[Gear VR] Multiview is buggy in 4.14.1

Hello!
multiview is a beta feature that is very dependent on the device and OS you are running on, due to driver support issues. 
Could you please provide us your device model, OS version, and UE4 logs of a multiview app running?
thanks a lot

rpalandri said:

Hello!
multiview is a beta feature that is very dependent on the device and OS you are running on, due to driver support issues. 
Could you please provide us your device model, OS version, and UE4 logs of a multiview app running?
thanks a lot

I am running Galaxy S6 SM-G920T with Android 6.0.1 and all Oculus software up-to-date.

I'll get you logcat as soon as I can.

I just wanted to explain something real quick as well on what exactly multiview is and does.

Multiview enables single CPU engine traversal, which means that instead of rendering all the objects on the left eye, then all the objects on the right eye, it renders all the objects, and every object render will render left eye and right eye at the same time. This allows to gain a lot of CPU time by going through the scene only once, which can be seen in the engine by typing "stat unit" and looking at the CPU timings. In an app that is CPU bottlenecked, like a lot of GearVR apps are, this will help the app get more frames out by moving the bottleneck to the GPU.

However, if your app is GPU bottlenecked, this will not help at all, since the GPU's work is the same. Worse, if we look at the actual shaders, there is more vertex work to be done on the vertex shader stage to unpack the view we're running on (before, the CPU was gonna handle telling the GPU which view it's running, but now since the CPU renders both eyes at the same time, the GPU has to do that work). We also have to do an extra full buffer blit to copy the multiview texture into the original one (an unecessary operation that I'll have a workaround for in SI1.11). So I wouldn't be surprised to have multiview slow your app down if you were at the limit of 60fps, and GPU bottlenecked.

@rpalandri

Well, this time my app just crashed and didn't even run. Here is logcat: https://drive.google.com/open?id=0BwE6dxM0O2PscTQ0Y0dyRzVOT3c

Apparently a nasty bug in UE 4.14.1. If I disable "Support Low Quality Lightmap shader permutations"  option in the Project Settings > Rendering (a way at the bottom), the app will crash.

I am gonna have to re-enable it and try getting you another logcat.

@rpalandri

There ya go: https://drive.google.com/open?id=0BwE6dxM0O2PsbERnaXV2ZGJFTVk

UE 4.14.1 Multi-view ES2 logcat log.

I'm not seeing anything blatantly wrong there, so I can't help you out further. 

In my post above, I was saying that we're doing an extra copy (copying the multiview result in the normal framebuffer) when running UE4 Multiview right now. I'm hoping to be able to land a parallel path to avoid that copy in the next SI release on our github, so hopefully that should help alleviate your issues. It still won't gain you any GPU perf though, only CPU.

I'll take a look at that multisampling issue. 

@rpalandri

Having more CPU performance is what I actually need, not GPU.

In our game the performance drop when upgrading it from 4.13 to 4.14 was really big. We are rolling back it to the 4.13 right now.
The multi-view gave us problems with billboard and the performance was really really bad, at least on 4.14.1.

@pekayatt

I didn't notice any performance drop at all, compare to 4.13. Are you using ES3.1 renderer ?

We are using ES2.0, when changing to ES3.1 we got a black screen on our game. Not sure if had anything to do with the multiview...

We got around less 10-20 fps on our game (The Rabbit Hole). The actual build (launched on store) runs smooth even on a Note4...