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I've imported a model from MagicaVoxel, and it uses UV mapping to assign vertex colors. I'm editing it in Blender 2.8, but I'm running into the issue that one vertex has 2 UVs in the same UV map! I thought this was impossible, and I need one vertex per UV. See the image below:

enter image description here

Here's a video of the same effect in motion (you might need to full-screen to see the detail)

As explained in the image, selecting that vertex in the 3D viewport auto-selects 2 UVs in the UV editor. Alternatively, selecting one of those 2 UVs auto-selects the other, but the stats bar at the bottom says I've only picked 1 vert.

Question:

How can I split these type of vertices that have multiple UVs? I'd like one vertex for the light-gray UV, and one vertex for the navy blue UV. My scene has 135k verts, so I would be very grateful for a solution that doesn't require I hand-pick them.

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  • $\begingroup$ A vertex has a UV for each face it belongs to. All the UVs for a vert can have the same value (and hence appear as one in UV editor) If on a seam it is likely the vertex has more than one unique value. Related blender.stackexchange.com/questions/158259/… $\endgroup$ – batFINGER Jan 30 '20 at 4:49
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A vertex exists in five-dimensional space, with coordinates (x,y,z,u,v), so while two vertices may share coordinates in 3D space, they can still be in different places in UV space.

Every time you create a UV seam, or have two different shading groups (flat shading), under the hood you are splitting those vertices along the seam into multiple vertices. It's confusing though because Blender will still treat these as a single vertex in many circumstances.

The way you can get around with for UV editing is to turn off UV Sync Selection: enter image description here

The down-side is that only elements of your mesh that you've got selected in 3D view will show up in UV view, but this will allow you to manipulate vertices separately in the UV editor, even if they appear to be the same vertex in the 3D view.

enter image description here

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  • $\begingroup$ Thank you for the explanation, now I understand what's going on. But is there a way to select all those vertices that point to 2+ UVs in separate locations? I'd need to rip those vertices so each one only has 1 UV. I tried Select Similar > Amount of adjacent faces, but in a cube that selects all 8 verts. $\endgroup$ – Marquizzo Jan 30 '20 at 23:51
  • $\begingroup$ I'm not 100% sure I know what you're asking. Take the vertex with the red arrow in your pic. What specifically are you trying to do with that? Are you trying to separate that in the 3D view, or in the UV view, or combine them? $\endgroup$ – Rekov Jan 31 '20 at 1:39
  • $\begingroup$ The vertex I have selected in the 3D viewport: I need to split it into 2 separate vertices; one for each UV. I achieved this with "Rip Vertices", but I have to select each vertex manually, and with 135k verts, it's really slow. How can I batch-split vertices like these so it's always one independent vertex per face? $\endgroup$ – Marquizzo Jan 31 '20 at 1:45
  • $\begingroup$ It looks like you mostly need to be able to separate groups of faces based on material. You can accomplish this by selecting all the faces of one material from the Material Properties tab. Then separate them from any other geometry by hitting Y. But the main point of my answer was supposed to be, you don't actually have to separate them in the 3D viewport in order to separate them in the UV editor. If you turn off UV sync selection, they will behave as two separate verts in the UV map, even if they're still one vert in 3D. $\endgroup$ – Rekov Jan 31 '20 at 2:15
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You need to turn off UV Sync Selection (upper left corner of UV editing window). When turned on it behaves in Shared vertex mode i.e. it selects UVs that share a mesh vertex, even if they are in different UV locations.

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