I recall that years ago I couldn’t use mesh box for skybox and had to use skysphere that came with UE4. While it’s not a problem generally, every triangle counts in mobile VR.
I am wondering how I can have a mesh box and equirectangular image married, so that I could have a skybox with minimal polycount.
The skysphere mesh isn’t that dense. A good rule of thumb to remember is that triangles only get really expensive if they start to go smaller than a pixel, for UE4’s skysphere this isn’t the case unless you look straight up maybe, and then there probably shouldn’t be enough onscreen to slow it down, at least in most titles.
I’d worry about other things first. Another rule of thumb is optimization is generally tackled by going after one biggest problem to the next until the title runs well enough. If your biggest problem is somehow the skydome mesh, then even on a mobile VR title you’re probably already doing great and likely good to ship.