UE4 performance - viewport drain

Attempting to output a double-wide 1920x1080 (so 3840x1080) render target from UE4 and split it between two projectors in LA – got it working, but it dropped our fps from ~90+ to ~10fps.

One thing I noticed was because our player controller is still outputting to the UE4 viewport, it’s functionally rendering a 1920x1080 and the 3840x1080 views at the same time… is there any way to turn off the main UE4 viewport render for the player?? With the Scenecapture2D output going to LA, we don’t need to have UE4 outputting a separate view of the player camera to the viewport…

I assume there’s no way to send the player camera to LA from UE since render targets are exclusively mentioned in the tutorials? Also was beginning to attempt two 1920x1080 SceneCapture2D’s to output to each projector, but figured that was likely even less performant…



Hmm, that sounds like a very big FPS drop indeed. It makes me think the reason might be somewhere else, but I’m not sure obviously. Does the performance drop happen also if you are not actually sending Spout out? Just using Scene Capture with render target? Also, what’s your VRAM usage?

Just as a comparison, we ran this scene on NVidia P6000, 32GB, Intel Xeon Gold 6126. I don’t remember the exact framerate, but it definitely didn’t drop that much. We used Scene capture component with a Render target with 4K resolution (3184x2160).

To your questions: I wouldn’t consider myself a UE4 expert and you might be better off asking this in UE4 forums, but afaik there is no way to ‘disable rendering’ as such. You might be able to do the same thing to play with the settings of the active camera. Alternatively, if you are using a packaged or standalone game, you will decrease the performance cost of viewport rendering by decreasing its resolution. There is a console command for that (I think it’s something like r.SetRes 640x480, but you should check that).

Lastly, UE4 has very good profiling tools that might help you pinpoint the biggest load.

Let me know what you find out.

Good luck!

Hey there, following up on this – the drop from SceneCapture2D / Render target usage is still a major hurdle to using Light Act in Unreal.

Just having the SceneCapture2D running drops a 50fps scene to around 25/30fps (this time outputting a 1080p texture rather than 4K), and I’m wondering if there’s any way to avoid this?

I’m not sure the VRAM usage (any suggestion on how to view the usage?) but I see 6144MB dedicated video memory, if that’s relevant.

These are my settings for the capture / render target:

Thank you!

Hi Sam,

Sorry for the late reply. I was out of the office.

The performance drop happens because UE has to render everything twice - once for the viewport and once for the Scene Capture.

Therefore, the goal is to limit how much resources viewport rendering consumes.

As suggested by this thread, when reproducing your problem, I created a box with a black unlit material and placed it just in front of the viewport camera.
This helped to increase the FPS of the game, however, it didn’t reach the same level as when SceneCapture2D was switched off.

I hope this helps.

That does help, thank you – so sounds like this will be a performance hurdle for the foreseeable future then, since there aren’t any plans/ability to take output from a UE4 camera instead of the render target?


There are several approaches we are discussing internally and with Epic. When any one of them proceeds to a sufficiently developed stage I’ll let you know. In the meantime, please use the above suggestions.

Best regards,

Hey there Mitja, apologies to keep following up on this topic but it’s the main hurdle to us being able to use LightAct in our projects – still getting 60fps projects dropping to 20fps just from having the LightAct scene capture running (when I set it to hidden in game the fps bounces back up).

Wanted to make sure my above settings are correct and that there aren’t any other ways to optimise the scene capture while still keeping it 1080p and functional with LightAct? Thanks so much for the help!


Is there any chance you could send me the whole UE project? I would really love to have a look at it and to see if there is any way to avoid the issues you are experiencing.

Best regards,

Hello again,

I’ve got one additional question though: does this happen in all of your projects or just some? If our Unreal Engine Sample Project works fine on your end, then I’m afraid there might be no other way than to optimize the problematic project itself.

What’s a bit confusing is that while there obviously is an additional load that happens when you insert a Scene Capture component, we or any of our other users, that we would know, haven’t experienced anything resembling your performance drop. Especially if your render target is just 1080p. So maybe there is something in your project that doesn’t work well with Scene Capture component?

Just trying to find the root cause of all of this… :no_mouth:

Good luck!

There’s a drain regardless, it’s just less of a fps drop in the simpler projects – I’m not sure why more complex projects would exponentially increase the fps hit by Scene Captures but it’s the case across all of my projects.

Regardless, via the Spout Plugin GH page I noticed that there’s a setting to send viewport, removing the need to use a Scene Capture if you’re doing a single output (also simplifies camera switching since with Scene Captures if I’m moving cameras I have to set up activating/deactivating Scene Captures as opposed to the simpler Set View Target With Blend) :

I haven’t tried this with LightAct yet but will report back if it circumvents the FPS issue.