Dynamically shaped plane cannot cast "Ray Tracing shadows"

Development Rendering
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1

Hi everybody,

I previously [created a post here][1], but I have found where the problem comes from since then. However, I am still unable to correct this problem, and it turns out to be a whole lot different because the problem seems to come with the Raytracing.

So as I said in my previous post, I have a terrain which is generated from noise and then applied to a Plane through a Material with World Displacement. This seems to be working as I obtain the expected results shape-wise : I have a “mountain” looking landscape. I also use the NormalFromHeightmap material node, for my terrain to react correctly to light which, once again, seems to be working all fine.

Now the problem is that I can’t make the Ray Tracing Shadows work with my terrain. The picture below is when I deactivate the Cast Ray Tracing Shadows in the Light settings of my terrain.

341470-2021-06-14-18h53-59.png
341470-2021-06-14-18h53-59.png760×499 293 KB

And here it is if I activate the Cast Ray Tracing Shadows.

341471-2021-06-14-18h54-10.png
341471-2021-06-14-18h54-10.png695×461 265 KB

The shadows simply disappear when I chose the Raytracing as the casting method. I have no idea where this problem comes from as I have already checked all of the followings :

  1. My light ( DirectionalLight) has got the Cast Ray Tracing Shadows enabled ;
  2. My terrain ( a Plane so a StaticMeshActor) has got the Evaluate World Position Offset and Visible in Ray Tracing options enabled ;
  3. My Material has got the Cast Ray Traced Shadows option enabled as well ;
  4. The cube you can see on the picture is also casting Ray Tracing Shadows and it works perfectly with it ;
  5. The terrain, the light and the cube are all Movable ;

I am really at loss here. Shadows work properly without but nothing happens when Raytracing is enabled.
Does anybody have an idea ? Thank you very much in advance.

2

I suppose I got my answer there :
https://answers.unrealengine.com/questions/983057/view.html#comment-983057-form

“I never found a solution. I suspect it’s an RT optimization where they’re tracing against the original mesh as it’s cheaper.”

And there :
https://kristoflovas.artstation.com/blog/Ko9e/what-nots-with-ray-tracing-in-its-current-state-in-unreal-engine

“Displacement with tessellation and world position offset causes incorrect diffuse and specular lighting (They are not visible to the ray tracing).”