What happens with UVs of joined objects?


I’m working on a project in which I have a prop divided in a few different objects. I’d like to join them together, but I don’t know if that could affect their UVs, since their materials are set up independently. I’ve tried to actually Ctrl+J the objects, and the materials seem to behave as they did before merging them. But I don’t see different UV maps in the new joined object, so I fear to be missing something “under the hood” that could cause me problems in the future.

So, how does Blender handle UVs when joining objects exactly?


You can set multiple UVs on an object under the data tab. I think if you were to unwrap though, after joining together, then you would end up with one UV map. I do this for things like bottles where you have one UV for the bottle and another for the label, but its one mesh.

Do you really need to join? Would grouping instead be enough?

To add to what Jimbo said, if the UV maps have different names before joining, they will remain separate maps once joined.

If, however, the UV maps are not named (all have the default “UVMap” name.), the UVs will be joined into one UV space. In this case, so long as the joined parts are given separate materials and/or textures, it is completely fine that the UVs overlap within the same UV space.


Yeah, I’ve been thinking about making a group, but I wanted to fill that gap and know what happens there. The thing is, if after joining them I end up with a unique, aparently unmodified UV map, shouldn’t the textures of the different materials overlap? Why do they look correct? I know maybe this is a stupid question, since I have not an actual problem, but as I said I wanted to know how the software handles this.

EDIT after @cgCody’s comment: Oh, I see. I guess that answers my question! Thanks! :smiley:

I cant swear to it because ive never done it this way round, but i think if you select your object and go in the data tab in the properties panel, you will see a list of UVW maps. I think when you joined them you have ended up with multiple UVs for the object and your just seeing the top most one in the list when you go into the uv editor. You just need to switch to the others.

The neat things you can actually feed diffrent things to different UVWs in the node editor… like i said… i do it with bottles, so I can feed an image texture to the label UV and a glass shader to the rest of the bottle on another UV and mix them… You get the idea :slight_smile:

Oh, no, it’s what @cgCody said. When I joined the objects I didn’t see only the top one of the list: they worked correctly! And I had a single UV map! :stuck_out_tongue_winking_eye:

I would also like to know how Blender knows which UV map corresponds to each material without using an Attribute node.

If you have several UV maps on one object, then you’d use the UV Map node. The attribute node is no longer needed.

In your scenario where all joined objects are sharing and overlapping within one UV map, you have different materials applied to the various parts. It doesn’t matter that the UVs are overlapping as the UV map is nothing more than reference on how the 3D objects relate to a 2D coordinate space. That space is being referenced by the different materials for different parts.

On the other hand, if all of the joined objects were sharing one texture and/or material, then the overlapping could be an issue.

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Yeah, I realised it a moment ago and came to answer my own question, but you were faster. It was so obvious! Ha, ha. Thank you anyway, Cody, helpful as usual :slight_smile: .

No problema, man.

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