Costly Full FrameBuffer Copy in GLES3 Renderer about godot_oculus_mobile HOT 5 OPEN

cc @BastiaanOlij @akien-mga

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Indeed, this has always been an issue for VR. While on one side people will want to be able to do some amount of post effects requiring this copy anyway, that we already take the render buffer and copy this to the final output through a lens distortion shader just seems wasteful. It can be more then one depending on the post effects used.

A more efficient way would be to skip the post effect step in Godot and either accept that as a limitation of the platform (on the Quest you're already trying to minimize unneeded overhead) or see if the platform allows some post effects in the lens distortion shader. The problem for other VR platforms Godot has to support is that for say desktop VR you still want to output to screen, something we have to keep in mind if we implement support for this in the core.

The final problem that needs to be resolved if we make it optional to skip the post effects is that the render buffer that Oculus wants us to create, and the render buffers that Godot needs in order to do its rendering properly, don't match up. Godot adds a number of extra effect buffers and requires the main color target to be a RGBA16F based buffer, not all platforms support.

Things will change radically for Vulkan anyway so it may be smart to wait and see where that ends up and then decide how we want to move forward with the new render pipeline.

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From my perspective I think waiting is also probably the best option. When I opened the issue I thought that using GLES3 would be the preferred choice for Mobile/Quest (assuming that OpenglES 3.0/3.1 features would allow better performance). But even with the option to disable post processing I see there are too many performance pitfalls due to the tiled render architecture for mobile with the current GLES3 renderer.
The ES2 renderer seems to be already very well optimized for mobile; maybe in the future some of the optimizations can be introduced by using extensions or allow the ES2 renderer to create an ES3.1 context if available.
I think the general recommendation for users should be to use only the ES2 renderer or wait for the mobile optimized vulkan path in godot 4.0.

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@NeoSpark314 Can you elaborate on the performance pitfalls you're seeing with the GLES3 renderer.
I'm also trying to identify the optimal Godot renderer, and so knowing ahead of time what I should be looking out for would help speed up the evaluation. It'll also be worth listing that information on the project pages since other users might be interested and/or want to weigh in.

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From what I remember at the moment (I stopped using GLES3 after I opened this issue :-) the main issue I saw with GLES3 is that there are many options that require additional full screen passes that are extremly expensive (mostly due to bandwidth): auto exposure, glow, SSAO, and screen space reflections that can easily be turned on in gles3;
On the material side I think sub surface scattering requires a buffer readback (refraction the same).
Mip map generation of the render buffer in GLES3 is also very expensive.

All in all just the demo project with no additional post processing enabled is already close to maxing out the GPU with GLES3 (gpu running at level 4); while in GLES2 it runs comfortably at level 2:

From my current experience I would list the following performance suggestions at the moment:

• Use GLES2
• Always use the ovr_metrics_tool during development
• No Postprocessing (for fadeout use the color multiply from the godot_oculus_mobile API)
• No alpha discard
• Limit alpha blending
• No Screen buffer readback
• Maximum 1 Dynamic Lightsource (not sure yet how Environment Lighting is implemented; maybe its not costly)
• Try to avoid realtime shadows (use only baked lighting)
• ~70000 Triangles visible per frame
• Use 4xMSAA (once it's fixed; the performance impact is quite low on tiled render architectures)

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