RTX Reflection and Metallic material: More Roughness = going darker?


i have a question abouth RTX and metallic materials, or rough materials in general: As soon as i increase the roughness, the materials reflection tends to go darker and darker, until it becomes pitch black at 100% roughness. Why is that so?
Shouldn´t the reflection just get a more average intensity, as one pixel gets the color from more than one spot, or not from the one spot, it would get with zero roughness? So while a rougher material will reflect a ray in an unknown direction, the overall intensity of the surrounding don´t change and the seen reflection intensity will be more an average value from the surrounding.
There is also a noticeable difference between pathtraced and rtx reflection in that regard.

I did a comparsion scene in Cinema4D with a simple material, that have reflection only and gets from 0 to 100% diffuse/roughness, recreated it in UE4, set raytraced max roughness to 1 (so that it should not switch to SSR). What am i missing? Or is this a bug?

The spheres at top row go from 0.1 to 0.4 roughness (in C4D 10% - 40%), the second row from 0.5 to 1.0, and the single sphere at the bottom have zero roughness.


UE4 RTX Reflection:

UE4 Pathtraced:

Do you have reflection capture actors in the scene? My guess is RTX falls back to the reflection actors when a material is too rough to notice. And if there’s no reflection capture actors it fades to black. There might be a bug or some other issue that causes this even when roughness is set to one.

No there are no reflection capture actors, have deleted them, because RTX should do everything. But there was a noticeable increase in intensity after setting rtx max roughness to 1, os i thought, it worked as intended.

What does the material look like? The graph.
It doesnt look right at all, 1 roughness should still give diffuse color.

@Suthriel the Realtime Raytracing is being released as Early Access, so it is expected some features are not correct or complete. The last Livestream Epic did last week, we were told that a lot of improvement were made in terms of quality and speed. Also features like translucency, landscape and foliage which were not present and/or working will be OK at release 4.23 which we might see the 1st preview in 3 weeks approx. I would greatly recommend what that livestream as lots of info is given and you can find the videos at or YouTube at the Unreal Engine 4 channel.

It´s probably the most basic material you can get: just white as color, and two scalar parameters for metallic and roughness input. I made a quick video with a completely fresh and empty default project and played with the roughness value.

Yeah, i am aware of that, was just confused, if i did something wrong on my end or misunderstood something aout roughness. Shall i file a bug report for this behaviour?

I would submit and also send a small project zipped along or pictures to state the issue. Better 4.23 come out without the issue then not submitting the bug and see it there (I don’t think it will appear thou). It would be curious maybe, pick the source at github with the latest rendering branch, build and give it a try, this way you might find more useful and working material for comparison than what we have with 4.22

Got an answer from UE Support, they created a report for this issue:

Great! Voting on it.

Just to add to this, the main issue is when the surface becomes completely black. The material getting darker as roughness increases is a known issue with the current microfacet shading model not including energy compensation, but it is being researched/integrated elsewhere at the moment so perhaps we may get it in UE4 the future.