ask and i'll make a quick one.

[dylan86.exe]

No, there is not anymore.
Set all your lights and shaders' cutoffs to 0.0, and V-Ray will (slowly, likely) be equally efficient across the whole lighting range.
Remember than post 3.3 the Noise Threshold is a percentage, not an absolute value.
The CutOff value, however, expresses the minimum absolute value below which to stop sampling for that light or shader.
CutOffs will catch the sampler before it will have a chance to do its job, and when doing very low-lighting shots, many areas will get within that cutoff range.

However, it's really not efficient an approach (not just for V-Ray, either.), as it'd also force low NTs and high sampling across the rest of the image.
It's likely better to expose up a bit the whole image (with the "allowed" part of pre-rendering colorMapping: the Camera Exposure.), to then undo the operation after the render has completed (the old "Wax on, Wax off" approach.).

below: the original render, a close-up, overexposed crop of cutoffs in action, and the same bit with cutoffs to 0.0 and noise threshold and max AA correspondingly lowered and raised.

Thanks LeLe, but you might have to tone it down for me a bit as I'm still relatively a noob. So to sumarise, you over-exposed by ramping up the exposure on the camera, then reduced the exposure after render using the sliders, but what is this cutoff you're talking about? Where do I find it??
And what should I be doing with my Min shading rate parameter? Leave it on 6?

[dylan86.exe]

This is the issue I'm having. Notice the noise on the carpet.

[^Lele^]

Yep, it's cutoffs and/or noise threshold.
Try executing these two lines, and rerender.
If it still shows issues, lower noise threshold further until it's gone.
I do not think you'll need to raise the max subdivs, as where V-Ray was able to find stuff it cleaned it very nicely.

for m in (getclassinstances vraymtl) do m.option_cutOff=0.0001
for l in (getclassinstances vrayLight) do m.cutoffThreshold=0.0001