Dark volume with glass problem

JoeDDD · Post by **JoeDDD** » Mon Oct 21, 2019 1:39 pm

This is glass material with ior = 1, opacity = 0 with homogenous volume

And this is glass material with ior=1 (air) and opacity = 100% and with the same volume setup.

Both pictures should look the same but when using glass with ior 1 volume turns dark

Post by **Dade** » Mon Oct 21, 2019 2:02 pm

Air IOR is 1 not 0

IOR is the ratio between light speed in vacuum and light speed in your volume. Air has a light traversal speed very near to vacuum so IOR air is about 1: https://en.wikipedia.org/wiki/Refractive_index

A IOR of 0 can not exist by definition of IOR (it would require a volume with an infinite light speed). Just any value < 1 is not possible aside in Start Trek (it requires to have light traveling faster then itself

).

JoeDDD · Post by **JoeDDD** » Mon Oct 21, 2019 2:14 pm

Dade wrote: ↑Mon Oct 21, 2019 2:02 pm Air IOR is 1 not 0

IOR is the ratio between light speed in vacuum and light speed in your volume. Air has a light traversal speed very near to vacuum so IOR air is about 1: https://en.wikipedia.org/wiki/Refractive_index

A IOR of 0 can not exist by definition of IOR (it would require a volume with an infinite light speed). Just any value < 1 is not possible aside in Start Trek (it requires to have light traveling faster then itself ).

sorry my mistake. ior is set to 1 in both cases

Post by **Dade** » Mon Oct 21, 2019 2:23 pm

It depends on what you are trying to achieve but you may want to use NULL material, not glass material: in your case, a path tracer can not render the light received by the inside volume (because it is going trough a specular material like glass). While it can render the amount of light received trough transparency.

The other option is to use new shadow transparency: viewtopic.php?f=5&t=1474
Or render real SDS paths: viewtopic.php?f=5&t=1449

Egert_Kanep · Post by **Egert_Kanep** » Mon Oct 21, 2019 2:45 pm

So if I was rendering a sea I could also benefit from transparent shadows?

Post by **Dade** » Mon Oct 21, 2019 2:58 pm

Egert_Kanep wrote: ↑Mon Oct 21, 2019 2:45 pm So if I was rendering a sea I could also benefit from transparent shadows?

Only if you are looking for lighting some under water object. In your case, you need to use glass because of water refraction (NULL material is similar to a glass with IOR = 1 without the problem of specular materials and path tracing).

I think the best solution for you would be to use an OpenVDB to describe the heterogeneous volume and PhotonGI caustic cache to render under water light. But I have no idea if there is out there a tool to model such volume. I have seen this kind of stuff over time so I assume some one has looked into the topic but I'm not sure if there is a general available tool.

I think most people are just faking water by using some colored texture for the surface.

Post by **Dade** » Mon Oct 21, 2019 3:35 pm

Dade wrote: ↑Mon Oct 21, 2019 2:58 pm I think the best solution for you would be to use an OpenVDB to describe the heterogeneous volume and PhotonGI caustic cache to render under water light. But I have no idea if there is out there a tool to model such volume. I have seen this kind of stuff over time so I assume some one has looked into the topic but I'm not sure if there is a general available tool.

I think most people are just faking water by using some colored texture for the surface.

This kind of stuff: https://80.lv/articles/beach-rendering- ... a-houdini/

JoeDDD · Post by **JoeDDD** » Mon Oct 21, 2019 3:46 pm

Dade wrote: ↑Mon Oct 21, 2019 3:35 pm
Dade wrote: ↑Mon Oct 21, 2019 2:58 pm I think the best solution for you would be to use an OpenVDB to describe the heterogeneous volume and PhotonGI caustic cache to render under water light. But I have no idea if there is out there a tool to model such volume. I have seen this kind of stuff over time so I assume some one has looked into the topic but I'm not sure if there is a general available tool.

I think most people are just faking water by using some colored texture for the surface.
This kind of stuff: https://80.lv/articles/beach-rendering- ... a-houdini/

I dont know why but when I use shadow color option specular highlights disappear:

This is black shadow color (0,0,0)

And this is white shadow color

Even if I use mirror material with dark gray shadow color lights in the reflection will turn black.
here is a picture:

Post by **Dade** » Mon Oct 21, 2019 5:13 pm

That is a MIS (Multiple Importance Sampling) bug, can you post the test scene ?

JoeDDD · Post by **JoeDDD** » Mon Oct 21, 2019 5:31 pm

Dade wrote: ↑Mon Oct 21, 2019 5:13 pm That is a MIS (Multiple Importance Sampling) bug, can you post the test scene ?

That picture is just a material preview screenshot of a mirror material and nothing else really.

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