Why is the mist missing from reflections...

Hi everyone,

I’m relatively new to Blender, and I’ve been giving myself projects to learn it better. I have one thing that I’ve been struggling with on and off for weeks and I’m beginning to wonder if I’m going mad or if there is just something I’m missing.

I created this scene a few weeks back for a Weekend Challenge… and overall I’m happy with it… I’m using it as a project to learn and working on refining the things I don’t like…

In this scene - The fog / mist works great… except where the neon is reflected in the puddle… in that reflection there appears to be no fog at all. I’m aiming for realism with this one and the lack of fog in the reflection is a dead give away.

Can anyone help me understand why this is and how to correct it?


The fog is being created by an object with a volumetric scatter material which envelops everything, including the camera. Nothing particularly fancy:


and my light path settings - which I thought might have something to do with it… but I’ve fiddled with these quite a bit to no avail:


I’ve followed a few fog / mist tutorials which is how I got to where I was… but the missing fog puddle has stumped me.

I would totally appreciate any explanations / pointers to things I could try / tutorials that might point me in the right direction.

Thanks!

It’s possibly caused by your anisotropy value.

Anisotropy scatters light forward towards the camera. Given your reflective surface is below the light source, less light may be scattered towards it and hence it is less visible in the reflection.

Try rendering the scene with it set at zero and see what happens.

Thanks for the recommendation. I tried it and, unfortunately, no dice.

At first I thought maybe it was something strange about my puddle material so I created a super-simple test file to demonstrate. It simply contains a text object set as neon, and a plane set to glossy and a Fog Cube. (I also added two fog materials, one with 0.0 Anistropy, and one with 0.07. Slight difference in the fog, no difference in the reflection)

I’m not sure if I should be happy or sad, but it demonstrates the issue perfectly. Fog around the object. No fog in the reflection. I even tried it on two different computers, just to be sure.

Have you seen anything like this before? Am I missing something?


You seem to be using a pretty aggressive clamping for the indirect light - might that be the culprit?

Hmmm, that does seem to have something to do with it. Setting that to 0, I get a little fog in the reflection. But the image is so covered in fireflies that it’s unusable. If I set it to the lowest setting blender will allow: 0.0000001, the fog is gone from the reflection again. :frowning:

Setting it to 0 is the same as setting it to infinite. If you want to clamp it (specially with sharp glossys around), you need at least a clamp bigger than 1.
Clamp cuts the light amount. If clamp is 0.1, then every ray carrying more than 0.1 will be reduced to 0.1.
If clamp is 2.0, then incoming light is clamped to 2.0. When Clamp is 0.0, Cycles assumes it is disabled and let any light amount coming in.

Ah HA! :smiley: You are awesome! That was it. Thanks to you and to ikariShinji!

I had my concept of Clamping inverted. (or more accurately, thinking they were relative values) So less clamping is actually a higher number… A quick adjustment to indirect clamping solves the problem. It’s still a bit noisy, but I think I have something to work from now.

Since clamping seems to be open-ended number-wise - can you share the ranges / values / bounds you generally find useful?

it’s completly scene dependent. I normally keep them at 0, but when I tweak with them I start by the intensity of the brightest light (or lower) and go the way down untill satisfied. I try to keep it above 2, but it depends also in the bounces you’re using… With bounce=0, it can be set to 1 with no stress.

and just another thing, scenes like this one, with very dark areas and small very bright areas, look more noisier much because your view is not calibrated to see all that is stored in the high range data, so the brightness of fireflies may be greatly reduced when the color space is correctly calibrated.

Good to know. Thank you! :smiley:

When you say color space is calibrated, what do you mean? Do you mean like using the Filmic Blender color management / contrast levels or something else?

Thanks again!

PS - feel free to point me at a tutorial / link if you have one handy. I really appreciate the info you shared!

I tend to use zero clamping as well initially, just to be able to identify that I have a firefly problem and no other ways to handle them. I will sometimes try to handle them using the “identify fireflies, copy good offset pixels” method in post. In render of yesterday I had a firefly problem limited to a small area, but wasn’t able to use the post method. But I initially start quite high, like clamping to 500 and then working my way downward (depends on scene lighting of course). My scene was completely firefly free at 100 direct and 50 indirect, and it didn’t impact the scene lighting pretty much at all. Setup was filmic low contrast exposure -2 with a quite bright extremely low sun peeking in a bunch of (fake) windows, mixed in with a bunch of interior lights.