Ray tracing - huge flickering. What is the problem? (VIDEO attached)

I have huge flickering (half of the screen gets black). I use Ray tracing. UE 4.24.1, 2080ti + latest Nvidia studio drivers. Please check out my video with this issue:

Thank you in advance!

Hm, havent seen that happening in my workflow. Are you sure that you dont have duplicates of the same mesh? Also when you play the level does that happen there too?

Hello! No duplicates, as far as I can see. Yes, it flickers even when I play it (and capturing movie). It looks like GPU switches between RTX and RTX OFF modes and back for a millisecond.
It can be a materials-issue. I’ve used Datasmith for this project. Another project (mush simpler) works ok.

Can you try to migrate the whole level to e new project and see if you get the same problem?


Same issue here…(when RTGI is on)


I’ve tried to migrate my level to the new EPIC “ArchvizRT” sample project. And the base level of the example project started to flicker (there were no issues with ArchvizRT before on my PC). Even when my level is not loaded. I’ve tried to capture youtube video, but at some point, it just stopped flicker.

I have also changed in my original project all materials to standard grey but flickering continues. Only removing half of movable RT lights helps. So, I thought that it can be because of the number of lights. I opened “ArchvizRT” example project and added 20 new different movable RT lights. I’ve expected that it starts flickering, but no, no flickering at all. Will write if I will find the real cause.

Hi Nico, what is your graphics card?

My first thought was about well-known hardware issues with RTX cards. I have Gigabyte Aorus Extreme 2080ti with Samsung video memory chips, that work on 1990MHz core frequency and 1400MHz memory frequency. I have tried to lower the core frequency to 1300MHz and memory frequency to 1000MHz. But it keeps flicker.

Also, I have downloaded and tried the newest game-ready and studio drivers with no success.

ahh, interesting. Post an update if you fixed the problem. :wink:

I know it’s a basic check, but are any of the movable lights overlapping? There’s a default of 4 overlapping lights noted in one of the lighting tutorials in UE’s learning portal, and the docs. It can be set to a higher number of overlapping lights.

Could it be reflection captures?

Hi mifa,

asus RTX2080 (1110mhz gpu clock, 1400mhz memory clock)… Preston42382 might have explain the pb :slight_smile:

Another thing to check is out of bounds pixels for meshes and lights. There’s a visualizer for it in the Show button. Go to Show > Visualize > Out of Bounds Pixels, and any blue, yellow, and white colors mean areas of the meshes / lights are extending beyond their bounding boxes (default color of bounding boxes is purple, and can also be shown / hidden in the Show button via Show > Advanced > Bounds). The directional light doesn’t count because it casts light through the whole scene, so mainly point, spot, rect, and other area lights. Here’s a link for it:…els/index.html

Would you be able to upload a project that exhibits the flickering issue somewhere for us to investigate?


****** HERE’S FLICKERING FIX *********************

Official 4.25 and 4.26 having some glitches in calculations, namely TemporalAAHistory structure was not filled out correctly

Here’s the only solution that will surely fix for both 4.25 and 4.26 versions (building from the source and fixing engine glitch), since 4.25 will not have any more updates and 4.26 also has the same issue:

  1. get Unreal 4.25 source, following link is a step by step instructions on how to get the source (no need for the building until you change buggy scripts, just when you get 4.25 source unzipped from git, run Setup.bat, after that **run GenerateProjectFiles.bat **): Building Unreal Engine 4 (UE4) From Source Code / GitHub - YouTube

  2. **change this 2 scripts **as explained in the below link, and finally, **build the source **…48f5a38dfb53ea

  3. compile your project from source (in the following link, instructions are around 8.50 min): Building Unreal Engine 4 (UE4) From Source Code / GitHub - YouTube

Enjoy again your UE4.25/26


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I found the flickering issue is always associated with a skylight actor in the scene that has Real Time Capture enabled. I suppose that was the main cause of the issue.

Same issue, but with HDRI map on Skylight. It works fine for a while, but then starts flickering. Additional settings that cause this:

  • Cast Shadows = True (on SkyLight either Stationary or Movable)
  • Cast Ray Tracing Shadows = True (on SkyLight either Stationary or Movable)

Ray Tracing Global Illumination (on PostProcess)

  • Type = Brute Force
  • Max. Bounces >= 1