Ray Tracing Translucency and other WEIRD BEHAVIORS

I would like to point out a couple of weird behaviors with the Ray tracing Translucency. I don’t know if it is a problem of the actual stage of development or if is something I haven’t figure out yet.
I’m using a Threadripper 1950x with a Nvidia 2080ti, 64gb ram

The backface of single sheet meshes are being rendered as two-sided-translucent even if it has a simple opaque material

When the ray traced translucency is disabled the back faces disappear as it should. In the example above the squares are planes, one facing the camera the other away. The tree in this example has a simple masked material (no translucency or two sided foliage).

Two sided foliage:
Another problem is setting up a material with two sided foliage. I’ve tried a lot of things but it seems that when the raytracing shadows (on my skylight and directional light) is on there is no two-sided-translucency, when off it seems to work.

In the example above I have the same planes with a Two sided folliage simple material. The SSS color is ser to pink. As you can see, it only, kind of, works only without the raytrace shadows from the light source.

Crazy Flicker:
Most of the time, after turning ray tracing on (global, relfection and translucency), the viewport suddenly starts to flicker and the only way I could figure out to make it stop is turning raytrace off or closing and opening the unreal editor

In the example above you can see a couple screenshots from the flicker, it seems to be changing between buffer passes or something like that, it affects everything with raytrace and even the sky . Really annoying.

Max Refraction Rays:
Using max refraction rays above “2” will wash out other raytrace passes

I’m using in the example above a simple glass material with “o” in the specular and roughness, a fresnel in the opacity and almos white color. This was the only way I could make a single sheet glass to work with raytrace, but, it also has some weird behaviors like the one above. If I set the Max refraction rays to 3 or more it will simply occlude the other raytrace passes for everything behind the glass.

More images bellow from the Flicker effect.

I also have been experiencing some flickering in the viewport. Usually it starts after I have been working in the scene for a while (20 minutes or so). I’m getting two types of flicker. The most annoying is a flashing effect, where it almost appears there is a plane flashing on and off just 10 to 20 feet in front of the current view. The other, more debilitating flicker is a series of colored lines that flash and flicker in the viewport. These actually appear in my saved video files when rendered out through a Level Sequence. Here’s an image that shows the lines, I haven’t captured anything with the flickering plane.


I don’t have this line flicker here, just the first one, very similar to your description

I’ve solved the line flickering! I had a Post Process Volume that set the Bloom Method to Convolution. Setting that back to the default Standard method cleared up the lines. The flickering of that mystery plane is so intermittent that I have not yet had a chance to see if it returns with this change. It usually doesn’t happen unless I’ve been working in the scene for a while.

Question for you @alexandrecollaco - are you using Datasmith files in your scene? We suspect there are issues when using Raytracing in scenes using Datasmith imports.

Yes, My test scene was modeled in rhino and imported via datasmith using the DataPrep to automatically assign material.
I also have another scene exported via datasmith from 3ds Max, same problem there.

Here is a video with the flicker

My team has noticed issues when using Datasmith and raytracing in Unreal. Back in 4.23 (maybe 4.22?) we noticed there was a memory leak, and after working in a scene for a half hour or so the software would abruptly crash. Things have improved, but I suspect there is still something between DS and RT that is not working right.