I’m finding that I can’t get Lumen to produce a surface cache at all on any HISM. All Hierarchical Instances produced at run time through a blueprint will render but do not show in the Lumen Scene and Cache. I just get the yellow debug color on the mesh in the Lumen Overview.
However - Instance Static Meshes produced from BPs work just fine with Lumen.
I’ve tested in a new empty project with no changes to defaults, using the engine assets and had no luck. See renders below.
Has anyone else been able to get Lumen to work nicely with HISM?
I’ve been working with lumen pretty extensively for a while (even ran through the entire CVar list once), and I still haven’t found any real explanation for why it has this problem. There’s a chance what you have is a VSM problem, in which case your only option is to boost the sample counts (I forget the CVar). Iff it’s lumen, then it’s an issue with indirect shadow casting, which I haven’t found a way to adjust as readily in the settings .
Didnt notice until now (and didnt check…), but I converted my normal ISMs to HISMs with custom data for colors etc… but never checked if it is fine, turns out… it isnt:
By the way, I have an incredibly specific series of technical questions, and I might tag the devs at some point, but don’t want to bother them unless I have to.
Lumen uses almost every single light transport method I’ve ever seen, SDF scenes, voxel cone tracing, hardware ray tracing, and a variety of clipmaps, caches, probes, and more. I see that there are options for adjusting ‘radiosity’, but I’m somewhat confused by this option.
Doesn’t radiosity only solve for entirely diffuse (Lambertian) light? And if so, what method does lumen use to handle specular and near-specular lighting? Just temporally reprojected and filtered ray-tracing? Is that why .4 is the roughness value when costs increase so much more for material evaluation?
Doesn’t radiosity require discretization of the surfaces it operates on (breaking it up into small plates whose visibliity can be compared to one-another), and if so, how is that done for SDFs? I can understand it for triangle meshes ,but figuring out how to discretize a distance function isn’t immediately clear to me.
How does the lumen scene fed by radiosity tie into the screen-space and world-space radiance probes? Is the Lumen scene just packing world textures into a low-res texture to solve memory access problems, and the radiance probes simply operate on this scene? And if so, what does the ‘evaluate lighting at hit point’ even do? If it’s ignoring the lumen scene, what is it using to let radiance accumulate frame-by-frame?
This is technical curiosity, if anyone can speak to this at all I’d be extremely curious, but not expecting any miracles.
Hello, I’m not sure if this is the right place to ask, but I’m having serious flickering issues with my reflections in UE5 and none of the recommendations online (among others console commands or change of reflection method) seem to have any effect.
Here is what I get when using Lumen Reflections and “Surface Cache”:
I hope the compression still makes this obvious, but inside the red box (the right hand filing cabinets) the reflections ‘wobble’ like crazy. This doesn’t look good and isn’t usable for final delivery.
Having said that though, this is by far the nicest rendition when it comes to reflections and highlights.
Next up is Lumen Reflections with “Hit Lighting for Reflections” enabled:
Definitely less ‘wobbling’ but it’s still happening, especially on the handles of those filing cabinets (see the red boxes). Also the highlights have become dull and the whole scene looks a lot less attractive.
Finally the “brute force” method, using “Standalone Ray Traced” instead of Lumen:
In terms of ‘reflection wobbling’ this gives the cleanest result, but again the scene is super dull and now I’ve got some crazy shadow fringing around the door knob (see red box). Maybe the dullness is due to Global Illumination not being factored into reflections when using the RT or HLfR options.
Either way, I like the first option, i.e. Lumen Reflections with Surface Cache, but I want to get rid of the wobbling, flickering, noisy reflections. What should I do?
Thanks for the response. I’ve been messing around with the VSM’s as well and nothing seems to fix it either, even the CVars I’ve tried. The only way I’ve been able to get rid of the artifact is to either switch to Stand Alone Raytracing (Deprecated), or to switch to path tracing. Also turning up the base colors values to something brighter seems to hide it to an extent but that’s not a fix. Those are the only things I’ve found that get rid of the artifacts or at least hide it. Which leads me to believe it’s lumen, or something directly related to lumen. If you find anything interesting or manage to fix it, please let me know
Two more issues I found while doing HISM-Investigations.
1: Even normal ISMs dont appear in Lumen Scene after a certain distance, a too close distance to be more precise:
Especially in my case, it makes a difference if a surface thats directly hit by the sun is black or colored, its not just missing light… its missing color too. (in certain contexts this can be very visible.)
Metallic surfaces/reflective (ISM!) stuff is always black instead of using the color of the metal, therefore “eating” the light instead of reclecting as metals usually do.
Aside from that, the trees are waaay too dark too, it doesnt even remotely match the color of the leaves. (and for some reason my tree is on fire? xD)
These “dividers” are now missing from lumen and the MDF completely, despite having worked fine in EA1 and EA2. (There is nothing special about them, 2 ISMs, stupidly basic metal material.)
I asked about the grainy shadow issue on Twitter and Wright who is working on lumen graphics in UE responded:
try ‘r.Lumen.ScreenProbeGather.StochasticInterpolation 0’
High scalability sets this to 1, which makes Lumen slightly faster but noisier. You can override in your game’s DefaultScalability.ini
I will, if I ever understand what exactly is causing it. In EA I once went through every single lumen CVar one by one, trying to understand what they did. I seem to remember a ‘reference mode’ that significantly increased the number of radiance probes while disabling denoising, and that improved quality at the cost of a performance nosedive.
I suppose it’s partly a use-case question. I’ve discovered that for offline, Lumen is excellent as a sort of viewfinder for pathtracing, allowing quick but imperfect iteration. Lumen and the PT largely match on medium and large scale, but I’ve noticed lumen struggles a lot with objects bigger than screen-space but smaller than the surface cache can fully pick up, like your situation.
Regarding Foliage Problems and Lumen, posted this on Twitter:
“I had to roll back Two Sided Foliage shading model support in Lumen last minute because of some nasty side effects, and I ran out of time to get it in the 5.0 release. =( Will be revisiting soon.
However the foliage over-occlusion fixes remained”
@_gkaradayi This is me We’re discussing the matter with … I guess no one realizes what is and how consistent it is. There is no need to delve further into the matter here.
I really appreciate understanding the reasoning, definitely. I imagine something similar might have played out with Lumen mirror translucency, but I just wouldn’t know. That being said, translucency is already way complicated in Unreal, and no rendering method short of full path tracing seems to get it exactly right, so I’m certainly not surprised it needs work.
Kind of related, but they got lumen reflections working on SLW and it is amazing. It even works on the Niagara fluids, although why the ray-tracing cost doesn’t explode with that massive particle count is beyond me.
