Reflection pass sample noise vastly different between GI modes

[smbell]

We're working on a 4k stereoscopic 60fps animation that's 5 minutes long, so needless to say, I've been working on lowering the render times and noise, as we don't have the time or hardware to use the denoiser for this project. In my messing around with the different GI modes I noticed that the reflection pass is quite a bit more noisy with IM/LC than with BF/LC, like a lot more noisy. I tried a few things to see if I could clean it up a bit and nothing at all seemed to even change it in the slightest. I turned on local subdivs and cranked the mult up to 8, as well as increase the local subdivs of the shaders, I also tried all the presets for the Irradiance map. The only thing that worked was turning off glossy effects all together, but obviously, this isn't a solution here. I can't think why the GI calculation would affect the reflection pass. It's just using the sun/sky setup at default settings. Honestly, it seems noisier than using the other setup I was playing with which was an HDR image, I would have expected the sun/sky to be cleaner.

I've attached some images that show what I'm talking about. L side is reflect, right side is GI

The first image is BF/LC. The reflections have a bit of noise on the plane, but underneath and the tower in the back are not too bad, GI has some noise I was trying to eliminate with IM.

The second image is IM/LC. GI is great, all the noise is gone and the render times are cut in half, but the reflections are crazy noisy on the plane and the tower, especially underneath the plane.

This is basically negating all the noise reduction from the IM mode in my beauty pass. Is there a way to clean the noise up in the reflection pass. I would have thought turning on local samples and upping the reflection samples would do it, but it did not.

[smbell]

so I forgot about the divide shading subdivs switch, when I turned that off, I was able to adjust the reflection subdivs on my shaders and remove the noise, but my render times are really long now... back to my original question though, why did the GI mode affect the subdivs of my reflections so much?

After looking at the lighting and reflection pass back to back, it looks like the IM/LC mode is not taking the shadowed areas into account as much and they are very noisy, whereas the BF/LC mode has a distinct difference between the noise levels in the parts that are lit by the sunlight and the parts that are in the shadows and it cleaned the dark shadowed parts up more.

[stezza]

Maybe a quicker method for render times is to turn off GI and just place some very specific lights to make it look like you have a GI solution, colored light to simulate colour bounce, that sort of thing.

[smbell]

Maybe a quicker method for render times is to turn off GI and just place some very specific lights to make it look like you have a GI solution, colored light to simulate colour bounce, that sort of thing.

It's signed off on as far as the whole look, so I was just trying to optimize as best as i could. Seems like a GI solution shouldn't affect the reflection pass in that way.

[Morbid Angel]

what are your dmc samples? usually noise like that means not enough dmc samples. I would also check if using local subdivs is helping the render time.

[vlado]

V-Ray only adaptively samples based on the RGB channel. If the overall result is smooth enough even with low reflection subdivs, V-Ray will not make any efforts to clean up reflections specifically. With brute force GI, V-Ray is forced to add more samples anyways, which is why the reflections come out cleaner.

It may help to increase the min. subdivs for AA, or alternatively to use local subdivs and increase the subdivs for the reflections only.

Best regards,
Vlado

[smbell]

Thanks, increasing the local subdivs of my reflections seemed to do the trick!