Current Lumen Mirror Reflections

Unreal Waterscape Banner
Unreal Engine Water

Originally, I was going to explain the effect in the header image. I even had a part about the requirements I had for this small scene, but instead, I'm interested in the Lumen reflections here. These comparisons are from Unreal 5.0.3 running on a pre-raytracing-hardware GPU, so take these images with a grain of salt.

I had wanted to create an underwater hotel scene and being able to see the ocean stretching out in front of the player is kinda important for selling the feeling, as too are the reflections of the interior. There's a couple of techniques that would have worked out-of-the-box for this, but I wanted an effect that could support a day and night cycle, and if possible, use Lumen.

While the volumetric effect looks decent to my eyes, I'd like to add a bit more noise to the particles that driven it, and there is also an unfortunate blocking artefact around the higher rock formation at more extreme angles, which I'm kindly not shown as it's not very appealing. But this was content was from before I changed this post.

You may notice that the reflections are a little odd. The look like screenspace reflections, but they aren't: they're Lumen. I'm not sure if this is due to my aging GPU not supporting hardware raytracing, but this shows up in a few other scenes too, such as a planet scene I made a few days ago. Initially, I thought that it may have something to do with the lumen scene quality as seen below, on further inspection, I don't think it does. The lumen scene quality seems to affect the on-screen reflections, but not the reflections of faces not visible in screenspace.

Visual Aid for Lumen Scene
Lumen Scene Quality set to 1.5

For the avoidance of doubt, that black void isn't meant to be black. The room is considerably bigger than the image shows. Underneath is the reflection view that shows the boundary for the scene.

Lumen Reflection Scene showing Lumen's bounds
Lumen bounds in Reflection view

This view distance seems to be the same as the reflection distance, and it could well be an issue with the reflection mode wanting hardware acceleration. Either way, Lumen does run on pre-RTX and -RDNA2 GPUs, and it's worth noting if it has significant differences.

Especially not when planar reflections, which admittedly do raise base pass cost, result in frankly excellent mirror reflections. They aren't quite at the level of render target mirrors, but they are close, and more than passable in game. The biggest issue with the below example is that the reflection doesn't quite align with the floor, but that's my fault for not aligning the glass plane nor the planar capture with any semblance of care.

Earth Scene showing non-Lumen, planar reflections
Test Scene with Planar Reflections

The difference between Lumen and Planar reflections here is stark. The below image has multiple issues, such as a blob where the occluded face of the step should be and the front faces of the mannequin being somewhat mangled. Interestingly, the top face is reflecting near the portion visible in screenspace. To be clear, real screenspace reflections would probably look worse than Lumen, but this highlights that Lumen is not the game in town.

Lumen Non-visible reflections
Test Scene with Lumen Reflections

So, to sum up. Lumen is an awesome global illumination and reflection solution, but it's not always a silver bullet, not yet at least.