See all parts of this breakdown: Photogrammetry: making Nanite meshes for UE5
This is, in my opinion, one of the most tricky parts of the process. In theory, one could think that now, thanks to Nanite, it’s possible to directly import the assets produced by Metashape into UE5. Although this is certainly an option, I don’t use it for several reasons. First at all, I don’t like the way Metashape produces UVs and I prefer to re-uv my meshes. Second reason is that, even if Nanite supports very high-poly meshes, one shouldn’t go too high, so I try to keep my high-poly assets to something still very high but reasonable for the Nanite scene I’m building. In theory I can reduce the number of polygons in Metashape, but as I need new UVs too, I prefer to do all this part in zBrush. I don’t know if this is the best way to proceed and I’m still trying a few variations, so, if you have some ideas, please write them in comment.
In zBrush I have experienced different approaches, the one shown here is the one that works for me in most cases. First, I import the mesh in zBrush and check if its topology is OK (no flying polys). Then, I duplicate the mesh, so I can work on this copy and project from the original one. Even if my target points is around 1 million in order to import the asset as Nanite asset in Unreal, in order to do the UVs in a simple way I need something low poly to start with. When reducing from a very high poly and triangulated mesh, as Metashape produces, to a low poly quad mesh using zRemesher a lot of time it causes zBrush to crush (at least in my experience). Also, I have noticed that zRemesher causes sometimes a loss of sharpness of the object shape. For this reason, I use the Decimation Master plugin to reduce the mesh to something around 10-15k ActivePoints. The mesh produced by the Decimation Master plugin is triangulated and its density isn’t uniform, but matches the shape of the mesh at its best. Then I use zRemesher to produce a quad mesh with around the same number of ActivePoints and I use the UV Master plugin to generate some nice UVs. Once this is done, I project from the original mesh to the new one subdividing the latter a few times until I reach the level of details I want. In a few cases, I’ve skipped the retopology with zRemesher and directly did the UVs, subdivisions and projections on the triangulated mesh.