[HtoA] Transmission Depth on top of a subsurface unwanted outline

[Adam_Chabane]

Hey there. Im using:

Houdini: 18.5.499

HtoA: 5.6.0.2

Arnold: 6.2.0.1

Im not finding a way to solve this unwanted visual artifact. I've got a ground surface with a subsurface shader applied (randomwalk). On top of this surface I have a mesh with a liquid shader using tranmission and transmissive depth. Where the edges of the liquid contact with the subsurface ground I get a harsh dark line. This only happens when my liquid has transmissive depth and the ground uses subsurface.

Here is a screenshot displaying the unwanted effect. My liquid has a green depth and the ground has a pink subsurface.

Anyone got any ideas how I can get rid of this effect? I presume its because of the subsurface returning a colour under the surface with the depth in play it becomes darker than expected.

Here is the same ground without subsurface:

Many thanks, Adam

[Stephen.Blair]

Do you have a simple repro scene? Or maybe just the shaders, and I can use a plane and a box?

[Adam_Chabane]

Yep sure thing Stephen - i've recreated it on my home machine and here is a zipped hip file:

sss_depth_unwantedInteraction.zip

You can recreate it with a sphere scaled in the Y so its very squashed - with a transmissive shader that has depth in it. Then a grid that has a subsurface (randomwalk) shader intersecting midway through the sphere - if you add a red SSSr you can see the blue outline where they intersect is prominent. No depth or SSS and there is no intersection outline.

[Stephen.Blair]

Hi Adam

Thanks for the scene file.

I think you're hitting a limitation of the current SSS implementation, and we're getting sss rays entering the sphere, and then exiting as transmission rays, and picking up a tint from the light absorption.

We've already got some similar tickets, but I think this should be a new, related ticket.

I was able to make the seam a lot less visible by making the SSS scale a lot smaller (0.00001)
Or conversely, I wasn't seeing the seam at first because my test geo was huge in comparison to the geo in your repro scene.