For sure there is some geometrical reason why it acts like that but please take a look at these two meshes, both created in Rhino 7, both with the same unwelded edges/seams, but one of them was exported to Blender and it was unwrapped there.
(BTW, this is a shower cord, and all I wanted to do was to give it a properly scaled shower cord texture.)
As you can see in the UV layout, unwrapped meshes differ a lot and thus texture mapping. Mesh unwrapped in Rhino has very unequal spacing - which means that I failed to map texture on it. The same mesh unwrapped in Blender has it right - the texture is distributed uniformly.
So, if this kind of uniform mapping (correct? mapping) is not automatic, is it even possible to achieve in Rhino?
In short, _ExtrudeCrvAlongCrv with options (splitattangent no, )
or _Sweep1 and rotate the UV mapping island in the same direction.
If you use unwrap but give you bad result, you can extract the surface and “Delete the Mapping” so that it turns it back to the default. You can end up adding a lot of work time.
Or If you what to deal with the mesh or more complex objects, use more dedicated soft RizomUV Real Space not Blender.
When you make the surface, to achieve a correct UV mapping, try to use different surface command tools to get the correct UV mapping. I mean, each surface solution has its own default UV mapping projection. So there will probably one that fits this case. But later since that polysurface will be connected to another polysurface surface (join or boolean) you lose the original default surface UV mapping. You can use _unwrap select the correct seams to take out the surface and restore to default the UV mapping.
Dealing with the actual mesh is more complex in Rhino because it is lacking fast multiple mesh edge selection for selecting the seam, point constraints for not moving edges, and correct reprojection tools. So is difficult to work moving the points of the final UV mapping mesh.
I try UV mapping in Blender there is a nice course for that. But when you start making UV mapping in complex objects you will understand that UV mapping is a 3D science and the work involved is the same as making the 3D model itself. So a dedicated 3D software exclusively made for UV mapping is necessary.
Unfortunately, there is no software for making UV mapping projections for surfaces (I mean NURBS or similar). Like Rhino is trying to do unless (maybe) Alias.
So in my case after experimenting with a lot of software I end up using RizomUV Real Space for dealing with mesh UV mapping. Since the object we make in Rhino is often hard surfaces with radius, do not use Blender because it will not preserve your vertex normals properly making incorrect average vertex normals.
When you apply UV mapping in Blender, Blender makes a new average vertex normal instead of preserving the super awesome Rhino vertex normals.
Imagine a transition from a flat surface to a constant radius surface (corner edge with radius). The mesh in Rhino is visually the same as the surface. but in Blender, after UV mapping is not. There will be a bad soft transition between flat and radius. And the radius mesh in Blender will not look like a radius. It looks like a potato especially if there is few mesh poligons.
Hi @Czaja, Rhino uses LSCM for unwrapping. Angle based flattening has been requested earlier and I believe that is what you have used in Blender for the better of the two results.
I will add reference to this thread for RH-42693 Unwrap: option to use angle based flattening algorithm
I’m not a programmer so I hope I am not mistaking anything.
UVPacker provides a free C++ library (even for the commercial products). Plugin does a great job inside Blender, so maybe they could be used inside Rhino?