Seeing this came up on the Discord a few days ago, figured I’d put the info we have so far here.
Most stand alone XR devices today are running Android often with Qualcomm GPUs like the XR2 chipset (Quest, PICO 4, HTC Elite, etc.). While very impressive hardware their architecture and constraints due to the mobile format mean that there are strict limits to what these GPUs can do. A lot of techniques Desktop GPUs have no difficulty with, these mobile GPUs will struggle with.
Below is a list of suggestions and considerations to be aware of when making XR applications for these platforms.
Note: this is just a start, hopefully people will add many suggestions as reactions and I’ll try and update this top post with new information as time progresses.
- Do use the compatibility renderer. While more limited in capabilities, its simplicity works well on stand alone devices. You can use the Mobile renderer, especially in the upcoming 4.3 release but there is still work to bring it in line with the compatibility renderer.
- Do not use screen space effects such as glow, DOF, etc. Be aware that Godot will auto enable glow, you must turn it off. Not only do mobile GPUs struggle with these effects, they also turn off various optimisations.
- Do use foveated rendering. For the compatibility renderer check the OpenXR settings and set
Foveation levelto high and enableFoveation dynamic. For the Mobile renderer enable VRS on your main viewport by adding the following line in your startup code:get_viewport().vrs_mode = Viewport.VRS_XR(note: will only be active on devices that support this). - (4.3+) Do use MSAA x2, it’s practically free on mobile GPUs.
- Do not read from screen texture or depth texture nor enable effects like refraction on materials. These are absolute performance killers on mobile GPUs.
- Do optimise your materials, this is something you will want to do late in your development cycle and only if you discover bottlenecks but converting your materials to shader materials and removing unwanted texture lookups, Godot materials are made for convenience and will by default have overhead and can often be simplified. Converting to shader material is also a great way to learn about Godot shader code and getting a feel for what are costly shader options.
- Do limit texture usage per material, it’s tempting to set that roughness, and metallic, and normal, and AO and depth texture, but mobile GPUs struggle when reading from multiple textures. If you can, combine them (roughness, metallic and AO can easily be one texture by using each color channel for each texture).
- Avoid UV calculations in the fragment shader, calculate your UVs in the vertex shader and use
varyingvariables for your texture lookups. - Avoid lights and especially shadows. They are very expensive on mobile hardware. Bake lighting if you can, 4.3 is getting some really good improvements for this. If your player has a flashlight, use a single spotlight. Use omni lights only with static shadow buffers.
Note: A clarification on point 4, MSAAx2 is practically free on mobile GPUs due to the unique nature of tile based GPUs but only under heavy restrictions. E.g. not enabling any effects that require Godot to perform multiple render passes. These effects have already been named as don’ts but they are extra don’ts when combined with MSAA. Enabling effects such as: glow/bloom, scaling and certain color corrections will have a exponential performance degradation that can easily reduce your framerate by over 30%.